I’ve been told that the unfertilized egg contains cytoplasmic determinants (e.g. proteins, transcription factors, mRNAs) that are divided unevenly during the early stages of cell division in an embryo. Thus, each embryonic cell has a different cytoplasmic composition with a different set of transcription activators, allowing the expression of only certain genes.

By this token, it seems to me that this uneven partitioning of cytoplasmic determinants occurs in the very first division of the zygote. Thus, the first blastomeres are already no longer totipotent. Is that correct?

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    $\begingroup$ Search on fraternal/ maternal twins $\endgroup$ Mar 19 '16 at 13:00
  • $\begingroup$ Initial 3 divisions are equal. $\endgroup$ Mar 19 '16 at 13:01
  • $\begingroup$ Can you explain why my reasoning is incorrect? What really happens to maintain the totipotency of the first few cells? $\endgroup$ Mar 19 '16 at 13:30
  • $\begingroup$ Did you search on fraternal twins? Well I know this much only. $\endgroup$ Mar 19 '16 at 13:33
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    $\begingroup$ If this were actually the case, identical twins would be impossible (yet they are far from rare.) There may be a bit of confusion regarding the identicality of two blastomeres (they are not always - maybe even never - completely identical), but totipotentiality means they have everything needed to develop into complete beings. Also, the zona pellucida plays a role in survival of divided blastomeres (e.g. 8 cell stage cells have been seperated, and have developed into normal mammals (horses, cows) if supplied with a zona pellucida. $\endgroup$ Mar 19 '16 at 13:47

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