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I would like to understand which mechanism triggers the first cell differentiation after n divisions.

I read previous articles on SE and Wikipedia articles on cellular differentiation and embryogenesis but still fail to understand what exactly makes it so that starting from a given division cells suddenly start to be different.

Wikipedia claims that

In the first hours after fertilization, this cell divides into identical cells. In humans, approximately four days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst

but do not explain why they begin to specialize.

I could imagine

  • that a cell has a "counter" on the number of divisions which triggers differentiation after a given amount of divisions
  • or a chemical substance (either cell-borne or external) forces a change in the division

but why some cells would become "cell A" and some others "cell B"?

I am fairly sure that the biochemical mechanisms which regulate the life of a grown up mechanism can explain cellular differentiation (through hormones for instance) -- I am however interested by this specific moment, this n-th division where identical cells become differentiated.

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There are two different questions here. One is how "identical" cells become different types of cells. The other is why do the cells start differentiating when they do. –  Bitwise Jul 15 '13 at 4:36
I am interested in the why more than the how –  WoJ Jul 15 '13 at 10:02

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