The first morphogenetic movement of teleost embryos is epiboly, when the embryonic cell mass spreads over the yolk. Kane and co-workers now provide new insights into this process in their analysis of zebrafish mutants with arrested epiboly (p. 1105). These mutants were found to have different versions of the half baked (hab) locus, which the authors report here encodes the zebrafish homolog of E-cadherin. They identify two cell layers in the epiblast, the outer of which expresses hab mRNA at higher levels,and show that during normal epiboly, inner layer cells radially intercalate into the outer cell layer and flatten, thus expanding the area of the epiblast. In hab mutants, the interior cells intercalate normally but fail to flatten and sometimes return to the inner layer. The researchers conclude that E-cadherin is required for the cell movements of epiboly and possibly for similar movements in mammalian embryos....

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