Abstract
The morphological maturation of nerve-muscle junctions in hindlimb muscles of the mouse was studied during the first month after birth. At birth the adult population of muscle fibers and motor axons is nearly complete and the latter are about to become myelinated in their extramuscular region. Each neonatal muscle fiber is innervated within a well-defined junctional zone by numerous fine processes of several motor axons. This zone is occupied roughly uniformly by a high density of acetylcholine receptors (AChR, labeled with a fluorescent conjugate of α-bungarotoxin). Polyneuronal innervation of individual muscle fibers is lost during the second week after birth. Following the elimination of redundant synaptic contacts the surviving preterminal axons grow and become myelinated and their terminals expand within the junctional zone which begins to grow more rapidly. The distribution of AChR changes markedly during the third week after birth. Spots of low AChR density appear which enlarge to form the nonsynaptic regions of the mature junctional zone as the receptors become closely associated with the maturing branches of the motor axon terminals. The subsequent increase in area of the junctional zone closely parallels that of the muscle fiber as a whole.
Published Version
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