Tissue interactions in the organization and maintenance of the muscle pattern in the chick limb.

Recent investigations on a hereditary muscular dysgenesis (cn/cn) in the chicken (Kieny, Mauger, Hedayat & Goetinck, 1983) have suggested that limb muscle pattern development and subsequent maintenance are two independent steps in the formation of the musculature. The respective activities or muscle cells and connective tissue cells in the ontogeny of the musculature have been investigated in avian embryos 1) by in ovo administration of drugs interfering with collagen biosynthesis, and 2) by heterogenetic somite-exchange experiments between normal and mutant embryos. None of the drugs administered to the chick embryo caused any disturbance of muscle pattern formation or maintenance whether treatment occurred before (5 days) or after (7.5 days) the muscle splitting period. Heterogenetic implantations were performed at 2 days of incubation either at the leg or at the wing level. Somitic mesoderm from non-mutant quail embryo was grafted to replace a piece of somitic mesoderm in putative mutant (cn/cn) chick embryos. The introduction of normal myogenic cells into a mutant leg or wing led to a normally patterned musculature, which demonstrates that the muscular dysgenesis cn/cn results from a defect of the somitic myogenic cell line.