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.