DESMIN TRANSGENE EXPRESSION IN MOUSE SOMITES REQUIRES THE PRESENCE OFTHE NEURAL-TUBE

Transgenic mice were used to study the effect of the neural tube on so mite 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 beta-galactosidase. In order to determine whe ther the developmental fate of cells, specifically the prospective myo genic population, in newly developed somites was pre-determined, newly formed somites were isolated from the caudal region of day 9.5 transg enic embryos and transplanted into 8.5 day non-transgenic host embryos . Even though the implanted somites were not oriented in the host embr yos, all the specimens examined developed normally at the graft site f orming a dermatome, myotome and sclerotome in the correct anatomical p ositions. The myotome even expressed the desmin transgene. In addition , we isolated the 3 most caudal somites, that is, the most recently de veloped somites, from day 9.5 transgenic embryos and maintained them o n gelatin-coated coverslips in culture for up to 4 days. While these s omite explants did not develop myoblasts, it was possible to induce my ogenesis by introducing pieces of neural tube into the explant culture s. These results suggest that the developmental fate of cells within t he newly formed somite is not predetermined, but is dependent on the m icroenvironment surrounding the developing somite.