Chronology and Process Features
Unlike many other developmental stages, human gastrulation is temporally split and clearly divided into two phases.
- The first phase starts on the 7th day of development. Notably, this process within the embryoblast does not occur in isolation, but strictly simultaneously with the implantation of the embryo into the endometrium of the uterus (a process physically mediated by the trophoblast).
- The second phase of gastrulation begins much later—around the 14th to 15th day of development.
Between these two phases, a distinct resting period occurs for the embryo proper. However, this rest is relative: it is precisely during this time window that critical extraembryonic organs required for the subsequent survival and nourishment of the embryo actively form and develop.
Mechanism of Two-Stage Delamination
The primary mechanism of the first phase of gastrulation is delamination (splitting of cellular sheets). In humans, this process represents a two-stage delamination which, unlike avian development, occurs in two consecutive cycles.
- First cleavage cycle. Driven by active cell division, the uniform embryoblast mass separates into two independent sheets: the upper (outer) layer is called the epiblast, and the lower (inner) layer is the hypoblast.
- Second cleavage cycle. At this stage, small cavities begin to appear within the formed epiblast. Soon they coalesce to form a single amniotic cavity. The appearance of this cavity definitively divides the epiblast into two unequal parts: the floor of the resulting vesicle becomes the embryonic epiblast, and its roof, lying directly adjacent to the trophoblast, becomes the amniotic ectoderm.
As a result of these two delamination cycles, the initial embryoblast transforms into three distinct cellular sheets, each possessing its own morphology and developmental fate.
Characteristics of the Formed Layers
Each of the three formed layers plays a strictly defined role in subsequent embryogenesis.
1. Embryonic Epiblast Occupies an intermediate position, lying between the amniotic ectoderm and the hypoblast. Morphologically, it is represented by large, tall columnar cells visually resembling pseudostratified epithelium. At this stage, while the embryo is flat, this structure is referred to as the embryonic disc (or shield). This is a critical layer: during the second phase of gastrulation, it will serve as the source of all three primary germ layers (ectoderm, mesoderm, and endoderm) and will also give rise to cells forming the extraembryonic mesoderm.
2. Hypoblast Characterized as the lowest (innermost) layer. Unlike the epiblast, it is formed by small, pale, cuboidal cells. The hypoblast is classified as extraembryonic endoderm. Subsequently, these cells proliferate along the inner surface of the trophoblast (extraembryonic ectoderm) and directly participate in the formation of the yolk sac.
3. Amniotic Ectoderm Represents the uppermost layer forming the roof of the vesicle. It consists of small cells morphologically very similar to hypoblast cells. Together with the trophoblast, this layer belongs to the extraembryonic ectoderm. Its main biological role is participating in the formation of the amnion.
Summary
Summarizing the first phase of gastrulation, two key events can be highlighted. First, there is a clear segregation of cellular material from which the embryo proper will subsequently develop (formation of the embryonic epiblast). Second, the foundation is laid for the development of extraembryonic structures necessary to sustain the embryo at later stages.