ig. 8. Establishment of sharp boundaries between differentially specified regions by switch-like cell fate decisions or compartmentalization. (A) Transcription factors A and B, which are involved in specification of two different domains (e.g. neural crest and placodes or two different individual placodes), may be induced by two different inducers (IA and IB, respectively) and have initially overlapping expression domains followed by sharpening of the boundary. (B) Overlapping expression domains as depicted in panel A may be due to coexpression of A and B proteins (red and blue dots) in individual cells (circles). Sharpening of the boundary in this case involves the differential upregulation of one but downregulation of the other transcription factor in individual cells (switch-like cell fate decisions), for example, due to their direct or indirect mutual transcriptional repression (lower panel). (C) Alternatively or additionally, overlapping expression domains as depicted in panel A may be due to mixing of cells expressing different transcription factors. Sharpening of the boundary may then involve the sorting of cells by non-random cell movements (arrows) (compartmentalization), for example mediated by mutually repelling cell adhesion molecules (CA, CB), activated by the different transcription factors (lower panel). (D) However, occurrence of cell movements per se does not necessarily imply cell sorting as depicted in panel C since random cell movements (arrows) may also occur before ectodermal areas are differentially specified to express different transcription factors.
Image published in: Schlosser G (2006)
Copyright © 2006. Image reproduced with permission of the Publisher, Elsevier B. V.
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