Initial State and First Phase
By the onset of gastrulation, the avian embryo is a discoblastula—a sheet of cells located on the surface of the uncleaved yolk.
The first phase of gastrulation is called delamination. This process involves the splitting of the initial single-layered blastoderm into two independent strata via cell divisions:
- Upper layer (epiblast) consists of tall columnar cells. Nuclei within this layer lie at varying heights, creating a pseudostratified appearance. The epiblast serves as the sole source for the formation of the embryonic shield. All three classical germ layers—ectoderm, mesoderm, and endoderm—ultimately develop from it.
- Lower layer (hypoblast) is formed by flat cells and directly covers the yolk mass. Current evidence indicates that the hypoblast does not contribute to the tissues of the embryo proper.
Second Phase: Immigration and Formation of Axial Structures
The second phase is termed immigration. Due to continuous active cell division within the epiblast, a cellular surplus arises, prompting complex cell migrations.
In the first stage of migration, epiblast cells move across the surface from the margins of the blastodisc toward its central axis (midline), and then turn inward.
As these migrating cells accumulate in the central region of the disc, characteristic elevations appear:
- Primitive streak (primitive streak).
- Hensen's node (primitive node) — a thickening at the anterior margin of the primitive streak.
At the center of these structures, migrating cells literally "stream" inward, forming depressions: the primitive groove runs along the primitive streak, and the primitive pit forms in the center of the node. Evolutionarily, this entire complex (streak, node, groove, and pit) is homologous to the amphibian blastopore.
Germ Layer Differentiation
Upon migrating inward, cells alter their trajectory, spreading from the center to the periphery to form the germ layers:
- Endoderm: The first wave of migrating cells displaces the hypoblast laterally to form the embryonic endoderm. The displaced hypoblast cells become the extraembryonic endoderm.
- Mesoderm and notochord: A second cohort of cells spreads as a loose layer between the epiblast and the newly formed endoderm. The axial structure—the notochordal process—is established in the center, flanked by "mesodermal wings" forming the mesoderm.
- Ectoderm: Cells that do not participate in migration and remain on the surface (derived from the original epiblast) become the ectoderm.
Microscopic Anatomy
A cross-section of a chick embryo at the primitive streak stage reveals the following features:
- The embryo retains the flattened shape of a discoblastula.
- The center of the section shows a thickening with a depression—the primitive groove.
- The ectoderm appears as the thickest, most massive layer with pseudostratified nuclear arrangements.
- The mesoderm lies as a loose aggregation of individual cells, resembling mesenchyme.
- The endoderm is represented by a thin, single layer of flattened (squamous) cells.