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.