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C. elegans
Life Cycle (Kyanna)

The C. elegans life cycle has an embryonic stage, a set of four larval stages and an adult stage as the last stage. First, the C. elegans embryos are and left to hatch. After hatching, the larval stage begins. Within the larval stage, there are four molts that take place for the organism to enter the adult stage.

During the development of the embryo following fertilization, two meiotic divisions take place and the cytoplasm that was within the cell moves towards the posterior and the cortical cytoplasm is moved to the anterior region. After cleavage continues, and the cells continue to divide, gastrulation begins to take place. Gastrulation is marked by the stage where two cells move into the interior as part of the invagination process.

Fertilization (Kyanna)

The fertilization process in C. elegans begins with an amoeboid sperm cell. The sperm enters any part of the outer surface of the oocyte. While the point of entry of the sperm is known to be random, it later determines the posterior of the zygote. In the C. elegans that contain hermaphrodite oocytes, the mature oocyte goes through ovulation and travels to the spermatheca. This results in fertilization.

When fertilization takes place in the hermaphrodite C. elegans, the process begins near either the intestine or anus portion of the worm. The oocytes mature in a single file line depending on their stages in development. They are formed by budding from the syncytial gonad. Every oocyte bud created contains one nucleus. The oocytes are fertilized once they enter the spermatheca. After passing through the spermathecal, the embryos develop in the uterus and get discharged through the vulva.

Another route of fertilization occurs when a male nematode inseminates a hermaphrodite. The male sperm found in C. elegans is used to fertilized oocytes. The sperm of the C. elegans consists of a nucleus with mitochondria tightly packed around it. There are no flagellum on the sperm of C. elegans.

Cleavage(Alex)

C. elegans exhibit rotational holoblastic cleavage. When cleavage begins 4 founder cells are created. The cell division in C. elegans is Asymmetrical. This means that during the first cleavage the cleavage furrow is located asymmetrically along the axis of the egg. Axis determination in C. elegans is determined by elongated axis found during cell division. The decision of which end of the cell will become anterior or posterior is determined by which end the sperm cell enters the egg. When the sperm enter the oocyte Cytoplasm the centriole pushes the male nucleus to the nearest end of the oblong oocyte this end is the posterior pole. The second anterior to posterior asymmetry is seen shortly after this. P-granules move towards the posterior end of the zygote, so they can only enter the blastomere. P-granules are ribonucleoprotein that function in specifying the germ cells.

Gastrulation(Alex)

Gastrulation in C. Elegans starts early compared to other organisms, it begins just after the formation of the P4 generation in 24-cell Embryos. After the 28 cell stage, 2 daughter cells will migrate from the ventral side of C. elegans into the center of the embryo. When this occurs they will divide and cause the formation of a gut consisting of 20 cells. The inward migration of Ea and Ep cells creates a tiny blastopore.

Neurulation(Alicia)

Neurulation is the stage in which the nervous system begins development following gastrulation. The beginning of neurulation occurs when the cells that will later make up the nervous system become internalized.4 The cells of the ventral epidermis begin invagination into the gastrula and division occurs in order to produce neuroblasts. The ventral surface of the embryo is comprised mainly of neuroblasts at this point and the neuroblasts continue to undergo division. The neuroblasts then form organized groups of cells within the embryo to create the structure of the nervous system. Following the last round of divisions of the neuroblasts, the surface of the embryo begins to be covered by the hypodermis that makes up the chitinous outer layer of C. elegans6. The end of neurulation is called the comma stage which occurs 300 minutes after the beginning of fertilization.

Axis Signaling (Alicia)

Axis development in c. elegans is crucial in determining how differentiation will take place. To determine the anterior-posterior axis in C. elegans, the cortical PAR genes are needed. PAR-1 and PAR-2 proteins determine the posterior potion of embryo while PAR-3 and PAR-6 determine the anterior. Both of these are present following the first mitotic division of the C. elegans embryo. The dorsal-ventral axis of C. elegans is formed following the division of the AB cell during cleavage. The dorsal/ventral axis is determined following the second mitotic division when the AB cell changes orientation of the mitotic spindle. The dorsal and ventral axis of the C. elegans embryo is determined by maternal genes apx-1 and glp-1. Glp-1 is used to determine the ABp and Aba cell fates during cleavage along with the maternal gene apx-1 .11 The left right axis of the C. elegans embryo is determined as a result of the anterior-posterior and dorsal-ventral determination. This occurs after the division of the 4-cell embryo and involves rotational cellular rearrangement and Wnt signaling.

Advantages/ Disadvantages as a Model Organism

Before you can get to the advantages and disadvantages of using the C. elegans as a model organism, it is important to understand why this organism was first chosen for these specific studies and experiments. Specifically, Sydney Brenner was the scientist responsible for the discovery of C. elegans. Brenner proposed that biological research would need to require a model system that could grow in vast quantities in the lab, were cheap to maintain and had a simple body plan.

When performing experiments on the C. elegans, the advantages of using this species as a model organism vary with each specific scenario. So many questions about C. elegans development and its close relation to that of human developmental processes have been answered when specific analyzes are performed on these organisms. These advantages range from simple knowledge about cellular structure and development to getting answers to vaccine possibilities for neurodegenerative diseases. Because humans and the C. elegans organism both possess similar molecular signaling responsible for development control, many of the organism’s genes have been studied and manipulated. The C. elegans genes have proved to be useful as a prime model for human diseases research. Benefits of C. elegans also include that the entire genome is sequenced and annotated, the availability of an RNAi library comprising approx. 80% of the genes in the genome, the ease of generating transgenic strains and the recent development of gene-targeting approaches. Lifespan has also been an element that has been studied in close relation between C. elegans and overall human development.

While there are many advantages of using C. elegans as model organisms, the organism is not without disadvantages. Because the organism has a very simple body plan, its structure may lack many defined organs/tissues including a brain, blood, a defined fat cell, internal organs, and is evolutionary from humans. The size of the C. elegans organism is also a disadvantage when being used in an experiment as a model organism. The organism’s small size leads to questions about whether the size would be appropriate for biochemistry experiments. Biochemistry covers so many aspects of these studies and it is important that those aspects are similar on both small and large scale. Because of this size difference, it has led to a limited understanding of any tissue-specific signaling such as whether a gene is expressed in the hypodermis or the intestine. Lab experiments with these simple organisms is also very limited due to lack of space and nutrients in cell cultures.

1� Klass, M. R. (1977). “Aging in the nematode Caenorhabditis elegans: major biological and environmental factors influencing life span.” Mechanisms of ageing and development, 6: 413-429.

2� Klass, M. R. (1977). “Aging in the nematode Caenorhabditis elegans: major biological and environmental factors influencing life span.” Mechanisms of ageing and development, 6: 413-429.

4� Harrell, J. R., & Goldstein, B. (2011). Internalization of multiple cells during C. elegans gastrulation depends on common cytoskeletal mechanisms but different cell polarity and cell fate regulators. Developmental Biology, 350(1), 1–12. http://doi.org/10.1016/j.ydbio.2010.09.012

6� Sulston, J., & Horvitz, H. (1977). Post-embryonic cell lineages of the nematode, Caenorhabditis elegans. Developmental Biology,56(1), 110-156. doi:10.1016/0012-1606(77)90158-0

Zebrafish
Lead:

Zebrafish have proved to be an effective model organism in a number of different fields of studies. Their transparent and externally mass-produced embryos are affordable. Zebrafish are similar to the human genome, due to the fact that they are also vertebrates. They have a short life cycle, reaching maturation at 90 days past fertilization. Fertilization in zebrafish happens after oocyte maturation. Eggs are fertilized externally, which initiates embryonic development. Cleavage begins soon after fertilization, a process of mass cell division. Gastrulation in zebrafish forms the three germ layers; ectoderm, mesoderm, and endoderm. Neurulation is the process of neural tube formation. Axis signaling consists of multiple genes responsible for the formation of the anterior-posterior and dorsal-ventral axes. There are both advantages and disadvantages of the zebrafish model system.

