Growth of axons into developing muscles of the chick forelimb is preceded by cells that stain with Schwann cell antibodies.

A study has been made of the development of limb and muscle nerves in relation to the first appearance of Schwann cells in the flexor digitorum profundus (fdp) and flexor carpi ulnaris (fcu) muscles of the avian forelimb. Schwann cells were identified by immunofluorescent techniques with antibodies to the glycoprotein HNK-1. Myotubes and nerves were identified by using antibodies to myosin and to neurofilament, respectively. At stage 24/25 the brachialis longus inferior (Bli n) and superior (Bls n) nerve trunks within proximal regions of the forelimb were surrounded by Schwann cells. These cells extended in a column for a distance of approximately 100 microns beyond the growing ends of nerves. At stage 26 both interosseus nerve (in n) and the medial-ulnar nerve (m-u n) had formed from the Bli n; each of these branches was surrounded by Schwann cells, which again extended approximately 100 microns beyond the growing ends of the nerves. By stage 26/27 the fdp and fcu muscles were clearly delineated by groups of myotubes. No nerves were detected within these groups; however, Schwann cells were observed between the myotubes. At stage 27 axons had left the in n and m-u n and grown into the fdp and fcu muscles, respectively. These axons were surrounded by Schwann cells. The present observations show that Schwann cells are located ahead of the main limb and muscle nerves as they grow into the fdp and fcu muscles of the limb. It is possible that these Schwann cells play a role in guiding nerves to their correct muscles in the developing chick forelimb.