Second phase of gastrulation
At week 3 of embryonic development (days 14–17), the second phase of gastrulation begins. Unlike delamination, this stage occurs via immigration — the active migration of cells.
The germ disc, consisting of the epiblast and hypoblast, elongates in the craniocaudal direction, acquiring spatial polarity. On the epiblast surface in the caudal part, the primitive streak (primitive streak) forms, and at its cranial end, Hensen's node.
Cells migrate through these structures, settling between the epiblast and hypoblast to form the mesoderm and definitive endoderm, while the remaining superficial layer becomes the ectoderm.
Germ layer derivatives
During further differentiation, each germ layer gives rise to specific tissues and organs:
- Ectoderm: forms the nervous system (neural tube, neural crest), body integument (epidermis, nails, hair), tooth enamel, and sense organs.
- Mesoderm: forms the dermis of the skin, musculoskeletal system (skeleton, muscles), circulatory, lymphatic, excretory, and reproductive systems, as well as serous membranes (coelom, pleura, peritoneum).
- Endoderm: gives rise to the epithelial lining of the middle part of the digestive tube, the respiratory system, as well as the parenchyma of the liver, pancreas, and endocrine organs.
Development of the nervous system: neurulation
The formation of the nervous system is divided into primary and secondary neurulation.
- Primary neurulation (3–4 weeks): under the influence of chordamesodermal induction, the dorsal ectoderm thickens into the neural plate. It invaginates into the neural groove with neural folds that fuse to form the neural tube containing a lumen (neurocoel).
- Neural crest: forms from the material of the neural folds and gives rise to the peripheral nervous system, adrenal medulla, and melanocytes.
- Secondary neurulation (weeks 5–6): the caudal cell mass undergoes regressive differentiation with cavitation, forming tissues distal to the S2 level and the conus medullaris.
EMBRYO: Ectoderm — external (brain, skin), Mesoderm — middle (muscles, blood, skeleton), Endoderm — internal (GI tract, lungs, glands).
Class and exam questions
What mechanism underlies the second phase of gastrulation?
The second phase of gastrulation occurs via immigration — the migration of epiblast cells through the primitive streak and Hensen's node.
What structures are formed from the neural crest?
Structures of the peripheral nervous system, adrenal medulla, skin pigment cells, and neuroendocrine cells are formed from the neural crest (ganglionic plate).
What is the difference between primary and secondary neurulation?
Primary neurulation (weeks 3–4) consists of the folding of the neural plate into the neural tube, while secondary neurulation (weeks 5–6) occurs via cavitation of the caudal cell mass and forms the spinal cord in the sacral and coccygeal regions.
What else to study on this topic
- Anatomic derivatives of mesenchyme and its histogenesis
- Developmental features of the foregut and hindgut
- Somite formation: dermatomes, myotomes, and sclerotomes
- Intermediate mesoderm and the primordium of the urogenital system
- Sturge-Weber syndrome and neurocutaneous syndromes
- Spinal dysraphisms and neurulation disorders
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