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There are in principle two ways that two cells with the same mother cell can differentiate/specialize/diverge:

  1. Because they are not perfect identical copies (= different internal influencing factors). I explictly don't mean "non-identical on the DNA level", but on the level of other cellular components!

  2. Because they are exposed to different external influencing factors (chemical, mechanical).

Can it be said, which of these two reasons of cell differentiation is the more common, frequent, resp. "important"? Or does it completely depend on the (type of mother) cell and some general circumstances?

If internal factors (= non-identical copies) are an important source of differentiation, I have two (optional) questions:

  1. How does the mother cell manage to cleave into two non-identical copies in a rather fine-tuned and determinstic way? Or is any kind of asymmetry enough?

  2. Which is the first cell (starting with the zygote) that cleaves into two non-indentical cells, different enough to give rise to differentiation? (From this answer I believe to have learned that this might happen already after 1-4 divisions.)

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I found a partial answer to my and this question at Quora:

What regulates initial cell differentiation?

Ian Driver says:

In many organisms the initial cell has mRNA (or protein) partitioned in one half of the zygote. One daughter cell inherits most of the mRNA or protein and goes on to activate or repress genes in that cell, but not in the other. This has been visualized and well studied in C. elegans: Asymmetric cell division and axis formation in the embryo.

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This question is kind of bad. There are common internal influencing factors that are unrelated to improper replication, most commonly:

Methylation. Addition of methyl groups to DNA regions inhibits RNA transcription from that region.

Imprinting. Can't recall exactly how this works but it's another mechanism for enabling/inactivating genes.

Histone structure (wrapping/unwrapping). DNA coiled tightly around histones is more unlikely to be transcribed.

External influencing factors include cell signaling, substrate detection, and large-scale endocrine signaling. Most important are growth factors and growth inhibitors (which let the cells know if they need to replicate quickly (repairing damage/large-scale growth) or slow down replication (overcrowding). Other external factors could include metabolism (produce these digestive enzymes when exposed to these foods) or endocrine signaling.

Non-identical copies at the DNA level (mutation) are thankfully relatively rare in practice, particularly among higher-order species with longer lifespans.

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  • $\begingroup$ I didn't mean "non-identical on the DNA level" but on the level of cellular components - and just made this clear in the question. Is the question still bad, then? $\endgroup$ – Hans-Peter Stricker Nov 7 '17 at 15:13
  • $\begingroup$ confirmed your question is no longer bad. $\endgroup$ – user1258361 Nov 7 '17 at 15:21
  • $\begingroup$ Puh, I am relieved. $\endgroup$ – Hans-Peter Stricker Nov 7 '17 at 15:24

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