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.