AN79.1-6 | 3rd to 8th week of development — Glossary

A midline thickening of epiblast appearing at Day 14-15 in the caudal embryonic disc; the site of gastrulation where cells ingress to form mesoderm and endoderm

The cranial thickening at the end of the primitive streak; the embryonic organiser that establishes body axes and gives rise to notochordal cells

The process converting the bilaminar disc to a trilaminar disc by cell migration through the primitive streak to form ectoderm, mesoderm, and endoderm

A midline rod of cells extending cranially from the primitive node; induces neural plate formation; persists as the nucleus pulposus in adult intervertebral discs

The process by which the neural plate folds to form the neural tube, the precursor of the brain and spinal cord

A thickened area of ectoderm induced by the notochord on Day 18; the first sign of nervous system development

The hollow tube formed by fusion of neural folds; becomes the brain (cranial end) and spinal cord (caudal end)

Cells that detach from the dorsal neural tube and migrate to form dorsal root ganglia, melanocytes, Schwann cells, adrenal medulla, and craniofacial skeleton

The cranial opening of the neural tube; closes on Day 25; failure causes anencephaly

The caudal opening of the neural tube; closes on Day 27; failure causes spina bifida and myelomeningocele

A paired block of paraxial mesoderm that differentiates into sclerotome, myotome, and dermatome; 42-44 pairs form between Days 20-30

The ventromedial portion of the somite that migrates toward the notochord and neural tube to form vertebrae and ribs

The intermediate portion of the somite that gives rise to skeletal muscles of the body wall and limbs

The dorsolateral portion of the somite that gives rise to the dermis of the skin and subcutaneous tissue

The body cavity formed by splitting of lateral plate mesoderm; partitioned into pericardial, pleural, and peritoneal cavities

A neonatal tumour arising from persistent primitive streak remnants in the sacrococcygeal region; contains derivatives of all three germ layers

The jelly-like centre of the intervertebral disc; a remnant of the embryonic notochord; herniation causes disc prolapse (sciatica)

A lethal neural tube defect caused by failure of anterior neuropore closure; absence of cranial vault and cerebral hemispheres

A neural tube defect where meninges and spinal cord herniate through a vertebral defect; the most common clinically significant NTD

Failure of vertebral arch fusion without herniation of meninges or cord; usually asymptomatic; marked by a tuft of hair or dimple

Herniation of abdominal contents into the thorax through an unclosed pleuroperitoneal canal (usually left, Bochdalek foramen); causes pulmonary hypoplasia

An agent that causes congenital malformations; most dangerous during weeks 3-8 (organogenesis); examples include rubella, alcohol, valproate

A B vitamin essential for DNA synthesis and neural tube closure; periconceptional supplementation (0.4-5 mg/day) reduces NTDs by 70%

Mirror-image reversal of normal organ asymmetry caused by defective nodal ciliary flow at Hensen's node; associated with Kartagener syndrome