Differential proliferation in morphogenesis of lateral body folds

Morphometric analysis and thymidine autoradiography were used to test the hypothesis that differential growth in the form of proliferative asymmetry is a major contributor to formation of lateral body folds in somatopleure. Chick embryos provided an opportunity to study both spatial and temporal differentials within an individual stage of development. Ectoderm and mesoderm were studied as separate tissues. Thymidine labeling index ranged from 34% to 49% in ectoderm and 43% to 66% in mesoderm and was assumed to reflect rate of proliferation in populations with a 100% proliferation index. Density of cells ranged from ten to 13 cells/1,000 micrometers2 in ectoderm and ten to 15 cells/1,000 micrometers2 in mesoderm, with a mean density of 12 cells/1,000 micrometers2. Cellular proliferation was important in folding, because ectoderm showed highest frequencies of labeling and highest cell density in folding levels. Correlation of proliferative differentials with morphogenesis suggests that proliferation is the form of growth contributing to formation of lateral body folds in 20-somite chick embryos.