Lifecycle:

The life cycle of a zebrafish is extremely fast. It normally takes about 3 months for the zebrafish to become an adult. It starts out as an embryo, which all offspring is considered embryos until they are born or hatch. (Parichy, D. M., Elizondo, M. R., Mills, M. G., Gordon, T. N., & Engeszer, R. E ,2009) The hatching of the embryo normally occurs around 48-72 hours after fertilization. The larva isn’t considered an embryo anymore, but it’s not considered a juvenile yet. (Parichy, D. M., Elizondo, M. R., Mills, M. G., Gordon, T. N., & Engeszer, R. E,2009) The juvenile state normally lasts about 4-12 weeks after fertilization, it just depends on the living situations. This state is where the fish has all the adult characteristics, except it is still not sexually mature. (Parichy, D. M., Elizondo, M. R., Mills, M. G., Gordon, T. N., & Engeszer, R. E ,2009) The Zebrafish can’t reproduce at this stage. The adult stage is when the fish can breed or reproduce. Metamorphosis is the step where a juvenile is formed into an adult fish. They lose some larval features during this stage. (Parichy, D. M., Elizondo, M. R., Mills, M. G., Gordon, T. N., & Engeszer, R. E ,2009) The post-embryonic development is basically any growing that occurs after embryogenesis. You can see in figure 1 all the individual steps of life cycle in the Zebrafish.

Fertilization:

The zebrafish must go through processes for oocyte maturation before fertilization can begin. The female oocyte goes through four stages before it is fully matured (Howley and Ho, 2000). Stages I, II, and III the oocyte is growing, and some maternal mRNA is distributed throughout the oocyte and other mRNA is specifically localized (Howley and Ho, 2000). There is a difference in the size of the mature oocytes between species. The zebrafish oocytes are anywhere from 7-700 micrometers (0.74 m). Zebrafish oocytes are transcriptionally active (Slack, 2013). Zebrafish egg form a Balbiani body, which is a visible cloud of mitochondria. The gene bucky ball (buc) is essential for the Balbiani body formation in zebrafish (Gupta et al., 2010). A study has shown that without buc, the zebrafish egg is unable to form a Balbiani body, which affects the animal-vegetal polarity (Figure) (Gupta et al., 2010).

The oocyte forms yolk granules for preparation of fertilization. The mature oocyte in zebrafish is released when a rise in gonadotrophin and a steroid is produced (Slack, 2013). The nucleus resides in the animal pole of the oocyte. Fertilization in zebrafish are external, meaning the eggs and sperm are released outside of the body for fertilization. The zebrafish sperm doesn’t have an acrosome. The sperm enter through the micropyle usually near the animal pole on the oocyte, which allows the sperm to penetrate the chorion (Sharma and Kinsey, 2008). At this point, fertilization has begun. After the sperm enters the oocyte through the micropyle, there is a rise in calcium in the newly fertilized egg. The calcium transient spreads from the sperm entry to the animal pole (Sharma and Kinsey, 2008).

zebrafish oocyte, and when the calcium is released. The rise in calcium is important for the rise of animal pole after oocyte has been fertilized. This figure shows the rise in calcium by calcium green fluorescence. At 2.0 min after sperm was injected, the calcium was most apparent. (Sharma and Kinsey, 2008)

The sperm can only enter through this small hole, known as the micropyle on the female egg. The micropyle allows for only one sperm to enter inside the oocyte. The gene buc prevents polyspermy from happening, by not forming multiple micropyles. (Pelegri et al. 2017) Once the male sperm haploid genome is released into the zygote, the gene futile cycle (fue) is required for female nuclear migration toward the sperm nucleus (Lindeman and Pelegri, 2012). After the egg is fertilized and the nuclei are fused together forming a diploid zygote, a cytoplasmic cap is formed at the top of the animal pole, in preparation for cleavage. (Slack 2013)

Cleavage:

Cleavage in zebrafish happens in the blastodisc. (Gilbert,1970) The divisions don’t divide the egg entirely (Gilbert,1970). The cytoplasm of the blastodisc becomes the embryo which is called discoidal. (Gilbert,1970) Figure 2 shows the process of cleavage in many different steps. Cleavage happens every 15 minutes and the first 5 divisions are vertical and creates 32 cells. (Jonatan M.W. Slack, 2013) The actin is activated and changes the embryo shape from spherical to pear shaped. The first 12 divisions form a mound of cells in the animal pole in the yolk cell. (Gilbert,1970) These cells make up the blastoderm. During the 10th cell division the zygotic gene transcription begins, and you can see three distinct cell populations.

The Yolk syncytial layer (YSL) is found during the 9th or 10th division, there are two parts of the yolk syncytial, which is external and internal. The internal is formed from the nuclei going under the blastoderm. (Gilbert,1970) The external is formed in front of the blastoderm. The enveloping layer is the outer cells of the blastoderm. The enveloping layer will later transform into the periderm which is just protection for the cell. The cells in between the enveloping layer and the yolk syncytial layer are called the deep cells. (Gilbert,1970) The blastoderm is set on what it will become before gastrulation happens.

Gastrulation:

Following cleavage, gastrulation occurs. The three cell movements that occur during gastrulation are: epiboly, involution, and convergent extension. The standard and desired result of gastrulation is the formation of three germ layers: ectoderm, mesoderm, and endoderm.

Beginning with epiboly, the cells of the blastoderm thin out and spread over the yolk. The blastoderm cells (deep cells) covering the yolk in the animal pole is termed the internal yolk syncytial layer, or internal YSL. The yolk cell in the vegetal pole where blastoderm cells have not enveloped is termed the external yolk syncytial layer, or external YSL (Bruce, 2015). Epiboly of cells continue and eventually cover the animal hemisphere of the cell. Once this occurs, the mesendodermal cell layer forms. The second cell movement is involution of cells. Cells start to move inward near the blastoderm of the cell. This involution creates the epiblast and hypoblast. Cells of the epiblast and hypoblast create the embryonic shield. As cells continue to involute, the endoderm and mesoderm form. During convergent extension, cells narrow and elongate perpendicularly. Slb/Wnt11 genes are the genes that play a significant role during

Neurulation:

Neurulation in Zebrafish is the development of a neural tube from the ectoderm layer, which is stated in gastrulation, and the neural plate (Blader and Strahle, 2000).

After gastrulation, the mesodermal layer meets the ectoderm, inducing neural induction. Neural induction is the first step in the neural tube formation (Schmidt et al. 2013). Some signals which are expressed early in neurogenesis is Wnt, Fgf, Nodal proteins, and retinoic acid. These are pattern signals that help with axis formation during the formation of the neural plate (Schmidt et al. 2013). A prechordal plate and the lateral region form from the central part of embryo. The dorsal ectoderm later becomes the neural plate (Slack, 2013). The neural plate is a flat sheet of neuroepithelium cells, which comes from the thickening of epiblast cells (Lawson and Schoenwolf, 2009). The lateral ends of the neural plate begin to thicken, and fuse together forming a neural keel. (Harrington et al. 2009). The neural keel then fuses at the midline, forming the neural rod (Schmidt et al. 2013). This process happens at about 16 hours of embryo development, forming a solid neural rod from the neural plate. There are some structural proteins, Pard3 and Rab11a involved in the midline polarization, during the formation of the neural rod (Schmidt et al. 2013). The neural rod goes through a process called secondary neurulation to form the neural tube. During neurogenesis, the mesoderm contains somites that are continuously differentiating for future organ development (Slack, 2013).

Early Development Leading to Axis Formation

At ten hours post fertilization, the zebrafish embryo has clearly recognizable anterior-posterior and dorsal-ventral axis (Schier, 2001). To generate this basic body plan, the zebrafish embryo undergoes rapid development and morphogenetic changes (Schier, 2001). A blastodisc is then formed on top of the yolk after fertilization, during the following three hours of development, rapid, synchronous cleavage occurs within the blastodisc to generate a blastula embryo of around 1000 cells (Sokol, 1999). After the mid blastula transition, at about four hours post fertilization (hpf), cell rearrangements reshape the blastoderm into what will be a characteristic vertebrate body plan. In the process known as epiboly, cells interpolate radially, thinning the blastoderm over the yolk. After gastrulation, epiboly movements have spread the blastomeres so that the blastoderm covers the entire yolk cell. There are also three other movements that contribute to the formation of the axis. At five hours post fertilization, cells at the margin internalize and form the hypoblast, the precursor of the mesoderm and endoderm (Westfall et al, 2003).

