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 prod uced evidence to show that these two groups of cells have significant differences (morphology, origin, fibronectin content, reaction to pean ut Lectin, communication properties) but the significance of these dif ferences has yet to be established. It is not yet clear whether the ep ithelial somite stage of development is merely a time for cell prolife ration, or whether it is a time when significant differences develop w hich have consequences in subsequent morphogenesis. Certainly, there a re indications that the two groups of cells might form different struc tures related to the vertebral column based on their position in the s ubsequent sclerotome. In this study, we have examined the number of ce lls that are present in both the epithelial ball and the somitocoele a t various stages of maturity. The results show that later-formed semit es contain significantly more cells in both the epithelial ball and th e somitocoele. Furthermore, while the density of cells in the epitheli al ball remains constant (accounting for an increase in dimensions of the somite), there is a significant increase in density of cells in th e somitocoele. This suggests that there is an important distinction be ing created between the cells of the epithelial ball and those in the somitocoele. The results also illustrate that somite development is no t 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 fo r as long as the earlier-formed somites.