Formation of the Blastocyst
Around the 5th day of embryonic development, an internal space—a cavity—begins to form within the dense cluster of embryonic cells. From the moment of its appearance, the embryo officially enters the blastocyst stage.
On days 5–6, the embryo descends into the uterine cavity. There it remains in an entirely free (unattached) state for about two more days. In parallel, cell division—known as cleavage—continues within it. At this stage, cleavage is characterized by the following features:
- Occurs asynchronously (cells do not divide simultaneously).
- Is unequal in nature.
- Leads to a continuous increase in the total number of embryonic cells (blastomeres).
Morphological Structure
Beginning at the 58-cell stage, which temporally corresponds to 4.5–5.5 days, clear cellular differentiation occurs within the embryo. Morphologically, the blastocyst acquires a complex structure and consists of three key elements.
| Element | Structural Description | Derived Structures |
|---|---|---|
| Trophoblast | The outer single-layered wall of the vesicle, consisting of small, light-colored cells. | Extraembryonic organ — chorion. |
| Embryoblast | Inner cell mass. A cluster of large, dark blastomeres forming a nodule at one of the poles on the inner surface of the trophoblast. | All tissues of the embryo proper, as well as the amnion, yolk sac, and allantois. |
| Blastocoele | The internal fluid-filled cavity of the vesicle. | — |
Hatching and Preparation for Implantation
To attach to the uterine wall, the embryo must undergo 'hatching'—the release from the zona pellucida. This stage occurs directly within the uterine cavity.
The mechanism of shell destruction includes three sequential steps:
- Volume increase. Cells of the outer layer (trophoblast) begin to actively absorb fluid from the uterine cavity, inflating the vesicle.
- Metabolic activation. Synthetic processes sharply intensify within the blastomeres themselves.
- Barrier destruction. Specific projections form in the trophoblast. A few hours before implantation, they rupture and break down the zona pellucida.
The biological significance of this process is the removal of mechanical constraints. Following the shedding of the zona pellucida, a full G1 phase (growth phase) is activated in the cell cycle. The embryo gains the ability to increase in size, and intensive mass growth begins.
Cellular Potency and Reversibility of Differentiation
As cells divide and form the different layers of the blastocyst, their developmental potential (potency) progressively decreases. However, it is important to understand that this reduction in potency is not associated with any irreversible damage to the genetic apparatus.
It has been established that if the nuclei of already differentiated cells are exposed to special inducing agents, they can be reprogrammed. This fact fundamentally proves the absolute reversibility of cell development processes during early embryogenesis.