Blastocyst Embryonic Development - Anatomy & Physiology

Pre-Implantation Development

Blastocyst - Courtesy of Wikimedia Commons

The sperm enters the the oocyte forming the ootid.
In the ootid, male and female pronuclei along with the first and second polar bodies are present.
Fusion of the male and female pronuclei results in a single diploid nucleus (syngamy)
The zona pellucida then becomes an imprenetrable layer that prevents polyspermy and so polyploidy.
Zygote (diploid) undergoes mitotic divisions (cleavage).
Cleavage gives rise to daughter cells called blastomeres.

Cleavage divisions continue and the 4-celled embryo gives rise to an 8-celled embryo.
These cells are totipotent and divide asynchronously.
After the 8-celled stage, a ball of cells is formed. This stage is referred to as a morula.
Tight junctions form between outer cells of the morula.
Gap junctions form between inner cells of the morula.
Thus, two groups of cells are created within the morula.
Outer cells pump sodium into the intracellular spaces and water follows osmotically.
Fluid begins to accumulate within the morula.

Cells of the morula continue to divide and a blastocyst develops, consisting of:
- Inner cell mass (ICM)
- Blastocoele cavity
- Single layer of trophoblast cells
As fluid accumulates, the outer cells become flattened and a cavity known as the blastocoele is formed.
Gap junctions connecting the inner cells of the morula enable them to polarise as a group.
As a result, two separate cellular components emerge - the inner cell mass and the trophoblasts.

Blastocyst continues mitosis.
Fluid accumulates to fill the Blastocoele and pressure within the embryo increases.
Concurrent with growth and fluid accumulation is the production of proteolytic enzymes by Trophoblastic cells.
Enzymes weaken the Zona Pellucida.
Blastocyst begins to contract and relax causing intermittent pressure pulses on the Zona Pellucida.
Zona Pellucida ruptures with the development of a small fissure just large enough for the Blastocyst to squeeze out.
Blastocyst 'hatches' from the Zona Pellucida.
- Rodent,Dog,Cat,Primate: 8 days after fertilization
- Ungulates: 12 days after fertilization.
Blastocyst free-floating within the Uterus, totally dependent on the Uterine environment for survival.