AbstractMuscular dysgenesis (mdg) is a recessive lethal mutation in the mouse which drastically affects skeletal muscle development during embryonic life. Physiologically, the disease is characterized by a complete paralysis resulting from a lack of excitation‐contraction coupling. Existing electrophysiological, biochemical, and genetic evidence shows that mdg/mdg mice express a basic alteration of L‐type voltage‐sensitive Ca2+ channels in skeletal muscle. Studies on mdg/mdg myotubes in primary culture have shown that +/+ fibroblasts or +/+ Schwann cells may fuse with them and correct their functional deficiency by genetic complementation. As the spontaneous formation of heterocaryons is thought to be an exclusive property of myoblasts, we asked whether fibroblasts may have changed their properties before fusion occurred. We used primary cells issued from sciatic nerves dissected from newborn transgenic mice carrying the pHuDes1‐nls‐LacZ transgene (Des‐LacZ cells) as non‐muscle cells. These cells were mainly fibroblasts (80%) positive for Thy 1.1 and Schwann cells positive for S100. The cultures were negative for myogenic markers (desmin, troponin T), did not form myotubes long‐term, and did not display significant activation of the muscle reporter gene (pHuDes1‐nls‐LacZ). After a few days in coculture with dysgenic or normal myotubes, the muscle reporter gene (β‐galactosidase) was detected both within dysgenic myotubes, correlating with the restoration of normal contractile activity, and normal myotubes. As well as confirming that fusion takes place, this shows that Des‐LacZ cells nuclei incorporated into recipient myotubes express their own myogenic genes. Moreover, individual mononucleated Des‐LacZ cells expressing β‐galactosidase were observed, indicating that myogenic genes were being expressed before fusion. This suggests a mechanism of myotube driven myogenic recruitment of cells during the in vitro myogenesis. Analysis of the distribution of the induced Des‐LacZ cells (positive for β‐galactosidase) in compartmentalized muscle cocultures showed that in the presence of dysgenic myotubes, these cells were equally distributed in both myotube free and enriched areas, whereas in the presence of normal myotubes, the positive cells remained in close vicinity of the myotubes. This difference could be explained by the fact that the dysgenic phenotype might include release of the induction process from its normal controls. Our results are consistent with the idea of a transcellular mechanism triggering myogenic differentiation in non‐muscle cells, and that myotubes themselves are able to drive myogenic recruitment of cells during the in vitro myogenesis. This phenomenon could be the result of either a myogenic induction in non‐muscle cells, imposing a phenotypic change, or the activation of pre‐myoblastic quiescent cells by the myotubes themselves. © 1995 Wiley‐Liss, Inc.