User:Thelmadatter/Sandboxes Group 2/Embryogenic germ disc

The embryogenic germ disc is the group of cells that will suffer changes to create the embryo(embryoblast), this group of cells are immersed in the trophoblast(placenta). After the implantation of the first week next the fecundation the trophoblast and the embryoblast would have important changes on the second and third week.(Carlson, p.40, 2009) The second and the third week will be the most important for the future fetus because this weeks will be the more sensible to the teratogens (physical and chemical agents that affect the fetus in his formation).On the second week the embryoblast will make two layers known as epiblast and hipoblast.The epiblast cells will gonna suffer a process known as gastrulation, this is the process in which the epiblast will turn into the three main layers to derive the organs. The three layers that we are referring to are the endoderm, mesoderm and ectoderm.(Moore,p.43,p59, 2011)

Abnormal implantation
Implantation occurs at the end of the first week. The trophoblast anchors to the endomentrial stroma from the uterus. Implantation may also occur outside the uterus, such as in the rectouterine pounch(pouch of Douglas), the mesentery, in the uterine tuve or in the ovary. All this abnormal implantation is also known as ectopic pregnancies.( Moore,p.47-50, 2011)

Second week: Bilaminar germ disc
After the implantation on the uterus of the group of cells (morula: the zygote divided on 16 cells) immersed in the trophoblast on the first week, this cells will start to the develop the embryoblast on the start of the second week.

This week is also known as the week of the pairs because from the embryoblast will produce two layers as well as the trophoblast.

The two layers differentiated of the trophoblast are the cytotrophoblast (the inner layer of mononucleated cells) and the syncytiotrophoblast (the outer layer made by multinucleated cells). The citotrophoblast will hold the chorion ( the fetal part of the placenta in which chorionic villi will be developt to feed the fetus) to the endometrium of the uterus. The syncytiotrophoblast will erode the maternal capillarity to start the fetal circulation.(Sadler,p.43-44, 2012)

Now talking about the embryoblast the one who gonna develop into a embryo, will also have 2 layers división. The first one is the epiblast that is the topcoat group of cells that in the third week will make the process of gastrulation. The second one is the hypoblast the undercoat group of cells that will make the yolk sac (from this structure will appear the first erythrocytes and germ cells for all the human systems) and the extraembryonic mesoderm.

Together the layers will flatten on a form of disc. At the same time cavities will appear, the first one two appear is the aminiotic cavity that will house the amnioblast then then this cells will produce the amnion (A membrane from the epiblast that will protect the fetus with the amniotic fluid).

Fort he 9 day the trophoblast will start a process known as lacunar stage in which the trophoblast have lacunar spaces and will erodes the maternal capillaries to produce an intercommunicating network between the maternal blood and the lacunar system then while the uteropacental circulation can start. Also in the day 9 the hipoblast cells invaginate and together with the exocoelomic membrane (cells from the cytotrophoblast) forms the primitive yolk sac.

For the day 13 the blood flow increased into lacunar spaces because this bleeding occurs near the 28th day of the menstrual cycle. Also is important to remember that the trophoblast is characterized by villous structures that are cells from the cytotrophoblast whom proliferate and penétrate to the syncitiotrophoblast forming the primary villi.

Meanwhile a different group of cells from the yolk sac form a connective tissue, the extraembryonic mesoderm.

The only place where extraembryonic mesoderm communicates with the chorionic cavity is the connecting stalk who will develop into umbilical cord. (Sadler,p.43-47, 2012)

Third week: Trilaminar germ disc
The third week is characterized for the formation of the primitive streak on the surface of the epiblast, the formation of the nothochord and the establishing of the 3 layers (endoderm, mesoderm, and ectoderm). The process of establish the three germ layers is also known as gastrulation. The gastrulation begins when the epiblast of the bilaminar germ disc form the primitive streak and the epiblast invaginate to this primitive streak making the endoderm and mesoderm, unlike the ectoderm are the cells that doesn’t migrate and stay on the highest place of the epiblast make the ectoderm.The gen Nodal establish and develop the primitive streak and the fibroblast growth factor 8 (FGF induce the migration of the cells to the primitive streak. The derivatives of the endoderm are epithelium of the respiratory system and digestive, liver and pancreas. The derivatives of the mesoderm are muscle layers, connective tissue, cardiovascular system , blood cells, bone marrow, skeleton and reproductory sistem. The derivatives of the ectoderm are epidermis, nervous system, eye and other sensitive structures. When cells move between the epiblast and hypoblast layers , they begin to spread laterally and superiorly then they establish contact with the extraembryonic mesoderm covering the yolk sac and the amnion forming the prechordal plate where prenotochordal cells will arrive and made the notochordal plate. As the hypoblast is replaced by endoderm cells they form the definitive notochord. The notochord is a column of cells at the midline and will induct the neural plate, the motor región of the brain and spinal cord, and the vertebrae. Also in this week alot of genes will establish the body axes ( anteroposterior, dorsoventral, left and right takes place before and during the period of gastrulation. The cells at the anterior margin of the embryonic disc is known as the anterior visceral endoderm (AVE), this cells expresses transcription factors as OTX2, LIM1 and HESX1 that are essential for the head formation, also important the factors cerberus and lefty establish the cranial end of the  embryo before gastrulation. The bone morphogenic protein 4(BMP4) with the Fibroblast growth factor (FGF) secreted by the embryonic disc induce the mesoderm to ventralized and contribut to divide the mesoderm on those cells migrating more posteriorly through the node and cranialmost aspect of the streak that will form paraxial mesoderm; those cells migrating through the next portion of the streak that will form intermediate mesoderm; those cells migrating through the more caudal part of the streak that will form lateral plate mesoderm; and those cells migrating through the most caudal that part will contribute to extraembryonic mesoderm. Each mesoderm will form different structures. The left side is establish by the expression of LEFTY and PITX2 in which for the left side brain will contributes the serotonin (5HT) that is a broken down by its metabolizing enzyme monoamine oxidase (MAO).From the right side is only known the factor Snail. Another important gene that function as barrier to prevent left-sided signals from crossing over is Sonic hedgehog (SHH). (Sadler,p.51-57, 2012).

Growth of the Embryonic Disc
The embryonic disc, initially flat becomes elongated with a broad top(for the neural plate) and narrow bottom end. Growth caused and elongation of the top part are caused by the continous migration of cells from the primitive streak until the end of the fourth week then the primitive streak rapidly shrinks and disappears and continue developing until the birth. (Bravo, 2005)

Development of the trophoblast
The trophoblast progresses rapidly. On the primary villi arise small capillaries when these villous capillaries make contact with capillaries in the chorionic plate and connect- ing stalk, the villous system is ready to supply the embryo with its nutrients and oxygen (García, 2005)

Teratogens affect gastrulation
The beginning of the third week, when gastrulation begins is highly sensitive stage for teratogenic atacks. At this time, fate maps can be made for various organs systems, such as the eyes and bran anlage(a precursor or forerunner or a more mature structure), and these cell populations may be damaged by teratogens. (Sadler,p.57, 2012)