On the spatial relationship between fibroblasts and myogenic cells during early development of skeletal muscles

The spatial relation between fibroblasts and myoblasts as well as myotubes in the extensor muscles of the forearm of mouse embryos was investigated with routine electron microscopic techniques. From day 13 of embryonic development onwards, fibroblasts and myogenic cells may be distinguished by their ultrastructure from each other. Above all the fibroblasts are characterized by their long cell processes. Specialized cell contacts between them are visible. Fibroblasts often encircle some myogenic cells indicating the division of the muscles into smaller bundles. Over long distances, the intercellular gap between both cell types is narrow and measures about 15 nm. Sometimes specialized intercellular junctions can be seen. They resemble simple desmosomes and seem to be mechanical cell junctions. After the establishment of the basal lamina around the myogenic cells such specialized cell contacts are no longer visible. It seems possible that the fibroblasts act as mechanical counterpart for the organization of the myogenic cells.