Blastocyst formation
At approximately day 5 of embryonic development, an internal space—a cavity—begins to form inside the dense cluster of embryonic cells. From the moment it appears, the embryo officially enters the blastocyst stage.
On days 5–6, the embryo descends into the uterine cavity. There it remains in a completely free (unattached) state for about two more days. In parallel, cell division processes—so-called cleavage—continue within it. At this stage, they are characterized by the following features:
- They proceed asynchronously (cells do not divide simultaneously).
- They are uneven.
- They lead to a constant increase in the total number of embryonic cells (blastomeres).
Morphological structure
Starting from the 58-cell stage, which corresponds to 4.5–5.5 days, clear cell differentiation occurs in the embryo. Morphologically, the blastocyst acquires a complex structure and consists of three key elements.
| Element | Description of structure | Derivative structures |
|---|---|---|
| Trophoblast | The outer single-layered wall of the vesicle, consisting of small light-colored cells. | Extraembryonic organ—chorion. |
| Embryoblast | The 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 cavity of the vesicle filled with fluid. | — |
"Hatching" and preparation for implantation
To attach to the uterine wall, the embryo must undergo a process called "hatching"—release from the zona pellucida. This stage takes place directly in the uterine cavity.
The mechanism of shell destruction includes three sequential steps:
- Volume increase. The cells of the outer layer (trophoblast) begin to actively absorb fluid from the uterine cavity, inflating the vesicle.
- Metabolic activation. Synthetic processes sharply intensify inside the blastomeres themselves.
- Barrier destruction. Specific outgrowths form in the trophoblast. A few hours before implantation, they rupture and destroy the zona pellucida.
The biological significance of this process lies in removing mechanical restriction. After shedding the shell, a full-fledged G1 phase (growth phase) is activated in the cell cycle. The embryo gains the ability to increase in size, and intensive growth of its mass begins.
Cell potencies and reversibility of differentiation
As cells divide and form different layers of the blastocyst, their developmental potential (potency) gradually decreases. However, it is important to understand that this decrease 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 induction agents, they can be reprogrammed. This fact fundamentally proves the principle reversibility of cell development processes in early embryogenesis.
Briefly: answers to topical questions
Into what two layers does the trophoblast differentiate before the start of implantation?
During development and invasion, the trophoblast differentiates into two layers:
- Cytotrophoblast—the inner cellular layer located on the basement membrane. It retains mitotic activity, serving as a source of regeneration (cambial reserve) for trophoblast growth, and also performs an endocrine function by initiating the production of human chorionic gonadotropin.
- Syncytiotrophoblast—the outer (superficial) layer, which is a single multinucleated structure (syncytium). It is formed by the fusion of cytotrophoblast cells and actively produces enzymes to destroy maternal tissue during invasion.
It is easy to remember the structure of the blastocyst by analogy with a walnut: the shell is the trophoblast (protection and nutrition), the kernel is the embryoblast (the embryo itself will grow from it), and the empty space under the shell is the blastocoele.
Class and exam questions
On which day does the embryo become a blastocyst?
The embryo reaches the blastocyst stage on day 5, when a cavity (blastocoele) appears inside it.
What is formed from the inner cell mass (embryoblast)?
All tissues of the embryo itself, as well as a number of crucial extraembryonic organs (amnion, yolk sac, and allantois), are formed from the embryoblast.
What is the purpose of embryo "hatching"?
"Hatching" is necessary to shed the zona pellucida. This removes mechanical restrictions, allows the cell growth phase (G1 phase) to be activated, and initiates intensive mass growth prior to implantation.
Is cell differentiation reversible at the blastocyst stage?
Yes, differentiation is fundamentally reversible. The decrease in potency is not associated with genome damage, and the nuclei of differentiated cells can be reprogrammed under the influence of inducers.
What else to study on this topic
- Features of the cell cycle before and after the activation of the G1 phase
- Mechanisms of fluid absorption by trophoblast cells
- Formation of the chorion from trophoblast derivatives
- The process of blastocyst implantation into the endometrium
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