By six hpf, movements of convergence and extension have begun, which results in dorsal accumulation of cells moving from lateral and ventral regions of the blastoderm (convergence) (Yan et al, 1999). Converging cells then insert with dorsal blastomeres and spread them along the animal-vegetal axis, which leads to the lengthening of the anterior-posterior axis (extension). This convergence of all these cells to the dorsal side leads to the first clearly apparent break in the radial symmetry and forms the shield, a thickening at the dorsal blastoderm margin that is equivalent of the amphibian Spemann-Mangold organizer (Yan et al, 1999).

Advantages/Disadvantages of model system:

Using the zebrafish for research experiments has both advantages and disadvantages. Zebrafish, unlike fruit flies and mice, are vertebrates and therefore they have a similar genetic sequence to humans. Humans and zebrafish share 70% of genes. They also have similar physiology such as a spinal cord, pancreas, heart, kidney, muscle, brain, etc. just like a human. Their gene homology for human diseases is 84%. This is significantly beneficial in studying human diseases and seeing the effects, causes, etc. on a model organism like a human being. Even though mice are much more similar to humans, zebrafish are cheaper and don’t require much space. Another advantage is that zebrafish embryos is that they are mass produced externally (around 200 eggs), rapidly developed, and transparent, which allows for researchers to see exactly what is going on in development when the embryo has been genetically manipulated in some way, especially in mutagenesis experiments. The embryos are also able to easily conform to chemical mutagens that have been added to the water they are simply placed in without physical disturbance to the embryo itself.

There are always disadvantages to any model system. Although zebrafish do have human organs such as the heart, kidney, pancreas, etc., they do not have breast tissue, lungs, or prostate. These exempt organs in zebrafish limit the human diseases they research in the model organism, such as heart or breast cancer. Humans are warm-blooded mammals whereas zebrafish are cold-blooded vertebrates, another difference in the physiology. Unlike mice and fruit flies, zebrafish do not have big library of mutants and transgenics for reference when conducting mutagenesis experiments.

Drosophila
= Introduction = Drosophila which are also known as fruit flies are one of the model organism systems that we use to understand genes and other organisms. Drosophila are very complex organisms. They have neural development similar to mammals which make them a great model system for learning more about development.

Lifecycle

Drosophila is a fruit fly seen on bananas, and its life cycle happens really quickly. This helps scientist to easily study them, it is highly preferred, and it helps answer the question of the link between genetics and developmental biology. Drosophila get to their adult stage by nine days. An organism that has larval and pupal stage without a nymph stage before being an adult is called holometabolous, and drosophila is one of them. Drosophila mates in the ten weeks of their life span, and the sperm is stored in the female's body in a seminal receptacle for internal fertilization. Even before an egg is fertilized in a female's body, females still lay eggs from 50 to 70 in a day. The ovum of drosophila had layers in the outer and inner. The outer layer is tough, and it is called the chorion, while the inner layer is a thin envelope. There are two small filaments at the anterior end of the egg called respiratory filaments that helps in gas exchange. The egg needs these filaments because the females usually lay the eggs almost half-way into a rotten fruit. Within 22-24 hours the eggs hatch at room temperature.

The larva then comes out as a little worm, and this first larva appearance is called the first instar larva. The food the instar larva feeds on is the substrate that the eggs were laid in. After about an additional   25 hours the larva, molts into a big worm which is the second instar larva. After enough feeding, it molts into the third instar larva which happens after 24 hours. The third instar larva is the largest of the three. As it feeds it also climbs out of the substrate to be clean and dry awaiting pupation. After about 30 hours the third instar larva molts into the pupa. At the early stages, the pupa is very immobile and yellowish- white in color. It darkens as it continues to develop. The pupal stage is where the development into an adult fly happens. The adult fly is also known as the imago. As it is metamorphosing, some of the larval organs are lysed but some are also kept. An example of an organ that is not lysed is the nervous system. Some and maybe most of the adult organs form fresh from two cells. These cells are what is referred to as the imaginal disc and the histoblasts. These cells lay dormant through the instar stages. Imaginal   discs, which are formed from epithelial cells, are what will later form wings, legs, eyes, etc. The histoblasts form the abdominal epidermis and internal organs. It takes about 3-4 days for the pupal stage and then the imago breaks out of the pupal case known as the eclosion. When they emerge, the female eggs are not ripe for reproduction until two days after eclosion for the cycle to begin again even though the male is very sexually active.

= Fertilization = Fertilization within Drosophila occurs a bit differently compared to other organisms. In Drosophila fertilization is internal and females store sperm Many post-mating barriers occur within fertilization that can exist at a variety of levels. These allow the process of unification with the gametes to  occur. Sperm must successfully enter the female and be transported to the storage organs, the spermathecae or ventral receptacle. The sperm must stay alive until they are utilized by the female. The sperm later will be retrieved from the storage organs, activated, and then will enter an oocyte. It must pass through the reproductive tract to complete this process. Entrance of sperm to the egg must travel through the micropyle to trigger the formation of a normal zygote. A failure at any of these steps prevents fertilization. This is one of the reasons that many sperm are withheld in case the fertilization are not successful. In order to help assortative fertilization, there must be some degree of heritable specificity in the male and female components in the process. Fertilization barriers help to remove abnormal, non-viable sperm, or ejaculates to ensure successful fertilization. The male must also signal the female to keep his sperm alive thus allowing her to utilize them in fertilization for many more eggs. Since the female can hold onto sperm from one or many males, then the female must be able to detect and respond to the sperm differences. This will also lead to differential fertilization within the eggs. Having sperm from different males will cause different potential signals to be sent out. These can signal different receptors and will further complicate the fertilization process.

Both the sperm and non-sperm components of the ejaculate are highly different in Drosophila. Sperm in Drosophila species have a high levels of sperm length variability when compared to other animal taxa. They range from 0.32 mm in Drosophila persimilis to 58.29 mm in Drosophila bifurca. In the bifurca, the sperm are approximately 12 times longer than the male itself. There is little to no variation when analyzing individuals of the same Drosophila species. Species also differ in how much of the sperm tail enters the egg. These differences are important when analyzing post-      fertilization isolation. Female sperm storage organs show interspecific variability as well; species   also differ in which of those two organs, the spermathecae or the ventral receptacle, sperm are stored. These differences lead to whether or not assortative fertilization may be mediated by a mismatch between sperm morphology or storage organs within the females. In four different species of Drosophila, sperm differ considerably in length. The sizes of the storage sites within the       females also differ in length. Females of Drosophila pachea and Drosophila wassermani store sperm only in the spermathecae. Drosophila nannoptera females use only the ventral receptacle.

Morphological differences may end up deciding storage site, sperm viability or its reduction. This can also result from biochemical interactions that occur while the ejaculate passes through the female reproductive tract.

Extensive variation has been found within research in the chemistry and function of the non-sperm component of the ejaculate. More specifically, the accessory gland proteins or Acps of the ejaculate. Approximately 80 of these proteins are transferred to females at the time of mating. Only 9               of these proteins have been described based on function and chemistry. It is clear from these studies that this group of substances are used to alter female behavior. This alters female behavior by stimulating oviposition and delaying the time that the female will mate to another male and facilitate the storage of sperm in the female.

It is clear that within species there is considerable sequence polymorphism at the loci encoding these proteins and that the proteins show considerable sequence divergence between species as well as species specificity of their functions. In D. melanogaster, variation in four accessory gland proteins examined appears to be associated with displacement abilities in ejaculate competition experiments, although these experiments were not designed to detect any interaction with female genotype. Once the sperm is inside the female, there are a variety of reactions that must occur for successful fertilization. Some of these reactions occur within the female reproductive tract itself, but others involve the action of male-derived substances in other sites in the female. Evidence of the reactions inside the female reproductive tract is both direct and indirect. Acp36DE has been shown to localize at the entrance to the sperm storage organs. This will help to put all of the sperm into storage without any diversion.

