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Cytoplasmic determinants are spread unevenly in the egg, and so when embryo starts forming (cells start dividing), the determinants are also unequally divided between cell. This unequal distribution later plays a major role in differentiation and gene regulation.

Protostomes undergo determinate cleavage. Their cells' fates are determined very early in development, whereas deuterostomes undergo indeterminate cleavage. So, if you take away a cell at early stage of development, it can be replaced.

Now if we compare the division of cytoplasmic determinants in protostomes and deuterostomes, what I said in the first paragraph seems true (to some extent) for only protostomes.

My questions:

  • Do deuterostomes' oocytes have even distribution of cytoplasmic determinants initially and divide unequally only after certain number of divisions?
  • Are there any studies showing correlation between the stage at which determination happens in deuterostome and beginning of unequal division of determinants?
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I dont have a definitive answer but the axis of cleavage can determine the nature of the daughter cells. Imagine an antero-posterior (AP) gradient of some determinant X. If the cell undergoes a division along AP axis the resultant daughter cells are identical. However, if the division is perpendicular to the AP axis then two different types of cells are produced (whose fate is in a way sealed). –  WYSIWYG Oct 13 '13 at 13:16
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