Gastrulation (first phase)

Chronology and features of the process

Unlike many other stages of development, human gastrulation is temporally discontinuous and clearly divided into two phases.

• The first phase starts on the 7th day of development. It is important to note that this process within the embryoblast does not occur in isolation, but strictly simultaneously with the implantation of the embryo into the uterine mucosa (this process is physically mediated by the trophoblast).
• The second phase of gastrulation begins much later—only on the 14th–15th day of development.

Between these two phases, a peculiar resting period ensues for the embryo itself. However, this rest is relative: it is during this time window that the active formation and development of vital extraembryonic organs, necessary for the further survival and nutrition of the embryo, takes place.

Mechanism of two-stage delamination

The main mechanism of the first phase of gastrulation is delamination (splitting of cell sheets). In humans, this process represents a two-stage delamination, which, unlike development in birds, occurs in two consecutive cycles.

1. First cleavage cycle. Due to active cell division, the single mass of the embryoblast divides into two independent sheets: the upper (or outer) layer is designated as the epiblast, and the lower (inner) layer as the hypoblast.
2. Second cleavage cycle. At this stage, small cavities begin to appear within the formed epiblast. Soon they fuse with each other, forming 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, adjacent directly 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 with its own morphology and developmental fate.

Characteristics of the formed layers

Each of the three formed sheets plays a strictly defined role in subsequent embryogenesis.

1. Embryonic epiblast Occupies an intermediate position, located between the amniotic ectoderm and the hypoblast. Morphologically, it is represented by large, tall prismatic cells that visually resemble stratified epithelium. At this stage, while the embryo has a flat shape, this structure is called the embryonic disc (or germ disc). This is a crucial layer: it is the one that in the second phase of gastrulation will become the source of all three primary germ layers (ectoderm, mesoderm, and endoderm), and will also delaminate cells to form the extraembryonic mesoderm.

2. Hypoblast The lowest (inner) layer. Unlike the epiblast, it is formed by small, light-colored cuboidal cells. The hypoblast is classified as extraembryonic endoderm. Subsequently, these cells proliferate along the inner surface of the trophoblast (extraembryonic ectoderm) and take a direct part 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 that are very similar in appearance to hypoblast cells. This sheet, together with the trophoblast, belongs to the extraembryonic ectoderm. Its primary biological role is participation in the formation of the amnion.

Summary

Summarizing the first phase of gastrulation, two key events can be highlighted. First, there was a clear segregation of the cellular material from which the embryo itself will subsequently develop (the embryonic epiblast formed). Second, the formation of primordia for extraembryonic organs necessary to support the embryo's viability at subsequent stages was initiated.

Class and exam questions

What else to study on this topic

• Cytological characteristics of embryonic epiblast cells
• Processes of yolk sac formation from hypoblast cells
• Development of extraembryonic organs during the resting period between gastrulation phases
• Morphogenesis of the amniotic cavity and amnion
• Interaction between the trophoblast and developing layers during implantation

Other topics in the section "Early stages of human intrauterine development"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.