Some reactions occur outside of the female tract come from three different protein: esterase 6, sex peptide and Acp26Aa. These small proteins are transferred quickly to the female hemolymph, even before copulation has terminated. They produce the same response in females. This is: increased oviposition and decreased receptivity to mating with another male. The 2 effects from sex peptides on female behavior are from the same unknown molecular target. The increase in oogenesis is mediated by the resultant increase in juvenile hormone synthesis in response to the sex peptide. An important direction for future research is the identification of the female targets of these proteins and the detection of variation in the targets that could provide mechanisms for differential fertilization. Species-specific morphological and biochemical features of the female tract are easily inferred from the larger size and longer duration of the insemination reaction mass. In house flies, female accessory gland secretions have been demonstrated to activate the sperm acrosome, enabling it to penetrate the micropyle. The actual substance involved has not been identified, nor is there any evidence as to species specificity in the activation process.

Cleavage

Most insect eggs undergo superficial cleavage, wherein a large mass of centrally located yolk confines cleavage to the cytoplasmic rim of the egg. One of the fascinating features of this cleavage type is that cells do not form until after the nuclei have divided. Cleavage in a Drosophila egg in the image below. The zygote nucleus undergoes several mitotic divisions within the central portion of the egg. In Drosophila, 256 nuclei are produced by a series of eight nuclear divisions averaging 8 minutes each. The nuclei then migrate to the periphery of the egg, where the mitoses continue, at a progressively slower rate. During the ninth division cycle, about five nuclei reach the surface of the posterior pole of the embryo. These nuclei become enclosed by cell membranes and generate the pole cells that give rise to the gametes of the adult. Most of the other nuclei arrive at the periphery of the embryo at cycle 10 and then undergo four more divisions at progressively slower rates. During these stages of nuclear division, the embryo is called a syncytial blastoderm, meaning that all the cleavage nuclei are contained within a common cytoplasm. No cell membranes exist other than that of the egg itself.

Although the nuclei divide within a common cytoplasm, this does not mean that the cytoplasm is itself. When the nuclei reach the periphery of the egg during the tenth cleavage cycle, each nucleus becomes surrounded by microtubules and microfilaments. The nuclei and their associated cytoplasmic islands are called energids. Following cycle 13, the oocyte plasma membrane folds inward between the nuclei, eventually partitioning off each somatic nucleus into a single cell. This process creates the cellular blastoderm, in which all the cells are arranged in a single-layered jacket around the yolky core of the  egg. Like any other cell formation, the formation of the cellular blastoderm involves a delicate interplay between microtubules and microfilaments. The first phase of blastoderm cellularization is characterized by the invagination of cell membranes and their underlying actin microfilament network into the regions between the nuclei to form furrow canals. This process can be inhibited by drugs that block microtubules. After the furrow canals have passed the level of the nuclei, the second phase of cellularization occurs. Here, the rate of invagination increases, and the actin-membrane complex begins to constrict at what will be the basal end of the cell. In Drosophila, the cellular blastoderm consists of approximately 6000 cells and is formed within 4 hours of fertilization.

After the nuclei reach the periphery, the time required to complete each of the next four divisions becomes progressively longer. While cycles 1–10 are each 8 minutes long, cycle 13, the last cycle in the syncytial blastoderm, takes 25 minutes to complete. Cycle 14, in which the Drosophila embryo forms cells (after 13 divisions), is asynchronous. Some groups of cells complete this cycle in 75 minutes, whereas other groups of cells take 175 minutes. Transcription from the nuclei (which begins around the eleventh cycle) is greatly enhanced at this stage. This slowdown of nuclear division and the concomitant increase in RNA transcription is often referred to as the mid-blastula transition. Such a transition is also seen in the embryos of numerous vertebrate and invertebrate phyla. The control of this mitotic slowdown appears to be affected by the ratio of chromatin to cytoplasm.

= Gastrulation = Gastrulation is after cleavage and its main contribution to embryonic development is by infolding of a band of cells in the midventral region. Before gastrulation happens, the drosophila embryo is divided into seven parts. There is the anterior midgut, posterior midgut, neural and epidermal ectoderm, hindgut ectoderm, stomodeal ectoderm, extraembryonic amnioserosa, mesoderm. The initial step in the process of gastrulation is the formation of a ventral furrow, where two cell groups invaginate to form  the endoderm and mesoderm. This happens in conjunction with the infolding of the mesoderm.

Mesoderm invaginates and the germ band and pole cells move forward. The furrow that is formed as  the cells move forward has an anterior and posterior end, and at these two positions there is an invagination of the endoderm which results in the formation of the anterior and posterior midgut respectively, the ventral endoderm invaginates and creates the anterior midgut, while the dorsal endoderm create the posterior midgut. The anterior stomodeum, also known as the foregut, and posterior proctodeum, also known as hindgut, are later formed by the invagination of the ectoderm and as the endoderm thickens. The stomodeum and the hindgut invaginate and get deeper. The anterior and posterior midgut start to extend and get longer. The cells around the ventral midline then change shape to form the ventral furrow. After everything is formed, the germ band and cells start to move back  down, and this creates the posterior gut or airduct opening. Anterior and posterior midget joins  together, and segmentation becomes obvious. Germ band moves to the head region and make head disappear to the interior. The process is very quick, and it turns epithelium cells that are identical to invaginated tube cells. Another form of infolding process called the cephalic furrow also happens in addition to that of the ventral infolding discussed above. This lateral infolding indicates the exact  position of the future head.

The consequence of gastrulation is the creation of three germ layers that extends around the ventral circumference of the egg. This multilayered germ band formed stretches along the dorsal side of the egg and with this the embryo performs, for the lack of a better term, acrobatic maneuvers to touch its head to the posterior end. Following this the germ band bounces back to its position. As this happens, major parts of the embryo appear. These include the mandibular, maxillary, labial, thoracic and abdominal regions. One important thing to note during gastrulation is the fact that at some point the head that forms disappears causing the thoracic regions to overgrow the head region. Only a little region of the head will appear.

= Neurulation = In Drosophila this is the process where neuroblasts generate the Central Nervous System (CNS). The neuroblasts divide asymmetrically and can eventually become glia cells or neurons. [7] After the division of neuroblasts they produce a daughter neuroblast and a ganglion mother cell or GMC. If the neuroblasts divide at an increasing rate they may very well be the cause of tumors that are malignant.

[9] The GMC produce two new cells during the single division that they have. The cells that are a result of their division can either become glial cells or neuron cells. The neuroectodermal cells are the cells that will end up becoming the central nervous system. The ventral nerve cord comes from the side of the neuroectoderm that is towards the tail and the belly side.

The neuroblasts form into segments. There are three segments for the brain, subesophageal and thoracic. The abdomen has nine segments for a total of 18 segments. After the division of neuroblasts they produce a daughter neuroblast and a ganglion mother cell or GMC. If the neuroblasts divide at an increasing rate they may very well be the cause of tumors that are malignant. Researchers have discovered the information they have about Drosophila from conducting experiments and searching for mutations then identifying them. By doing this they have discovered that sensory axons only grow towards the Central Nervous System. Photoreceptors are different than other parts of the body, they are connected to the brain by interneurons, therefore they don’t form axons.

There are groups of cells that Drosophila have called neural equivalence groups. These groups are the first to divide which is what starts neurogenesis. This occurs when one neuroblast splits or migrates into two. Proneural genes are expressed by certain cells within this zone. These will become neural cells  while the ones that don’t express proneural genes will end up becoming epidermal cells. Certain transcription factors allow Delta, a ligand, to bind to Notch. This then interacts with other genes which in turn cause transcriptional repressors that then act on the proneural genes. This is a process called  lateral inhibition, which means that a gene or neuron is reducing the activity and decreasing the amount of possible movement that its neighboring genes or neurons have.

The process of neurulation in Drosophila is a cascade of transcription factors. Hox genes have an important role in determining the formation of segmentation and specification of the identities for some motoneurons. They also seem to have a hand in segment-specific cell determination. This is something that they do with the help of their cofactors. At least that is what is believed to be the case there hasn’t been a lot of research completed on this.

= Axis Signaling = Localizing mRNA during oogenesis is important producing axes and maintain polarity in Drosophila. The mRNA gets localized due to the proteins within the organism. This process is done using microtubules. Localizing RNA also helps produce germline and somatic cells in the embryos future. Microtubules use localization to create the body axes by using three major genes. The first two genes are Bicoid (bcd) and Oskar (osk) which help create the anterior- posterior axis by localizing mRNA. The third gene is Gurken (Grk) and it produces the dorsal- ventral axis.

