Development and innervation of soleplates in the freely grafted extensor digitorum longus (EDL) muscle in the rat

The ultrastructural events in the establishment of the neuromuscular junction of the freely grafted extensor digitorum longus (EDL) muscle of the rat were studied 1-120 days after grafting. The original axons and muscle fibers, including soleplates, degenerated during the first few days, but Schwann cells and basal laminae persisted. Myofibers regenerated within the original basal laminae. Indentations of the sarcolemma, termed "presumptive synaptic clefts" (PSC), were found on myotubes from 7-day grafts. Schwann cells and residual acetylcholinesterase were invariably associated with the PSC, suggesting that the PSC developed at the site of the original soleplate. Nerves entered the grafts 10 days postoperatively and contacted the PSC of the regenerating muscle fibers on the 18-20th day. The secondary synaptic clefts of these "reconstructed" soleplates extended far beyond the subaxonal region. A second type of soleplate appeared on the 18-20th day. These soleplates were similar to those found in embryonic muscle and were considered to have been induced to form "de novo" by the presence of the nerves. When grafts were placed in permanently denervated limbs the "reconstructed" soleplates appeared, but the "de novo" type did not. These results show that information directing the morphogenesis and innervation of the soleplate persists after the original muscle fibers and axons of a graft degenerate and regenerate.