User:Kputman2/sandbox

'''PD added the following. This is your merged article.'''

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