Gastrulation

Gastrulation

The embryonic disc develops from bilaminar (2 layers) to trilaminar (3 layers).

The bilaminar disc comprises epiblast + hypoblast.

The trilaminar disc comprises ectoderm, mesoderm, endoderm.

All of which derives from the epiblast (none from the hypoblast).

Blastocyst formation

The blastocyst is a circular cyst; it's divisions are:

• Trophoblast: the outer cell mass.
• Embryoblast: the inner cell mass.

The blastocyst resides within the uterine cavity and eventually invades the uterus.

The uterine walls, from inside to outside, are:

• Endometrium
• Myometrium
• Perimetrium

Trophoblast division

Trophoblast divides into:

• Cytotrophoblast, the inner cell line, which maintains a similar shape as the trophoblast.
• Syncytiotrophoblast, the external cell line, which invades the uterine wall to lay the foundation of the placenta.

Within the cytotrophoblast, the embryoblast transforms into:

• The epiblast (which are columnar cells) – the original mass of inner cells
• The hypoblast (which are small cuboidal cells) – a new layer of cells underneath the epiblast.

The bilaminar germ disc exists where the epiblast and hypoblast meet.

At this stage, the syncytiotrophoblast invades into the uterine wall.

Epiblast divison

The epiblast generates cells that become:

(1) Ectoderm

• Amniotic cavity fills the cavity internal to the ectoderm.
  (2) Endoderm
• Yolk sac fills the cavity internal to the endoderm.
  (3) Mesoderm

Gastrulation

Ectoderm forms the primitive streak: a dimpling at the germ disc - the site of gastrulation.

• Gastrulation is a process of invagination, wherein ectodermal cells pass from the ectodermal surface to the primitive streak. Mesodermal cells spread out between the ectoderm and endoderm and also surround these cell lines.

The germ disc is now trilaminar.

In addition to the mesoderm mentioned previously, there also exists an additional mesoderm layer: the extraembryonic mesoderm just internal to the cytotrophoblast.

Germ layer derivatives

The key germ layer derivatives (note that these are numerous and we only list the highlights):

Ectoderm

• Skin + derivatives (hair, nails, etc...)
• Adrenal medulla
• Nervous tissue
• Sense organs

Mesoderm

• Musculoskeletal (including heart muscle)
• Adrenal cortex
• Testes + ovaries
• Kidneys + ureters

Endoderm

• Epithelial lining of: GI, Respiratory, Urinary, Reproductive systems