ANTERIOR-POSTERIOR

In the development of the A-P axis, Bicoid is localized to the anterior side while Oskar is on the posterior. Bicoid creates a protein gradient that determines the head and thoracic regions. Oskar helps produce the poles and the abdominal region with the help of Nanos RNA.

Oskar produces a long and short form which comes from start codons. The long form is just used for anchoring while the short form nucleates pole plasm. Bicoid is localized using Exu protein and involves several different proteins to keep it on the anterior side.

DORSAL-VENTRAL

There are two main genes that produces the dorsal-ventral axes and those are Gurken (Grk) and torpedo (Top). The two genes are used for signaling to distinguish dorsal cells. The Toll receptor is used for the asymmetry in the embryo. The Toll receptor is constrained to the ventral region and inhibited on the dorsal due to Grk-Top signaling. Wingless (Wg) and Decapentaplegic (Dpp) are also genes that control the D-V and A-P axes of the wings and legs. Wg is expressed in ventral cells while Dpp is inhibited the same cells, but it is opposite in the dorsal region. Dpp is expressed while Wg is inhibited.

PROXIMAL-DISTAL

While Wg and Dpp plays an important role in D-V and A-P axes, they also are genes that determine the proximal-distal axes of the legs of drosophila. The expression of the two genes are met in the middle of the leg. These two genes encode of three different genes that define the different domains within the legs. Distal-less codes for a protein (DII) that is used for early leg formation, but as the organism matures, DII is only expressed in the distal portions of the leg. Dachshund also codes for another protein that is partially shown in the same domain as DII but helps with the portion that contains the parts of the upper leg like the femur. Extradenticle (Exd) and homothorax (Hth) are expressed in the proximal regions around the hip and connective parts to the body.

Advantages/Disadvantages

Drosophila is an important model organism for many reasons. They have short generation times, inexpensive and they also have balancer chromosomes. One of those is that drosophila has been important for cancer studies. Researches have picked these organisms due to their variations within their genome, and they can also do large screenings. Drosophila has helped scientists understand many tumors forming genes like Hippo, Notch, Dpp and JAK-STAT. They have also been used to understand genes that cause heart failure. The genes of drosophila are similar to the genes of mammals. A large percentage of human diseases has been found with the drosophila genome. A disadvantage of using the fruit fly is that they are really small, only 3mm in length.

'''These references need to be added to the paper. (comment by PD)'''

12.) Leptin, M. (1999). Gastrulation in Drosophila: the logic and the cellular mechanisms. The EMBO Journal, 18(12), 3187-3192. doi:10.1093/emboj/18.12.3187

13.) Kam, Z., Minden, J. S., Agard, D. A., Sedat, J. W., & Leptin, M. (1991). Drosophila gastrulation: analysis of cell shape changes in living embryos by three-dimensional fluorescence microscopy. Retrieved from, http://dev.biologists.org/content/develop/112/2/365.full.pdf

14.) Development of the Fruit Fly Drosophila melanogaster. Bainbridge and Bownes, (1981).

Retrieved from,   http://web.as.uky.edu/Biology/faculty/cooper/Population%20dynamics%20examples%20with

%20fruit%20flies/08Drosophila.pdf

Chick Development
Lead Section:

The chick was the original model organism, dating back to the 1600’s. Due to the extensive history, the chick is an ideal organism to study. Its external development and transparent embryo make it easy to follow the stages through development. Eggs are also cheap and easy to obtain and the resemblance to human embryos at anatomical, molecular and cellular levels makes them a common organism to study.

The chick takes 21 days to hatch and becomes an adult three months after hatching. Once an adult, they live 8-10 years.

Gastrulation takes place after cleavage. It gives rise to the three germ layers. The ectoderm, which is the outer layer, gives rise to the to the skin, epidermis, and tissue that will eventually turn into the nervous system. The mesoderm, middle layer, gives rise to the circulatory system, the kidneys, skeletal compartments, and somites. The endoderm is the inner layer and gives rise to the respiratory and gastrointestinal tracts. The primitive streak marks the beginning of gastrulation. Convergent extension promotes the elongation of the primitive streak. The anterior region of the streak is referred to as Hansen’s node. The order that cells enter and pass through Hansen’s node will help determine which germ layer they become. At the end of gastrulation, Hansen’s node regresses and the notochord forms.

Neurulation occurs after gastrulation and will form the central nervous system in the chick. The notochord signals stem cells to orient themselves along the dorsal axis of the embryo, which creates the neural plate. The neural plate then elevates itself and forms neural folds. It also will invaginate to form the neural groove. Chick embryos perform both primary and secondary neurulation.

Axis Formation is the establishment of differentiated portions of the embryo. The basis for axis formation has been broken down into three types of axis formation. First, Anterior and Posterior axis formation where the anterior is the head region and the posterior is defined by the lower half of the embryo. Next, is Dorsal and Ventral axis formation where Dorsal is the back side of the embryo and Ventral is the belly side of the embryo. Finally, Right and Left axis formation which is crucial for determining how organs will form such as the heart and gut folds. All of these are established through gene regulation mechanisms and are crucial in the creation of a healthy functional organism.

Lifecycle

Similar to mammals, oogenesis occurs during fetal life and chicks have a lifetime supply of oocytes upon hatching. The oocyte becomes fertilized if the hen has mated recently, after ovulation when the oocyte is released form the ovary to the oviduct. In the roughly 20 hours the oocyte spends in the oviduct, uterus, its shell is formed. The egg remains in the vagina of the hen until it is laid. When the egg is laid, gastrulation typically begins. The beginning part of development is described by Eyal-Giladi and Kochav in series and the rest of the developmental life cycle of a chick is best described in 46 stages described by Hamburger and Hamilton. The chick takes roughly 21 days to hatch. The chick becomes an adult after 3 months and they live for 8-10 years, the overall cycle can be best seen here.

'''These references need to be placed in the text and in the reference list. (comment by PD)'''

Fertilization: (Tia)
 * 1) ^ Jump up to:a b c d e 1949-, Slack, J. M. W. (Jonathan Michael Wyndham), (2013). Essential developmental biology (3rd ed ed.). Chichester, West Sussex: Wiley. ISBN 9780470923511. OCLC 785558800.
 * 2) ^ Jump up to:a b c Hamburger, V.; Hamilton, H. L. (December 1992). "A series of normal stages in the development of the chick embryo. 1951". Developmental Dynamics: An Official Publication of the American Association of Anatomists. 195 (4): 231–272. doi:10.1002/aja.1001950404. ISSN 1058-8388. PMID 1304821.
 * 3) ^ Jump up to:a b c d Gandara, Carlos André Tarrio; Araújo, Eduardo Spadari; Motta, Ubirajara Indio Carvalho da (May 2008). "Chicken embryo as an experimental model for the study of gastroschisis". Acta Cirurgica Brasileira. 23 (3): 247–252. ISSN 0102-8650. PMID 18552995.
 * 4) Jump up^ Stern, Claudio D. (January 2005). "The chick; a great model system becomes even greater". Developmental Cell. 8 (1): 9–17. doi:10.1016/j.devcel.2004.11.018. ISSN 1534-5807. PMID 15621526.
 * 5) ^ Jump up to:a b c Vergara, M Natalia; Canto-Soler, M Valeria (2012-06-27). "Rediscovering the chick embryo as a model to study retinal development". Neural Development. 7: 22. doi:10.1186/1749-8104-7-22. ISSN 1749-8104. PMC 3541172  . PMID 22738172.
 * 1) ^ Jump up to:a b c Vergara, M Natalia; Canto-Soler, M Valeria (2012-06-27). "Rediscovering the chick embryo as a model to study retinal development". Neural Development. 7: 22. doi:10.1186/1749-8104-7-22. ISSN 1749-8104. PMC 3541172  . PMID 22738172.

Cleavage: (Tia)

After fertilization, the small disc of an egg goes through cleavage at the egg and only the egg. The first division in cleavage occurs centrally and continue to form a single layered blastoderm. Divisions later continue to add more layers. The divisions cause a small space to be formed in between blastoderm and yolk sac called the subgerminal cavity where the blastoderm cells absorb the albumen on the outside of the yolk, also known as the egg whites. When this happens in cells in the center of the blastoderm are shed creating a hollow space in the middle of the cells ready for gastrulation.

