Limb-somite relationship: myogenic potentialities of somatopleural mesoderm.

Since 1977, CHEVALLIER et al. had the idea that, although limb muscle cells are usually of somitic origin, somatopleural cells could give rise to myocytes under certain experimental conditions. New experiments were undertaken in order to investigate further the differentiating capacities of the limb somatopleural mesoderm. Two-day quail limb somatopleural mesoderm was ectopically implanted into two-day chick hosts (into the flank, in place of the neural tube or between the neural alar plates) and the specificity of the muscle cells in the resulting ectopic limbs was analyzed 7 to 10 days later. The grafts were grouped as a function of the stage of segmentation of the adjacent somitic mesoderm at the time of excision. Grafts, whose adjacent somitic mesoderm was still unsegmented (before stage 15 pairs of somites for the wing grafts and 26 pairs of somites for the leg grafts) or in process of segmentation (stages 15 to 20 pairs of somites for the wing grafts, only) were considered, after histological investigation, as devoid of somitically derived myogenic cells at the time of transplantation. In 6 out of 29 cases, such grafts expressed the ability to differentiate quail muscle cells. It can therefore be admitted that the limb somatopleural mesoderm contains a labile population of cells able to adjust its differentiative capacities to the environment.