Increases in the number of cells in different areas of epithelial somites related to changes in morphology and development

There are two distinct groups of cells in the epithelial somite: cells in the epithelial ball that form the periphery, and loose mesenchymal cells found in the central cavity (somitocoele). Recent work has produced evidence to show that these two groups of cells have significant differences (morphology, origin, fibronectin content, reaction to peanut lectin, communication properties) but the significance of these differences has yet to be established. It is not yet clear whether the epithelial somite stage of development is merely a time for cell proliferation, or whether it is a time when significant differences develop which have consequences in subsequent morphogenesis. Certainly, there are indications that the two groups of cells might form different structures related to the vertebral column based on their position in the subsequent sclerotome. In this study, we have examined the number of cells that are present in both the epithelial ball and the somitocoele at various stages of maturity. The results show that later-formed somites contain significantly more cells in both the epithelial ball and the somitocoele. Furthermore, while the density of cells in the epithelial ball remains constant (accounting for an increase in dimensions of the somite), there is a significant increase in density of cells in the somitocoele. This suggests that there is an important distinction being created between the cells of the epithelial ball and those in the somitocoele. The results also illustrate that somite development is not the same at all segmental levels and that development of each might need to be considered on an individual basis, especially as the later-formed somites are known not to remain in this stage of development for as long as the earlier-formed somites.