COMPOSITION OF NEWLY FORMING MOTOR UNITS IN PRENATAL RAT INTERCOSTAL MUSCLE

We have examined the composition rat intercostal motor units during th e period of late gestation, when most muscle fibres are formed, in ord er to see the pattern of the contacts initially made between single mo toneurons and myotubes. At this early stage, the muscle contains two t ypes of myotubes, primary and secondary myotubes, and a major aim was to see whether individual motoneurons preferentially made contact with a particular myotube type. The technique used to define myotubes cont acted by a single motoneuron was anterograde labelling of the neuron, followed by electron microscopic detection of labelled terminals and t heir postsynaptic targets. We find that prenatal motor units are inhom ogeneous with respect to their primary/secondary myotube composition, Most individual motoneurons show many permutations of contact with pri mary myotubes, secondary myotubes, and undifferentiated cells, includi ng single nerve terminals which contact both primary and secondary myo tubes. Our results are interpreted in terms of changes to the composit ion of both the muscle and of the motor units during the final 5 days of gestation. We demonstrate that motoneurons necessarily make their i nitial contacts on primary myotubes, but that these are surprisingly s parse. As secondary myotubes appear and become innervated, motor units are at first all similar and all heterogeneous, However, primary myot ubes are represented more often in motor units than in the muscle as a whole. This probably reflects the relative densities of polyinnervati on of primary vs. secondary myotubes. By embryonic day 20, motor units have become divergent in composition, with some dominated by primary myotubes and others by secondaries. We propose that motoneurons initia lly establish contacts at random on either myotube type, but then begi n to express preference for one type or the other and reorganise their periphery. Refining of motor unit composition towards homogeneity in the postnatal period probably involves other elements, such as mutabil ity of muscle fibre and/or motoneuron characteristics as a function of usage and muscle position, perhaps influenced by sensory feedback mec hanisms. (C) 1996 Wiley-Liss, Inc.