The pattern of avian intramuscular nerve branching is determined by the innervating motoneuron and its level of polysialic acid

Most skeletal muscles are composed of a heterogeneous population of fast an d slow muscle fibers that are selectively innervated during development by fast and slow motoneurons, respectively. It is well recognized that, in bot h birds and mammals, fast and slow motoneurons have substantially different intramuscular branching patterns, a difference critical for proper motor f unction. However, the cellular mechanisms regulating these differences in m otoneuron branching are unknown. In a previous study, we showed that the fa st and slow pattern of intramuscular branching, in a chick muscle containin g distinct fast and slow muscle regions, was remarkably similar to normal w hen formed by foreign motoneurons. Whether this was attributable to some pr operty of the innervating "fast" or "slow" motoneurons or to some property of the developing fast-slow muscle fibers was not determined. To distinguis h between these two possibilities, we performed chick-quail hindlimb chimer as to force slow chick plantaris motoneurons to innervate a fast quail plan taris muscle. The pattern of intramuscular nerve branching in the fast plan taris of these chimeras closely resembled the slow branching pattern normal ly observed in chick slow plantaris muscles. Enzymatic removal of polysiali c acid (PSA) from nerve and muscle during normal quail plantaris developmen t dramatically changed the normal fast pattern to more closely resemble a s low pattern. In contrast, removal of PSA from chick plantaris motoneurons a nd muscle fibers had little effect on the pattern of nerve branching. Toget her, these results indicate that the pattern of intramuscular nerve branchi ng is determined by the level of PSA on the innervating motoneurons.