Preparation for Gastrulation and Blastocyst Structure
Before gastrulation begins, the mammalian zygote undergoes total, unequal, and asynchronous cleavage. The culmination of this process is the formation of a specialized type of blastula called the blastocyst.
Structurally, the blastocyst is a fluid-filled vesicle consisting of three main components:
- Trophoblast — the outer layer of the vesicle.
- Embryoblast (inner cell mass) — a compact group of embryonic cells located internally against the trophoblast.
- Blastocoele — the internal cavity.
An important morphological event prior to the onset of gastrulation is the shape change of the embryoblast cells. They flatten, arranging themselves into a structure that is visually and functionally analogous to the avian germ disc. This structural similarity dictates that subsequent cell mass movements will follow an avian-like pattern.
First Stage: Delamination
The initial phase of mammalian gastrulation occurs via delamination. This process takes place exclusively within the embryoblast and involves two consecutive cleavage steps:
- Primary cleavage. The unified cell mass of the embryoblast splits into two layers. The inner layer facing the blastocoele forms the hypoblast. The outer layer adjacent to the trophoblast is termed the epiblast.
- Secondary cleavage. This step is a distinctive feature of higher mammals and humans. The newly formed epiblast undergoes a second delamination into two distinct layers:
- Amniotic ectoderm — the upper layer of cells remaining in contact with the trophoblast.
- Embryonic epiblast — the lower cell layer.
A slit-like space, the amniotic cavity, forms between these two layers. The primary outcome of the entire delamination phase is the formation of the germ disc (embryonic shield) from the embryonic epiblast cells. Similar to the avian epiblast, this disc serves as the universal source for the development of all three primary germ layers.
Second Stage: Immigration
The second phase of gastrulation—immigration—unfolds within the embryonic epiblast. The process begins with the active migration of cells across the epiblast surface from the periphery toward the midline.
Upon meeting at the midline, the migrating cells form axial thickenings:
- The primitive streak, along whose axis a depression called the primitive groove develops.
- At the anterior end of the streak, Hensen's node (primitive node) forms with a central primitive pit.
Following the formation of these structures, invagination begins: cells migrate beneath the germ disc through the primitive groove and pit. Subsequent differentiation depends on the timing and depth of migration:
- The first wave of cells migrates deeply, displacing the primary hypoblast to the periphery. These cells completely replace it, forming the embryonic endoderm. The displaced original hypoblast becomes the extraembryonic endoderm.
- The second wave of cells spreads out in the space above the newly formed endoderm. Proliferating, they form the mesoderm and the centrally located notochordal process.