Gastrulation

Gastrulation is critical to all organism’s survival. It turns a multicellular organism into fully functioning organs. It occurs about seven hours after fertilization and begins as soon as cleavage is accomplished. The blastula, a single layer of cells, doubles over to form two layers. Each of these layers will play a different role in the formation of the embryo. The formation of the primitive streak marks the start of gastrulation. Mesodermal precursor cells are the origin of the primitive streak. The primitive streak appears when the epiblast begins to thicken. [4] As cells are migrating to form the primitive streak, an indention forms called the primitive groove or the blastopore. Convergent extension allows for the elongation of the primitive streak. Once the primitive streak is formed, the embryo now has true anterior-posterior axis. The anterior region of the primitive streak is referred to as Hansen’s node, which functions are the organizer for the cells. The order that cells enter the blastocoel and through Hansen’s node will determine which of the three germ layers they will become. Cells differentiate and migrate to form the ectoderm, which is the outer layer, the mesoderm, the middle layer, and the endoderm, the inner layer. Ectoderm cells give rise to skin, epidermis, the neural crest and tissue that will eventually form the nervous system. Mesoderm cells will turn into the circulatory system, the kidneys, and skeletal compartments. It will also give rise to somites, which will form muscle, cartilage for the ribs and vertebrae, the dermis, the notochord, blood vessels, and bone for the chick. The endoderm cells give rise to the respiratory and gastrointestinal tracts, such as the liver and pancreas. As the primitive streak begins to descend, Hansen’s node migrates to the posterior region. This will eventually form the anus of the chick (Vasiev et al). As a result of the anterior-posterior division, cells progress through the stages at different rates. Typically, the anterior region is more advanced than the posterior. The mesoderm and endoderm cells migrate inward and surround the yolk by epiboly. No true archenteron is formed during chick gastrulation. As gastrulation comes to an end, Hansen’s node begins to regress and leaves behind the notochord and the ectoderm cells finally migrate and surround the yolk.

Neurulation

Neurulation is the formation of the central nervous system and the development of the neural tube in the ectoderm. Once the notochord has been formed during late gastrulation, signals are sent to stimulate stem cells in the embryo of the chick. This causes the stem cells to orient themselves along the dorsal axis of the chick, which creates the neural plate. The neural plate elevates itself, which forms neural folds. Then invagination of the neural plate creates the neural groove. Before the neural groove completely closes, a group of cells called the neural crest form above the tube. The neural crest contributes to the formation of cranial nerve ganglia and skeleton in the skull.

There are two types of neurulation, primary and secondary. Primary neurulation is when neuro-plate cells are directed to be proliferated, invaginated, and pinched off to form a hollow tube. This occurs when the neural groove, which is located in the ectoderm, closes to form the neural tube in the anterior region. As the neural tube closes, it creates the midbrain, forebrain, and hindbrain vesicles. The forebrain will give rise to cerebral hemispheres and optic vesicles. The midbrain will later form the optic receptors for optic nerves and the hindbrain will form the cerebellum and the medulla. Secondary neurulation is when the neural tube is produced by a solid cord of cells that sink into the embryo to form a hollow tube. Like most organisms, chicks perform both primary and secondary neurulation. The anterior portion of the neural tube is formed by primary neurulation. Everything that is posterior to the hind-limbs are made by secondary neurulation. The remainder of the neural tube will form the spinal cord.

Axis formation:

Axis formation is the determining of the different portions of the embryo that is detrimental in the developmental process because without this axis formation the embryo would not be successful. Genes are the establishers and determinants of axis formation. There are multiple different mechanisms utilized to control these gene gradients. From gap junction communication, movement of cytoplasm to move a gene to one side of the embryo, activating and inhibiting proteins, and Henson’s node in the case of the chicken. For the simplest explanation of how these are created and maintained is that specific genes are expressed in certain parts of the embryo and usually inhibit the opposite sides expression to maintain these genetic gradients.

Anterior/Posterior formation:

In chicken axis formation Dorsal and Ventral formation is closely related to Anterior and Posterior due to the disc embryo design that causes much overlap in these two cycles. The original step in these two axis formations is the centrifugation of the egg as it travels down the reproductive tract. This causes the proteins to be moved to appropriate locations in the embryo. The Posterior end of the embryo is highly expressed with BMP and B-Catenin coming from the PMZ in the vegetal pole of the embryo establishing the organizer. The next steps in anterior and posterior axis formation can be broken down into three steps.

The references from here down need to be fixed (PD)

First, in Chicken embryos, there is the use of Hensen’s node, which is their organizer.[3] Hensen’s node is in high concentration of anterior expression gene noggin.[3]  Hensen’s node begins at the most anterior part of the embryo and then moves down the notochord of the embryo to the posterior end of the embryo while, establishing the head and somites as it moves down as well as leaving behind a trail of gene expression.[2][3] These gene gradients that are established are detrimental in creation of axis formation.

The next step in anterior/posterior formation is when Smad1 comes in by upregulation through BMP binding BMPR1 and BMPR2 and then creates Smad1.[3]  While this is happening, BMP is being repressed by Noggin while Noggin is repressing BMP, with Noggin being highly expressed in the anterior and BMP being highly expressed in the posterior.[3]  While they both inhibit each other from being expressed on the wrong side.[3]

In the final step of Anterior and Posterior axis formation a signaling pathway is creating concentration gradients that have RA and FGF4 both expressing posterior strongly.[1][3]  While cryp26 is the prominent anterior gene.[1][3]

Left/Right axis formation:

In left and right axis formation of chick embryos there is a movement of proteins by cell gap junctions that signal for an intercellular current.[1]  This moves PitX2 to the left as well as Cerberus and Nodal that establish the heart and gut folds.[3] While, FGF8 is found to be the determinate of the right side because it suppresses all three of the left side proteins.[1] [3]
 * 1) Schlange, Thomas; Arnold, Hans-Henning; Brand, Thomas (July 2002). "BMP2 is a positive regulator of Nodal signaling during left-right axis formation in the chicken embryo". Development (Cambridge, England). 129 (14): 3421–3429. ISSN 0950-1991. PMID 12091312.
 * 2) ^ Jump up to:a b c d Grieshammer, U.; Minowada, G.; Pisenti, J. M.; Abbott, U. K.; Martin, G. R. (December 1996). "The chick limbless mutation causes abnormalities in limb bud dorsal-ventral patterning: implications for the mechanism of apical ridge formation". Development (Cambridge, England). 122 (12): 3851–3861. ISSN 0950-1991. PMID9012506.
 * 3) ^ Jump up to:a b c d e f g h i j k l m n o 1949-, Slack, J. M. W. (Jonathan Michael Wyndham), (2013). Essential developmental biology (3rd ed ed.). Chichester, West Sussex: Wiley. ISBN 9780470923511. OCLC 785558800

Advantages/disadvantages as a model system
The chick was the first organism used to study development. The long history of studying the chick is an advantage because it was the only focus for so long so a lot of time and research was spent on understanding it. Other advantages include: Despite all of the advantages, chicks also have disadvantages as a model system that include:
 * Cheap[3]
 * Easy to make room for[3]
 * Availability[3]
 * Resembles human embryo at anatomical, molecular and cellular levels[4]
 * External development allows for accessibility at all stages[5]
 * Cutting a small hole in the shells allows for manipulation in ovo
 * Advanced embryos can be used to transfer small pieces of tissue onto the chorioallantoic membrane[6]
 * Culture of some undeveloped or immature parts of organs is possible in vitro
 * Poor use for genetic work
 * Long life cycle
 * Take up a lot of space once they hatch[2]
 * Unable to successfully establish genetic modified chicken lines[7]
 * Different compositions of amniotic fluid and blood compared to humans and that can alter things in development and make them hard to use as research for humans[7]
 * Transgenesis and targeted mutagenesis do not have a routine protocol

Life Cycle
Wild mice can survive to be up to four years of age though prevalence of predators often shortens the average lifespan to within two years (Miller 2002;). Mice can begin breeding 50 days after birth, with females potentially having their first estrous cycle from twenty-five to forty days from birth. Mating occurs at night and initiates ovulation in the female (Slack, 2013). Early development follows fertilization with: cleavage, gastrulation, neurulation, and axis formation. Litter sizes range from about ten to twelve pups with each pup born nude, blind, and earless about twenty days after fertilization (Wiki Mouse; Slack 2013).

