Embryology: Gastrulation, Neurulation, and Embryonic Folding

Vocabulary practice flashcards covering the key concepts of gastrulation, germ layer derivatives, notochord development, neurulation, neural crest differentiation, neurofibromatosis, and embryonic folding.

Gastrulation

The developmental process commencing in week 3 with the formation of the primitive streak that transforms the bilaminar embryonic disc into a trilaminar embryonic disc and firmly establishes the cranial/caudal and right/left axes of the embryo.

Primitive streak

A linear band that appears in the epiblast at the beginning of week 3; epiblast cells migrate toward and through it to form the embryonic germ layers.

Definitive endoderm

The first embryonic germ layer formed during gastrulation when migrating epiblast cells detach and completely replace the hypoblast cells.

Definitive mesoderm

The second embryonic germ layer, formed around days 14–16 by migrating epiblast cells that position themselves between the epiblast and the newly formed definitive endoderm.

Definitive ectoderm

The third and final germ layer of gastrulation, comprised of the remaining epiblast cells that do not migrate through the primitive streak.

Epiblast

The layer of the bilaminar embryonic disc from which all three definitive embryonic germ layers (ectoderm, mesoderm, and endoderm) are ultimately derived.

Ectoderm derivatives

Structures formed from the ectoderm, including the epidermis of skin and epidermal derivatives (hair, nails, sweat glands, mammary glands), nervous tissue and sense organs, pituitary gland, adrenal medulla, enamel of teeth, and lens of the eye.

Mesoderm derivatives

Structures formed from the mesoderm, including the dermis of skin, muscle tissue, connective tissue (proper, bone, cartilage, blood), heart, kidneys and ureters, internal reproductive organs, spleen, adrenal cortex, and epithelial lining of blood vessels, lymph vessels, body cavities, and joint cavities.

Endoderm derivatives

Structures formed from the endoderm, including the epithelial lining of the respiratory tract, GI tract, tympanic cavity, auditory tube, urinary bladder, and urethra, as well as most of the liver, the gallbladder, pancreas, thymus, thyroid gland, parathyroid glands, and a portion of the palatine tonsils.

Notochordal process

A hollow tubular ingrowth formed by the primitive node as the primitive streak regresses.

Notochordal plate

A flattened plate of cells created when the notochordal process transiently fuses with the underlying embryonic endoderm.

Notochord

A solid cellular rod formed when the notochordal plate separates from the endoderm and fuses; it acts as the primary signaling center that induces neurulation.

Neuroectoderm

The thickened region of ectoderm overlying the notochord that forms in response to notochordal induction and gives rise to the neural plate.

Neurulation

The developmental process beginning around 22 days in which the lateral edges of the neural plate elevate to form neural folds, which then fuse beginning in the cervical (neck) region and proceed cranially and caudally to create the neural tube.

Neural crest cells

A specialized cell population that detaches from the lateral edges of the neural folds as they elevate and fuse, migrating laterally and ventrally throughout the body; sometimes referred to as the 4th germ layer.

Cranial neural crest derivatives

Structures arising specifically from cranial neural crest cells, including connective tissue surrounding the eye, pupillary and ciliary muscles, dermal bones of the skull, truncoconal septum, dermis and hypodermis of the face and neck, odontoblasts, pharyngeal arch cartilages, and some cranial nerve ganglia.

Spinal neural crest derivatives

Structures arising specifically from spinal neural crest cells, including preaortic ganglia, chain ganglia, dorsal root ganglia, and the adrenal medulla.

Shared cranial and spinal neural crest derivatives

Structures derived from both cranial and spinal neural crest populations, including Schwann cells, glial cells in peripheral ganglia, arachnoid and pia mater, melanocytes, and enteric ganglia.

Neurofibromatosis

A genetic condition caused by an inactivating mutation in the NF1 tumour suppressor gene, which removes inhibition on cell division and leads to benign tumours of the nervous system, skin, and cranial bones.

NF1 gene

A tumour suppressor gene encoding neurofibromin 1 that is 'switched off' in neurofibromatosis, resulting in increased cell division.

Embryonic folding

The 3D transformation during the 4th week wherein the flat trilaminar disc folds along craniocaudal and lateral axes, driven by rapid expansion of the amniotic sac while the yolk sac remains relatively constant in size.

Craniocaudal folding

Longitudinal folding occurring during week 4 that curls the embryo in the head and tail regions, establishing the foregut, midgut, hindgut, and repositioning the connecting stalk and allantois.

Lateral folding

Transverse folding occurring during week 4 that brings the lateral sides of the trilaminar disc ventrally to enclose the gut tube, form the body cavity, and separate the mesoderm into parietal and visceral layers.