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The Stages of Early Cleavage |
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Monday, 27 September 2004 |
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- Cleavage begins about 24h after the egg has been fertilized once the pronuclei have fused.
- We will discuss cleavage as if each division occurs synchronously because it is easier to understand this way
1st Cleavage
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Resulting embryo is 2 cells (i.e., 2 blastomeres or 2 cleavage cells)
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DNA has been synthesized: Embryo has double the DNA content of zygote
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Embryo has made membrane to surround both cells
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Growth has not occured so the 2 blastomeres together are approximately the same size as the original zygote
2nd Cleavage
Embryo is 4 cells
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DNA has been synthesized: Embryo now has 4x DNA content
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Embryo has made membrane to surround all four cells
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Growth has not occurred so the 4 blastomeres together are approximately the same size as the original zygote
3rd Cleavage
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Embryo is 8 cells
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More DNA has been synthesized: Embryo now has 8x DNA content
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Embryo has made membrane to surround all 8 cells
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The duplication of DNA and synthesis of membrane will continue as cells continue to divide but now other changes will begin to occur
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Growth has not occurred so the 8 blastomeres together are approximately the same size as the original zygote
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Cells still in loose association prior to compaction; this will change
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Embryo will now undergo compaction just prior to next division leading to 16 cell stage
This information was provided by Professor Danton H. O'Day - Univerisity of Toronto at Mississauga - Canada. Unless otherwise stated the information and graphics that are presented within this embryology section are the sole property of Danton H. O'Day, copyright 1998(c), 1999(c), 2000(c), 2001(c), 2002(c).
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Embryology 