Fertilization

Haploid gametes develop from meiosis of germ cells starting in embryogenesis. Post-meiosis, sperm cells have condensed their DNA and modified their organelles for fertilization. The nucleus is headed by an acrosome and tailed by centrioles, mitochondria. The overall shape of the cell is elongated; most of the cytoplasm is extended into the flagellum (source?). Egg, or oocyte, cells are arrested in metaphase II of meiosis; completion of meiosis does not occur until after fertilization (Slack, 2013). Egg cells are secreted into the oviduct following mating as cumulus cells, oocytes enveloped in a zona pellucida and surrounded by follicle cells (Slack 2013; Liu, 2014).

Following mating, sperm cells navigate to the oviduct, completing capacitation during this time (Slack, 2013). Fertilization of the oocyte occurs within the oviduct. Sperm cells bind to the zona pellucida by interacting with ZP3 and ZP2 (Slack, 2013; Howes, 2001). The initial interaction between the sperm cell and ZP3 activates the acrosomal reaction: the contents of the acrosome are released to digest the zona pellucida (Slack, 2013). Sperm-egg recognition is carried out at the egg surface with ADAM and integrin bindings (Slack, 2013). There is a preference for binding at the equatorial region, ninety degrees from the location of the first polar body; sperm binding influences cleavages at later development stages and unfavorable binding points reduces the ability to properly distribute transcription factors (Piotrowska-Nitsche, 2013). Once bound, plasma membranes of both gametes fuse and the remaining contents of the sperm are integrated into the egg.

Following fusion, intracellular calcium levels begin to oscillate and meiosis is resumed. Cortical granules are released and migrate to the plasma membrane and modifying receptors to block fusion by more sperm (Slack, 2013). A second polar body is expelled with the completion of the final meiotic division. DNA replication occurs within the paternal and maternal nucleus. The finished chromosomes align with the maternal spindle fibers prior to cleavage (Slack, 2013).

Cleavage

The developmental step after fertilization is cleavage. The zygote begins dividing into individual cells. There is no growth involved with these divisions. Forty-eight hours after fertilization, the first three cleavage have occurred. The genes that make up the zygote begin to be expressed once cleavage has resulted in two cells (Slack, 2013). The individual cells resulting from cleavage are called blastomeres. Once eight cells are reached the process of compaction begins. Compaction is important for increasing intracellular contact. Blastomeres flatten out and the embryo changes shape, looking more spherical. E-cadherin is a molecule that is heavily involved in the process of compaction(Larue, 1994). Gap junctions are formed during compaction as well, these are necessary for the diffusion of molecules in the embryo (Slack,2013).

Once compaction is complete the embryo is called a morula and is referred to as so until thirty- two cells have been reached. Cleavage continues, and the embryo will start forming a blastocoel. The blastocoel is a cavity full of fluid that begins forming after thirty-two cells are reached. For formation to begin, desmosomes and tight junctions must be present in the embryo creating a seal between the inside and outside. The embryo moves into the uterus during the process of blastocoel formation (Slack,2013). The tight junctions keep the fluid in the cavity as it expands (Echert, 2004). The blastocoel will continue expanding the embryo, into the blastocyst. The blastocyst is divided into two layers, one inner, the other outer (Slack,2013). The outer layer, the trophectoderm, contains tight junctions that help form a protective barrier over the inner layer. The inner layer is made up of the cavity fluid and a ball of cells called the inner cell mass (Morinaki, 2007). These newly formed layers will give rise to new tissues. Tissue formed in the inner cell mass is called the primitive endoderm. The trophectoderm forms two tissues, one of which is polar. The other tissue becomes polytene cells. These cells contain DNA which replicate and increase. There is no mitosis involved with this DNA replication (Slack,2013).

Gastrulation

The developmental process of gastrulation results in the embryo consisting of three layers, the ectoderm, mesoderm and endoderm. The formation of these layers all begin in the epiblast. One end of the epiblast, will give rise to what is known as the primitive streak. This streak lays out the future anterior-posterior axis. Where the primitive streak begins forming, marks the posterior end (Slack, 2013). The formation of the primitive streak also establishes the future left-right axis (Tam, 1997).

The streak is a series of cell movements that moves across the embryo from posterior to anterior. During these cell movements rapid cell division is occurring. These newly divided cells are then integrated into their respective layer. (Hegel 1995). At the anterior end of the streak, a node forms which is made up of two layers. These newly formed layers, along with the node will give rise to the body plan of the embryo later in development. (Slack, 2013).

Neurulation

Neurulation is defined as morphogenic movements within the primitive streak to form an enclosed neural tube.1 Morphogens can be defined as intracellular signals. These signals have the capability to inhibit or activate cell growth. In mice, the primitive streak extension begins on day 6.5.2 The primitive streak can be defined as cell growth to form the midline anterior-posterior axis of an embryo.2 Here, anterior and posterior are words that represent the future head and tail sites.

The anterior tip of the embryo denotes where head development will occur. To begin neurulation, the neural plate needs to develop. The neural plate will become the neural tube later on in development. To create the neural tube, the anterior cells within the primitive streak need to replicate. In order for the correct cells to replicate, an important mass of morphogens need to be inhibited.1 The inhibition morphogens are exported by a signaling center called the node. This correct cell replication causes a thickening in that region. The thickening grows into a U shape. In mice, this occurs on day 7.5 of development.2

Next, the U-shaped mass of cells needs to extend. The U-shaped mass becomes thinner and longer within the anterior region. In order to form a closed tube, the U-shaped mass needs to fold. The neural plate U is induced to fold. Morphogens are secreted by the notochord, which lay underneath the neural plate. Hinge regions are created within the neural plate and assist in the fold. The folding occurs during day 9-10 of embryo development.3 The folding that occurs due to the hinge regions allow the both ends of the neural-U, or neural folds, to touch along the dorsal midline of the neural plate.2 Fusion begins once the neural folds touch. And the neural plate is now a neural tube.

Advantages/Disadvantages

An advantage to using mice as a model organism for development is that they are easy to breed. They also don’t require a lot of space to maintain housing for them and their pups. Since mice are mammals, studying their development can provide insight to human development (Gibson, 2000). A disadvantage is that fertilization is internal (Slack, 2013). Another disadvantage is that the embryo develops inside of another organism instead of outside like other animals, who develop externally in their environment (Gibson, 2000).

Advantages to Mouse model Disadvantages to Mouse model
 * Fertilization mechanisms are more similar to humans than other model organisms
 * Lifecycles are relatively quick for mammals
 * Litters provide a larger experimental group than single births
 * Transgenic mice can be used to model human genetic defects including those that occur in embryogenesis
 * Challenges presented by internal fertilization and viviparity are also present in mouse models
 * Paternal and maternal contributions to the zygote differ from most mammals:
 * Centriole and mitochondria are maternal
 * Experiments may require more space and regulations than those on invertebrate models
 * Regulations and paperwork needed to operate on vertebrates

Xenopus
INTRODUCTION:

Development is highly conserved process, which includes fertilization, cleavage, gastrulation, nuerulation, and axis formation. Each of these phases are required in order for a species to develop from a single egg cell, into a fully functioning multicellular adult. While this process is highly conserved, there are variations in the mechanisms used by species to undergo each phase. This article will focus on the overall development Xenopus laevis, and the mechanisms used by the African clawed frog during each phase of development.

Life Cycle
The life cycle of Xenopus laevis takes roughly twelve months from start to finish. That is, from fertilization to adulthood. It is important to note that the earliest stages of the frogs’ life cycle only take about 24 hours to complete, it is the unique process of metamorphosis that takes the longest. The first stage of the life cycle is fertilization.

Fertilization
Fertilization is the event that sets the entire process of development into motion. In Xenopus it occurs externally, meaning that the male frog disperses his sperm across the eggs in the environment. A single sperm enters the egg and triggers it to revolve so that the animal pole is in its proper position. Without this rotation the gray crescent would not be properly set up and the next step, which is cleavage, would not occur correctly.

Cleavage
Cleavage is the second step of development and is important because it establishes the different poles and makes sure that the organizers are being put in place. Cleavage is what its name implies, separations of the egg without growth of the cells.

