I have been reading about Townes and Holtfreter's work in 1955, in which cells are dissociated from a blastocyst in an alkaline solution then mixed together and spontaneously reaggregates based on type, so epidermal cells around the outside and neural plate cells in the middle.
I understand enough about cell adhesion to understand why the cells will seem to attract cells of their own type, but would like to know how they can initially detect what to become and where they are needed in a specialised form, without something acting like a brain telling them what to become and where to go.
If the selection from the available types is random, as I suspect, what happens to blastocysts with too much epidermal tissue or vice versa? I'm struggling to imagine how organisms like this can develop without something taking the lead and actively coordinating what goes where.
Answer
Cell differentiation, cell fate and cell mapping is an interplay of accessible evolutionary strategies/programmes and responses to dynamic environmental cues such as specialized hormones (e.g. morphogens) and physical parameters and constraints. That is putting it very broadly. It is a complex issue, if L. Wolpert's PLOS assays are any indication. I compiled a few links to get you started.
Specifically, reappraising the topic of your cited classical experiment are R.Moore et al:
The classical cell sorting experiments undertaken by Townes and Holtfreter described the intrinsic propensity of dissociated embryonic cells to self-organize and reconcile into their original embryonic germ layers with characteristic histotypic positioning. Steinberg presented the differential adhesion hypothesis to explain these patterning phenomena.....
Regarding: ...without something taking the lead and actively coordinating...
On a hunch that you are not talking about complexity, attractors and polarity, I would like to share a few words. You may completely disregard them if you'd like.
My personal experience (from my country) is that few biologists delve into "that" branch of EvoDevo, because a profound understanding involves calculus, modeling and a knack for theoretical biology. Yet, afaik few struggled comprehending the evolutionary logic of making an organism based on Albert's book chapters alone.
See:
I personally recommend G.Müller's authored/coauthored papers:
(open access) http://homepage.univie.ac.at/gerhard.mueller/publications-papers.html
Disclaimer: I am not specialized in the field of EvoDevo or Theoretical Biology.
Views expressed here are my own.
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