Gastrulation

Gastrulation begins with the appearance of the primitive streak on the surface of the epiblast. The primitive streak consists of a groove, a node, and a pit at the cranial end. Epiblast cells migrate toward the streak, detach from the epiblast, and slip beneath it in a process called invagination. The first cells to migrate displace the underlying hypoblast to form the definitive embryonic endoderm. Subsequent cells settle between the epiblast and the newly formed endoderm to create the mesoderm layer. The cells remaining in the epiblast then become the ectoderm. This process proceeds in a cranio-caudal direction. During this time, the notochordal process also forms, which eventually becomes the notochord—the primary inducer for the overlying ectoderm to begin neurulation. Gastrulation establishes the three body axes (cranio-caudal, dorso-ventral, and left-right), which is essential for correct anatomical positioning of organs.

Molecular signaling, particularly via Fibroblast Growth Factor 8 (FGF8), controls the migration and specification of epiblast cells. Ectoderm gives rise to the CNS, PNS, skin epidermis, and hair. Mesoderm develops into muscle, bone, circulatory system, urogenital system, and the dermis. Endoderm forms the epithelial lining of the GI tract, respiratory tract, and glands like the liver and pancreas. The notochord serves as the structural basis of the vertebral column (largely forming the nucleus pulposus in adults) and provides signaling cues (like Sonic Hedgehog) for the differentiation of surrounding tissues.

Gastrulation is a highly sensitive period for teratogenesis. Errors in gasrtulation can lead to major structural defects. Situs inversus occurs when the left-right axis is incorrectly specified. Caudal dysgenesis (sirenomelia) results from insufficient mesoderm formation in the lower part of the body. If the primitive streak fails to regress completely by the end of the fourth week, it may form a sacrococcygeal teratoma, which is the most common tumour in neonates containing tissues from all three germ layers.

Prenatal ultrasound may detect gross structural abnormalities or tumours such as sacrococcygeal teratomas. Genetic testing and amniocentesis may be used for related syndromic presentations.

Management is mainly surgical for complications like sacrococcygeal teratomas post-delivery. Many gastrulation defects are incompatible with life or require lifelong specialist multidisciplinary care (e.g., for situs inversus complications or caudal syndromes).