CLEAVAGE:

Cleavage refers to a set of early cell divisions, these divisions occur without cellular growth and yield multiple smaller cells originating from one cell. Cleavage in Xenopus occurs in an orderly manner and the first cleavage splits the egg vertically down the middle, into anterior and posterior halves. The second cleavage occurs roughly perpendicular to the axis of the first and divides the egg into dorsal and ventral halves. Cleavage continues and around the 32 cell stage, the formation of the blastocoel is observed, the blastocoel is a cavity that forms in the Animal hemisphere. When the formation of the blastocoel is noted, the embryo is referred to as a blastula. Cells composing the blastula are joined by cadherins, which are calcium dependent adhesion's, and thus the removal of Ca2+ degrades the cadherins. Cleavage continues until the mid-blastula transition, this prompts for a decrease in cleavage divisions and promotes transcription of the zygotic genome.

Intracellular Ca2+ is essential for cleavage to occur in Xenopus laevis, Evidence indicates that cell cleavage involves narrowing of the region in which the cleavage furrow is located. The mechanism involved in the contraction of the area has been associated with that of muscle, and is thought to contain an actin-myosin based structure. This structure utilizes calcium, thus Ca2+ is required in order for cleavage to occur.

The formation of the organizer (Spemann's organizer) during the final cleavage divisions is a crucial step required for the remaining phases of development. The organizer is an area located in the posterior/dorsal area of the embryo, and is responsible for expression of many genes involved in axis formation. This area is crucial in gastrulation, as it forms the area known a the dorsal lip of the blastopore.

Gastrulation is a phase of development where the three embryonic germ layers (endoderm, mesoderm, and ectoderm) are constructed through various mechanisms of morphogenic cell movement. This stage is initiated at the blastopore of the dorsal lip, which is formed on the opposite side of the embryo from where the sperm enters. The dorsal lip is part of the blastopore located below the Spemann's organizer that becomes elongated and eventually circularized. The blastopore is the point of access for the outer cells to enter the embryo. The cells around the blastopore undergo apical constriction and are referred to as bottle cells because of their shape. Invagination occurs at the blastopore. This is where cell sheets of the marginal zone fold inward in order to construct a pocket. This pocket develops into a fluid-filled cavity located in the middle of the embryo referred to as a blastocoel.

Multiple types of morphogenic cell movement is involved throughout gastrulation. These processes do not rely on one another; however, the overall result of their actions are what drives the differentiation of the germ layers. Epiboly, invagination, involution, and convergent extension are clearly seen in Xenopus gastrulation. Epiboly can be seen in the animal hemisphere. These cells flatten and spread out to increase the surface area of the cell sheet in order to cover the whole surface of the embryo. The outer cells move towards the blastopore of the dorsal lip causing the marginal zone to invaginate, creating a circular blastopore. During invagination, the blastocoel moves up towards the top of the animal pole, away from the dorsal lip, and subsequently down to the vegetal pole. While doing this, its size decreases due to the infiltrating cells. Eventually, the blastocoel shrinks and the archenteron, which is the early structure of the gut, becomes the principle cavity. Because the mesoderm is already located on the inside of the gastrula, involution allows it to separate into a whole different layer forming between the endoderm and the ectoderm. Involution is a process where a cell sheet folds inward, forming an underlying layer of cells (mesoderm).

Convergent extension is a process where cells line up based on their polarity and move between neighboring cells through the use of lamellapodia, generating shape change at the tissue level. Cellular intercalation occurs in all three germs layers which moves the marginal zone internally, further closing the blastocoel. Convergent extension is one of the more well-studied cellular movements involved in gastrulation. The Wnt/β-catenin pathway is necessary for convergent extension to occur. Proper execution of this pathway leads to dorsal cell specification. The Xnr-3 protein is a major target of this pathway and if it is not expressed correctly, then the cells do not undergo proper convergent extension. Paraxial protocadherin (PAPC) plays an important role in cell movement as well regarding planar cell polarity and the Wnt pathway. It is dependent on three GTP proteins RhoA, Rac1, and JNK.

The end of gastrulation results in a structure called a gastrula. The three embryonic germ layers are now differentiated in the embryo. The innermost layer is the endoderm that was originally the vegetal tissue. The outermost layer is the ectoderm, which was once the animal cap. The middle layer is the mesoderm that was formed from the involution of the marginal zone.

The next phase of development is neurulation where the neural tube is formed giving rise to the central nervous system. During this stage, the embryo is referred to as a neurula. Neurulation is initiated when ectodermal cells thicken, forming the neural plate. These cells change in shape from cuboidal to columnar, causing the edges of the plate, referred to as neural folds, to rise towards one another which is caused by the constriction of F-actin during apical constriction. As these boundaries move, the middle of the neural plate descends to form the neural groove. The neural tube forms when these two folds meet to form a hollow cylinder that becomes covered with a layer of the ectoderm known as the epidermis. Deep cells, which are non-neural cells, help bring the two folds together. The Wnt pathway also plays a critical role in these cell movements. Wnt needs to inhibit BMP in the gastrula stage and then BMP needs to be activated in the neurula for proper neural crest formation. Sox2 is shown to be consistently expressed throughout neurulation. There are hinge points located along the neural tube that aids in closing it. This happens almost simultaneously. Convergent extension is consistently being carried out, especially in the notochord, resulting in the elongation of the entire body.

Gastrulation
Gastrulation is the next step of development and is where the cells in the embryo are moved into their designated locations. The three germ layers are also set apart during this time and the organizer is set up on the dorsal side of the embryo, this is vital to neurulation.

Neurulation
Neurulation follows gastrulation and utilizes the germ layers and organizer set up that occurred during that stage. The early form of the nervous system is set up during this time, it is called the neural plate.

Organogenesis
Organogenesis benefits from the earlier stages because it requires the organizer and nervous system. The three germ layers, which were established during gastrulation, now respond to signaling and set up the organ systems.

Research shows that there are cells that support the germ layers in forming the organs during this time. These cells come from the immune system assist in creating organs such as kidneys.

Metamorphosis
Metamorphosis is the last stage of the developmental process and is a defining moment for Xenopus. The defining characteristics of the tadpole, such as the tail and gills, shrink and are replaced by arms, legs, and useful lungs.

TR hormones have been found to play an active role in the changes that occur during metamorphosis. They are found in the head, middle, and tail regions of the changing tadpole both before and after the change.

Axis Formation
All axis formation is centered around the organizer, which is located on the dorsal side of the developing embryo. The organizer is set up in this location because of the Nieuwkoop center. Axis formation is important to the development of Xenopus because without it the frog would lack proper body formation and more than likely would not survive.

Dorsal/Ventral
The dorsal and ventral body axis are the product of mainly two organizers. The first is Wnt, which signals on the dorsal side of the embryo. The other organizer responsible for setting up this axis is BMP, which is found on the ventral side of the embryo.

Research has been done on the specific forms of Wnt within the embryo and during the setup of the axis. It has been found that Wnt11 work in conjunction with one another to establish the dorsal side of the embryo. It has also been found that Dkk1 assists the two forms of Wnt in influencing only where they are supposed to go. This is achieved by it repressing and initiating the function of Wnt.

Anterior/Poster
Wnt and BMP not only influence the development of the dorsal and ventral axis, they are also the key players in the establishing of the anterior posterior axis. BMP is credited with congregating in the anterior side of the embryo. Wnt, on the other hand is found in the posterior.

Left/Right
The final axis that is present in the developing embryo is the left and right axis. This axis is unique from the others because it is regulated primarily by rotating cilia, whereas the others are set up by repression/expression of organizers. Pitx2 is a left signaling organizer, and as mentioned, it is set up on the left side because of the rotating of the cilia. The cilia are in the node and as it receives the Pitx2 from the organizer is carries it over.

Research has been done on the importance of the placement of the cilia for the equal spreading of the organizer, and therefore the proper setup of the axis. The cilia traditionally develop on the posterior side of the embryo, and carry out their function from there. They receive the signals that they need to develop in this area from Vangl2. It was discovered that if the Vangl2 levels were not held at steady amounts, the cilia grew in a more central location. This completely upsets the spread of Pitx2 and therefore causes the left and right axis to form incorrectly, or not at all.

Arabidopsis
Arabidopsis is a model system for genetics and developmental biology.

Fertilization
For an overview of Arabidopsis fertilization see this review.