Third Week of Embryonic Development: Gastrulation and Neurulation

Flashcards covering key concepts from the third week of embryonic development: gastrulation, neurulation, germ layer derivatives, notochord, left-right regulation, and related congenital malformations.

What are the two main events occurring in the 3rd week of development?

Gastrulation and the beginning of neurulation (formation of the trilaminar disc from the bilaminar disc).

Define the primitive streak.

A narrow groove on the dorsal surface of the bilaminar disc that establishes the cranio‑caudal axis and contains a primitive pit.

Define gastrulation and describe how it occurs.

The process turning the bilaminar disc into a trilaminar disc by epiblast cells migrating through the primitive streak to form ectoderm, mesoderm, and endoderm.

What is the origin and main function of the notochord?

The notochord arises from axial mesoderm and induces formation of the neural plate while providing the axial skeleton framework.

What are the main derivatives of the ectoderm?

Skin, CNS, PNS, eyes, inner ear; neural crest cells contribute to bones and connective tissue of the face and part of the skull.

What are the main derivatives of the mesoderm?

Bones and connective tissue, urogenital system, cardiovascular system.

What are the main derivatives of the endoderm?

Gut and gut derivatives (e.g., liver, pancreas, lungs).

How is the neural plate formed?

The notochord induces thickening of the overlying ectoderm to form the neural plate.

When are the first blood vessels formed connecting the embryo with the placenta?

During the third week; blood vessels form from mesoderm in the chorion and embryo, establishing the first vascular network.

What marks the beginning of neurulation?

Formation and thickening of the neural plate with the elevation of neural folds and the neural groove.

What happens to the neural tube during neurulation?

Neural folds fuse to form the neural tube and detach from the overlying ectoderm (neural tube closes in the 4th week).

What are the two layers of extraembryonic mesoderm?

Visceral (splanchnic) mesoderm and parietal (somatic) mesoderm.

What forms the fetal component of the placenta in relation to the chorion?

Visceral (splanchnic) extraembryonic mesoderm contributes to the chorion, which becomes the fetal component of the placenta.

Which germ layer gives rise to neural crest cells?

Ectoderm.

What are some major derivatives of neural crest cells?

Bones and connective tissue of the face and part of the skull, among other derivatives.

Which gene regulates left-right laterality in the embryo?

Nodal gene expression on the left side of the embryo.

What is sacrococcygeal teratoma?

A tumor arising from residual pluripotent cells of the primitive streak after gastrulation.

What is Sirenomelia (caudal dysgenesis)?

Caudal region mesoderm deficiency leading to fusion of the limb buds and other defects.

What is the trilaminar disc?

An embryonic disc with three germ layers: ectoderm, mesoderm, and endoderm formed after gastrulation.

When does gastrulation create the trilaminar disc?

During the third week, as the bilaminar disc becomes trilaminar via formation of three germ layers.

What are the three germ layers and their general derivatives?

Ectoderm: skin, CNS, PNS, eyes, inner ear; Mesoderm: bones, connective tissue, urogenital and cardiovascular systems; Endoderm: gut and derivatives.

What signals initiate the formation of the neural plate and neural tube closure?

Notochord signaling induces neural plate formation; neural folds rise, fuse, and close to form the neural tube (CNS).