Human Embryology: Segmentation and Blastula Changes

Study Notes

The embryonic disc forms from the inner cell mass, which organizes into two layers: the hypoblast (primary endoderm) and epiblast (primary ectoderm).
Two cavities appear:
- Amniotic cavity, formed on the 7th or 8th day.
- Primary yolk sac, arising after the amniotic cavity.
Extraembryonic mesoderm forms as a connective tissue layer between the trophoblast and yolk sac/amniotic cavity.
Isolated spaces within the extraembryonic mesoderm fuse to create the extraembryonic coelom, dividing mesoderm into somatic mesoderm (lining trophoblast) and splanchnic mesoderm (covering yolk sac).
The connecting stalk connects the two mesodermal layers at the embryo's caudal end, which will form the future umbilical cord.
The chorion, formed by the trophoblast and somatic mesoderm, together with structures it encloses, is called the chorionic vesicle.

Characterized by changes in the bilaminar embryonic plate and the formation of three germ layers.
The embryonic plate transitions from circular to oval, and finally to pear-shaped as the notochord develops.
Notochord serves as the primary longitudinal support structure and plays an organizing role in the nervous system.
Prochordal plate forms from increased height of endodermal cells at the cranial end around the 14th day.
Primitive streak appears as columnar ectodermal cells multiply around days 15-16, leading to the formation of the primitive node.

The notochordal process develops on the 17th day from cells originating in the primitive node, extending cranially.
By the 18th day, a notochordal canal forms from an invaginated primitive pit, eventually connecting to the yolk sac.
On the 19th day, the floor of the notochordal canal fuses with the endoderm, leading to the formation of the neuroenteric canal.
On the 20th day, the notochord detaches from the yolk sac endoderm, solidifying its structure.
The cranial part of the notochord contributes to the base of the occipital bone and sphenoid bone, while other parts degenerate or contribute to the nucleus pulposus in intervertebral discs.

A process that transforms the bilaminar embryonic disc into a trilaminar embryo, starting around the 15th day.
Cells from the primitive streak migrate laterally and cranially, forming the intraembryonic mesoderm, which merges with the extraembryonic mesoderm.
The intraembryonic mesoderm separates ectoderm from endoderm, except at key areas: the oropharyngeal membrane, cloacal membrane, and median plane cranial to the primitive node.
The three primary germ layers are established:
- Ectoderm (outer)
- Mesoderm (middle)
- Endoderm (inner)

Ectoderm derivatives include the central and peripheral nervous system, sensory epithelium, skin epidermis, and dental enamel.
The notochord stimulates ectoderm to thicken into the neural plate, initiating neural tube formation.
Neural folds develop from lateral margins of the neural plate and converge to form the neural tube by the end of the 3rd week.
The neural tube separates from the surface ectoderm, with neuroectodermal cells forming the neural crest.

Folding converts the flat trilaminar embryo into a cylindrical shape, beginning in the 4th week.
Cephalo-caudal folding is driven by rapid growth of the central nervous system, while lateral folding is influenced by somite shape.
The result of folding creates the foregut, midgut, and hindgut divisions and results in the formation of the umbilical cord.
The foregut and hindgut correspond to sections of the yolk sac included in the head and tail folds, respectively, altering the positions of various embryonic structures.