Domains of differential cell proliferation and formation of amnion folds in chick embryo ectoderm.

Patterns of cell proliferation in ectoderm epithelium that will form avian amnion correlate with morphogenesis, but not in an obvious pattern with respect to large-scale folding. At sites where the pre-axial amnion folds will first appear in 4- to 8-somite embryos, patterns of proliferation do not separate into domains that presage location of the single pre-axial fold that is commonly described in embryology texts. Instead, increased cell proliferation occurs in a significant, bilateral pattern. In stages with 13 to 27 somites, when lateral amnionic folds are prominent, five paraxial domains of cell proliferation correlate with morphology and show decreasing levels of cell proliferation with distance from the neural axis. Slowly growing areas surround rapidly growing areas and could assist buckling of epithelium by providing constraints on expansion of faster growing areas. Proliferation domains in ectoderm correlate with morphology and morphological events when localized changes in cell shape are lacking and suggest a role for differential cell proliferation in formation of large-scale epithelial folds in early chick embryos.