Human Growth and Development: Weeks 3 to 8 Embryology

Vocabulary flashcards covering embryonic development from the third week to the eighth week, including gastrulation, notochord formation, neural tube closure, and developmental milestones.

Gastrulation

The process occurring during the third week of gestation that establishes all three germ layers: ectoderm, mesoderm, and endoderm.

Primitive streak

A narrow groove with slightly bulging regions on either side visible on the surface of the epiblast in a 15-16 day embryo, marking the start of gastrulation.

Primitive node

The elevated area at the cephalic end of the primitive streak that surrounds the small primitive pit.

Invagination

The inward movement of epiblast cells detaching from the epiblast and slipping beneath it after migrating to the primitive streak.

Embryonic endoderm

The layer created when invaginated epiblast cells displace the hypoblast.

Epiblast

The dorsal germinal layer in the bilaminar stage that gives rise to the ectoblast, mesoblast, and intraembryonic endoblast.

Hypoblast

The ventral germinal layer in the bilaminar stage that gives rise to the extraembryonic endoblast, including the umbilical vesicle and allantois.

Chordal process

A structure formed around the 19th day by cells invading the primitive node region and migrating cranially along the median line up to the prechordal plate.

Axial canal

A structure formed on roughly the 21st day through the invagination of epiblast cells migrating into the region of the primitive node.

Chordal plate

The structure formed at around the 20th day when chordal tissue bonds with the endoderm.

Notochord

A complete cord formed between the 22nd and 24th day when chordal tissue cuts itself off from the endoderm, positioning itself in the middle of the mesoderm between ectoderm and endoderm.

Neurenteric canal

A temporary communication at the caudal end of the notochord between the umbilical vesicle and the amniotic cavity.

Anterior neuropore

The rostral opening of the neural tube that closes by day 24 to 26.

Posterior neuropore

The caudal opening of the neural tube that closes by day 25 to 28.

Allantois

A structure where blood formation occurs during weeks 3-5, and whose blood vessels become the umbilical arteries and vein.

Embryonic folding

The process where the flat trilaminar embryonic disk becomes a cylindrical embryo through simultaneous longitudinal and transverse folding.

Otic pits

The primordia for the inner ears, clearly visible on Day 26 of Week 4.

Lens placodes

Ectodermal thickenings representing future lenses on the side of the head, seen at Day 28 (end of Week 4).

Finger rays

Digital ridges developing on paddle-shaped hand plates during Week 6 for future fingers.

Cervical flexure

The bending of the brain in the cervical region during Week 6 that causes the head to bend farther over the heart prominence.

Week 8 of development

The final week of the embryonic period during which human characteristics are present, tail bud disappears, eyelids begin to meet and fuse, and the head is about one-half of the embryo.