Blastocyst Embryonic Development - Anatomy & Physiology
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Pre-Implantation Development
- 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)
- This causes the zona pellucida to form around the zygote. This is 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.