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== Pre-Implantation Development ==

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== Pre-Implantation Development ==

* In the Ootid, male and female '''pronuclei''' along with the first and second '''polar bodies''' are present.

[[Image:Blastocyst.jpg|thumb|right|300px]] Once sperm has entered the the '''oocyte''', an '''ootid''' is formed. During early stages the '''ootid''' will contain male and female '''pronuclei''' along with the first and second '''polar bodies'''. Fusion of the male and female '''pronuclei''' will result in a single '''diploid nucleus''' or '''syngamy'''. Once '''syngamy''' has occurred, the '''zona pellucida''' then develops into an imprenetrable layer that prevents polyspermy and so polyploidy. Once the '''zona pellucida''' has developed, the ootid is now referred to as a '''zygote''' (diploid) and will begin undergoing mitotic divisions via a '''cleavage''' process that will begin to give rise to daughter cells called '''blastomeres'''. These cleavage divisions will begin to produce a 4-celled embryo and then an 8-celled embryo. <br> <br> All cells in the embryo up to this stage are totipotent and divide asynchronously. After the 8-celled stage, a ball of cells is formed and is referred to as a '''morula'''. A '''morula''' has '''tight junctions''' formed between outer cells and '''gap junctions''' between the inner cells enabling them to polarise as a group and effectively create two distinct groups of cells within the morula. The outer cells pump sodium into the intracellular spaces and water follows osmotically allowing fluid to begin to accumulate within the morula. <br> <br> Cells within the '''morula''' continue to divide and a '''blastocyst''' (see image) develops, consisting of an '''inner cell mass''' or (ICM), a '''blastocoele''' cavity and a single layer of '''trophoblast''' cells. As fluid accumulates, the outer cells become flattened and a cavity known as the blastocoele is formed. At this stage, two separate cellular components emerge - the '''inner cell mass''' and the '''trophoblast'''. <br> <br> Mitosis continues to occur within the '''blastocyst''' and 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 which begin to weaken the '''zona pellucida'''. The '''blastocyst''' then begins to contract and relax causing intermittent pressure pulses on the '''zona pellucida''' which eventually ruptures facilitating the development of a small fissure just large enough for the blastocyst to squeeze out. <br> <br> The '''blastocyst''' ''hatches'' from the '''zona pellucida''', although the period after fertilisation that this occurs is variable depending on species. '''Blastocyst''' hatching occurs in rodents, dogs, cats and primates 8 days after fertilization whilst in ungulates it is 12 days after fertilization. Once hatched, the '''blastocyst''' is free-floating within the uterus, totally dependent on the uterine environment for survival.  

* Fusion of male and female pronuclei to form a single '''diploid nucleus''' ('''Syngamy''')

* '''Zygote''' (diploid) undergoes mitotic divisions ('''cleavage''').

* Cleavage gives rise to daughter cells called '''Blastomeres'''.

 

* Cells  of the Morula continue to divide and a '''Blastocyst''' develops, consisting of:

[[Category:Pregnancy_and_Parturition]][[Category:A&P_Done|Category:A&amp;P_Done]][[Category:Developmental_Biology]]

 

 

** '''Rodent,Dog,Cat,Primate''': 8 days after fertilization

** '''Ungulates''': 12 days after fertilization.