Gastrulation, Neurulation & Somitogenesis – Core Vocabulary

Core vocabulary terms extracted from the lecture on gastrulation, neurulation, and somitogenesis, designed to reinforce key definitions for exam preparation.

Epithelial cell

Embryonic cell type that is tightly connected, polarized, and arranged in sheets or tubes.

Mesenchymal cell

Loosely connected, migratory embryonic cell that forms connective tissue matrices.

Epithelial-to-mesenchymal transition (EMT)

Process in which epithelial cells lose polarity and adhesion, becoming invasive mesenchymal cells.

Mesenchymal-to-epithelial transition (MET)

Conversion of migratory mesenchymal cells back into polarized, tightly joined epithelial cells.

Gastrulation

Early morphogenetic movement that forms the trilaminar embryo—ectoderm, mesoderm, endoderm.

Primitive streak

Longitudinal groove in epiblast whose formation marks the start of gastrulation and EMT.

Primitive (Hensen’s) node

Cell cluster at cranial end of primitive streak that organizes notochord and body axis.

Ingression

Movement of epiblast cells through the primitive streak to form new germ layers.

Endoderm

Innermost germ layer arising when epiblast cells displace the hypoblast; forms gut lining.

Mesoderm

Middle germ layer positioned dorsal to endoderm; gives rise to muscle, bone, blood, etc.

Ectoderm

Remaining epiblast after gastrulation; forms epidermis and nervous system.

Situs inversus

Left-right reversal of organ placement due to faulty axis signaling during gastrulation.

Sirenomelia (caudal dysgenesis)

Fusion or hypoplasia of lower limbs caused by inadequate caudal mesoderm production.

Sacrococcygeal teratoma

Tumor from persistent primitive streak cells, often containing teeth, hair, bone.

Notochord

Axial rod derived from notochordal plate; induces neural plate and later becomes nucleus pulposus.

Neurulation

Formation of neural plate, folding, and closure of neural tube to create CNS.

Neural plate

Thickened ectoderm over notochord that will fold to form the neural tube.

Neural groove

Median depression between elevating neural folds during neurulation.

Neural tube

Closed structure formed by fusion of neural folds; precursor of brain and spinal cord.

Cranial neuropore

Anterior opening of neural tube that closes ~day 25; failure leads to anencephaly/encephalocele.

Caudal neuropore

Posterior opening of neural tube that closes ~day 28; failure causes spina bifida.

Anencephaly

Absence of large parts of brain and skull from cranial neuropore non-closure (days 23–25).

Encephalocele

Herniation of brain tissue through skull due to cranial neuropore closure defect.

Spina bifida

Spectrum of vertebral and neural defects from caudal neuropore closure failure (days 22–28).

Noggin & Chordin

Notochord-secreted BMP inhibitors that induce ectoderm to become neuroectoderm.

Neural crest

Cell population at neural fold edges that migrates widely to form diverse tissues.

Neural crest derivatives

Include sensory & autonomic ganglia, Schwann cells, meninges, facial bone, melanocytes, adrenal medulla, conotruncal heart septum.

Embryonic folding

Lateral and cephalocaudal bending converting flat disc into a cylindrical body form.

Somite

Segmented block of paraxial mesoderm containing dermomyotome and sclerotome.

Somitomere

Whorl of mesenchymal cells in segmental plate that epithelializes to start a somite.

Dermomyotome

Dorsal somite region giving rise to dermatome (dermis) and myotome (skeletal muscle).

Sclerotome

Ventral somite region derived by EMT; forms vertebrae and ribs.

Myotome

Muscle-forming cells migrating from dermomyotome; split into primaxial and abaxial groups.

Primaxial domain

Muscle domain of myotome that stays medial, forming intrinsic back & intercostal muscles.

Abaxial domain

Muscle domain whose cells cross the lateral somitic frontier into lateral plate to form limb/body-wall muscles.

Lateral somitic frontier (LSF)

Boundary between somite and parietal lateral plate mesoderm crossed by abaxial myotome cells.

Parietal (somatic) lateral plate mesoderm

Dorsal split of lateral plate forming limb bones, sternum, dermis of body wall, parietal pleura/peritoneum.

Visceral (splanchnic) lateral plate mesoderm

Ventral split forming gut wall, visceral pleura/peritoneum, heart, smooth muscle.

Somatopleure

Parietal lateral plate + surface ectoderm; forms ventral body wall.

Splanchnopleure

Visceral lateral plate + endoderm; forms gut tube wall.

Gastroschisis

Anterior body-wall defect where viscera protrude; linked to parietal lateral plate failure.

Head mesoderm (prechordal plate)

Anterior paraxial mesoderm that does not segment; contributes to craniofacial skeleton, muscle, connective tissue.

Trilaminar embryonic disc

Three-layered structure (ectoderm, mesoderm, endoderm) formed by gastrulation.

Prechordal mesoderm

Mesenchyme migrating cranially from the node; participates in notochord formation and forebrain induction.