Blastocyst Embryonic Development - Anatomy & Physiology

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()Map REPRODUCTIVE SYSTEM (Map)
FERTILISATION , IMPLANTATION AND EARLY EMBRYONIC DEVELOPMENT'



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.