Desmin transgene expression in mouse somites requires the presence of the neural tube

Transgenic mice were used to study the effect of the neural tube on somite myogenesis. These mice express a transgene in which the 1 kb DNA 5' regulatory sequence of the desmin gene is linked to a reporter gene which codes for E. coli β-galactosidase. In order to determine whether the developmental fate of cells, specifically the prospective myogenic population, in newly developed somites was pre-determined, newly formed somites were isolated from the caudal region of day 9.5 transgenic embryos and transplanted into 8.5 day non-transgenic host embryos. Even though the implanted somites were not oriented in the host embryos, all the specimens examined developed normally at the graft site forming a dermatome, myotome and sclerotome in the correct anatomical positions. The myotome even expressed the desmin transgene. In addition, we isolated the 3 most caudal somites, that is, the most recently developed somites, from day 9.5 transgenic embryos and maintained them on gelatin-coated coverslips in culture for up to 4 days. While these somite explants did not develop myoblasts, it was possible to induce myogenesis by introducing pieces of neural tube into the explant cultures. These results suggest that the developmental fate of cells within the newly formed somite is not predetermined, but is dependent on the microenvironment surrounding the developing somite