The immediate fate of cells in the epithelial somite of the chick embryo

The precise origins and fates of cells forming the epithelial ball and those contained within the somitocoele at the centre of the somite in the chick embryo are unknown. In particular, it is not known whether the progeny of the proliferating cells in both the epithelial ball and the somitocoele simply mix with each other, or whether they represent two separate populations that remain isolated during proliferation. We tested whether cells found in both of these locations are the result of cell migration, cell division or both. To do this, we injected single cells in both the epithelial ball and the somitocoele with lysinated fluorescein dextran, a molecule that can only be transferred to other cells through cell division, and observed their development over the next 24 h. Control experiments were also performed to assess the incidence of error associated with the microinjection method and to provide reliable and valid comparative data. Our results showed that the progeny of the cells in the somitocoele, as well as any non-proliferating cells, appear to remain in the somitocoele. In contrast, the progeny of the cells in the epithelial ball either remain in the epithelial ball or migrate into the somitocoele to mix freely with the other cells present. These results suggest that the cells in the epithelial ball and the somitocoele develop as discrete groups, with the possibility of diverse fates accompanying further development.