Superficial migration of epiblast
The initial stage of the second phase of gastrulation involves the movement of cell masses even before they begin to invaginate into the embryo.
- Migration direction: Epiblast cells enter a phase of active division. Proliferating, they shift across the surface of the embryonic disc. The movement is directed toward the caudal (posterior) end of the embryo and its median axis.
- Primitive streak and node: The accumulation of moving cells along the median axis leads to the formation of a thickening in the caudal part — the primitive streak. At its most anterior pole, another important cluster forms — the primitive (Hensen's) node.
- Invagination: As soon as cells reach the midline, the process of inpouching begins. Cells plunge into the interior of the disc, causing characteristic depressions to appear on the surface. In the center of the primitive streak, the primitive groove forms, and in the very center of Hensen's node, the primitive pit.
- Prechordal plate: Concurrently, an isolated cellular thickening arises in the cranial (anterior) part of the embryonic disc. This zone is critical for the correct spatial topography of future organs.
Internal migration and formation of germ layers
After cells pass through the primitive streak and node into the interior of the disc, their fate and movement vector change dramatically. Once inside, cell streams surge forward (cranially) and diverge laterally.
The differentiation process occurs in several stages:
- First migration wave (formation of endoderm). The very first invading cells actively displace the hypoblast cells, pushing them to the periphery. Taking their place, these new cells form the embryonic endoderm.
- Second migration wave (formation of mesoderm). The next stream of cells wedges into the space between the epiblast and the newly formed endoderm. The fate of these cells depends on their position:
- Cells growing strictly forward along the median axis form the notochord.
- Cells located on either side of the notochord form wing-like structures — the intraembryonic mesoderm.
- Cells that migrate far beyond the boundaries of the embryonic disc become the extraembryonic mesoderm (which will build the walls of extraembryonic organs).
- Formation of the ectoderm. Those epiblast cells that did not participate in internal migration at all and remained on the surface of the shield differentiate into the embryonic ectoderm.
An important topographical exception is the prechordal plate zone. In this area, migrating mesoderm is completely absent, and two layers — ectoderm and endoderm — lie tightly against each other, forming a local bilaminar region.
Development of the axial primordia complex and notochord
From the end of the 3rd week and throughout the 4th week of development (days 18–28), the trilaminar embryonic disc undergoes transformations: the germ layers begin to transform into specific tissue and organ primordia (axial complex).
The key process of this period is the formation of the notochord:
- Cellular material penetrates deep into the disc through the primitive pit of the node.
- The notochordal process is formed, which actively grows forward along the longitudinal axis of the embryo.
- This growth occurs strictly between the layers of the mesoderm and continues up to the border of the prechordal plate.
As a result, a robust unpaired axial structure arises. The notochord acts as the main supporting rod of the embryo and serves as the spatial basis for the subsequent formation of the vertebral column. In the course of further development, the notochord regresses. In the adult human body, only small fragments remain of it — the nucleus pulposus (nucleus pulposus) located in the center of the intervertebral discs.
Briefly: answers to topical questions
What tissues and organs form from the embryonic ectoderm subsequently?
Derivatives of three differentiation directions form from the embryonic ectoderm:
- Neural tube — gives rise to the brain and spinal cord.
- Neural crest cells — give rise to peripheral nerve ganglia; as well as sensory spinal and autonomic ganglia, skin melanocytes, and cells of the neuroendocrine system.
- Surface ectoderm — forms the epidermis of the skin and its derivatives: hair, nails, sebaceous glands, merocrine and apocrine sweat glands. The epithelium of the initial section of the GI tract (oral cavity), the terminal section of the GI tract/anal canal, and the vaginal epithelium also form from the surface ectoderm.
What other structures, besides the notochord, are part of the embryo's axial primordia complex?
In addition to the notochord, the axial primordia complex of the embryo includes:
- Neural tube.
- Primitive gut.
- Somites — derivatives of the mesoderm, which divide into dermatomes, myotomes, and sclerotomes.
- Nephrotomes — structures in which tubules form.
- Splanchnotomes — represented by parietal and visceral layers separated by the coelomic cavity.
Mesenchyme is located between the primordia.
Class and exam questions
Which cells form the embryonic ectoderm?
It is formed from epiblast cells that did not enter the process of invagination, did not migrate inward, and remained lying on the surface of the embryonic disc.
What happens during the first wave of internal migration?
The cells that first plunge into the interior of the disc displace the existing hypoblast to the periphery and form the embryonic endoderm in its place.
Where is the mesoderm absent in the trilaminar embryo?
The mesodermal layer is completely absent in the region of the prechordal plate (at the cranial end), where ectoderm and endoderm closely adhere to each other.
What does the notochord become in an adult?
The notochord itself disintegrates during development, and its tissue persists only as gelatinous nucleus pulposus within the intervertebral discs.
What else to study on this topic
- Cellular mechanisms of proliferation and displacement in the epiblast
- Functions and derivatives of the primitive (Hensen's) node
- Differentiation of intraembryonic mesoderm (formation of somites)
- Development of structures from the prechordal plate
- Topography and fate of extraembryonic mesoderm
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