Myogenic differentiation in early chick wing mesenchyme in the absence of the brachial somites.

A controversy exists in the literature over the ability of wing mesenchyme of somatopleural origin to form skeletal muscle. Experimental approaches used in such studies leave open the possibility of postoperative accessibility of the experimental wings to somitic cell invasion. In the present study wing somatopleural tissue was isolated from HH stage-12 to -21 chick embryos and grown either in organ culture (OC) or on the chorioallantoic membrane (CAM) of host chicks, conditions under which postoperative entry of somitic cells is impossible. In the presence of axial and somitic tissues of the brachial region, the wing territories from all four stages underwent comparable growth and tissue differentiation. However, isolated wing regions showed a stage-dependency in the differentiation of skeletal muscle but not of other limb tissues. The incidence and amount of skeletal muscle was markedly reduced in HH stage-12 and -15 isolated wing regions while myogenesis in HH stage-18 and -21 wing buds was not affected by the absence of the somitic tissues. These results are consistent with reported stages of somitic cell migration into wings with the exception of HH stage-12 explants which should have been muscleless if somitic cells are the sole source of wing myofibres. The possibility that somitic cells had been included in these explants was investigated by testing the myogenic potential of lateral plate tissue adjacent to the wing, altering the dissection procedure for isolating wing territories and using antibodies to skeletal muscle myosin and actin to detect myotubes. The results from this series of experiments illustrate the need for extraordinary care in the isolation of wing regions when investigating limb-somite relationships and suggest that the myogenic capacity attributed to wing somatopleural cells in the past can be accounted for by either postoperative entry of somitic cells into experimental wings or inadvertent inclusion of somitic cells in the primordia when dissected. Overall, the results show that somitic cells are the sole source of wing myofibres for, in their absence, somatopleural cells from all mesodermally-derived wing cell types except skeletal myofibres.