The formation of discrete muscles from the chick wing dorsal and ventral muscle masses in the absence of nerves

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Abstract
It is well known that the different muscles in the vertebrate limb develop by a series of splittings and subdivisions of dorsal and ventral muscle masses. The mechanism for this process is not clear, and the suggestion from previous studies that tension exerted by the growing limb stimulates these splittings is now thought unlikely. It has also been proposed that nerves play an important part in the separations by physically pushing the muscle mass apart. There is also the possibility that nerves could stimulate differential contraction of parts of the muscle mass, leading to a shearing effect and resulting in separation. In this study, peripheral nerves are removed by the administration of the nicotinamide analogue 3-acetylpyridine before the start of muscle mass division. The resulting pattern of muscle is normal although nerves are completely absent. This clearly rules out any major role of nerves in the formation of the muscle pattern, although many authors have shown that innervation is important for the maintenance and later development of the separated muscles. While the mechanism of the process of division of the muscle masses remains unknown, it is suggested that there are changes in cellular behaviour at areas corresponding to the future spaces between the muscles. These changes may be specified by some aspect of positional information within the limb. They might involve muscle cells stopping or reversing their differentiation as muscle in the areas forming the future spaces, or perhaps change the adhesion properties of the muscle cells to cause them to ‘sort out’ and separate. Alternatively, a localized invasion of non-muscle mesenchymal cells at the future spaces could lead to separation of discrete muscles. These possibilities are at present under investigation.