Blastocyst Embryonic Development - Anatomy & Physiology

2009-07-20T11:42:44Z

Gregcovarr: /* Pre-Implantation Development */

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== Pre-Implantation Development ==
[[Image:Blastocyst.gif|thumb|right|300px|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.

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Blastocyst Embryonic Development - Anatomy & Physiology