Surface Migration of the Epiblast
The initial stage of the second phase of gastrulation involves the movement of cell masses prior to their internalization into the embryo.
- Migration Direction: Epiblast cells undergo active proliferation. As they multiply, they shift across the surface of the embryonic disc toward the caudal (posterior) end and the midline.
- Primitive Streak and Node: The accumulation of migrating cells along the midline leads to the formation of a thickening in the caudal region—the primitive streak. At its cranial tip lies the primitive (Hensen's) node.
- Invagination: Once cells reach the midline, invagination begins. Cells internalize into the disc, creating surface depressions: the primitive groove forms along the center of the primitive streak, and the primitive pit forms in the center of Hensen's node.
- Prechordal Plate: Concurrently, an isolated cellular thickening arises in the cranial (anterior) region of the embryonic disc, which is critical for proper spatial patterning of future head structures.
Internal Migration and Germ Layer Formation
After cells ingress through the primitive streak and node, their fate and direction of movement change drastically. Once inside, the cellular streams advance cranially and spread laterally.
Differentiation occurs in several waves:
- First Migration Wave (Endoderm Formation): The earliest internalizing cells actively displace the hypoblast, pushing it to the periphery. Taking its place, these new cells form the embryonic endoderm.
- Second Migration Wave (Mesoderm Formation): The next stream of cells wedges between the epiblast and the newly formed endoderm. Their fate depends on their final position:
- Cells migrating strictly forward along the midline form the notochord.
- Cells flanking the notochord form the intraembryonic mesoderm.
- Cells migrating far beyond the embryonic disc become extraembryonic mesoderm (contributing to extraembryonic membranes).
- Ectoderm Formation: Epiblast cells that do not participate in internal migration and remain on the surface differentiate into the embryonic ectoderm.
A key topographic exception is the prechordal plate. In this region, migrating mesoderm is entirely absent, and the two layers—ectoderm and endoderm—lie in direct contact, forming a local bilaminar area.
Development of the Axial Complex and Notochord
From the end of week 3 through week 4 of development (days 18–28), the trilaminar embryonic disc undergoes extensive remodeling as germ layers differentiate into specific tissue and organ rudiments (the axial complex).
A key process of this period is the formation of the notochord:
- Cellular material extends deep into the disc through the primitive pit of the node.
- The notochordal process grows cranially along the longitudinal axis of the embryo.
- This growth proceeds strictly between the mesodermal layers up to the boundary of the prechordal plate.
This results in a prominent midline structure. The notochord acts as the primary skeletal axis of the embryo and provides the structural template for the future vertebral column. During subsequent development, the notochord regresses; in the adult human body, only small remnants persist as the nucleus pulposus located in the center of intervertebral discs.