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Composition of newly forming motor units in prenatal rat intercostal muscle
Author(s) -
Sheard Philip W.,
Duxson Marilyn J.
Publication year - 1996
Publication title -
developmental dynamics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.634
H-Index - 141
eISSN - 1097-0177
pISSN - 1058-8388
DOI - 10.1002/(sici)1097-0177(199602)205:2<196::aid-aja10>3.0.co;2-6
Subject(s) - myogenesis , biology , myocyte , motor neuron , anatomy , neuroscience , microbiology and biotechnology , spinal cord
We have examined the composition of rat intercostal motor units during the period of lategestation, when most muscle fibres are formed, in order to see the pattern of the contacts initiallymade between single motoneurons and myotubes. At this early stage, the muscle contains twotypes of myotubes, primary and secondary myotubes, and a major aim was to see whetherindividual motoneurons preferentially made contact with a particular myotube type. The techniqueused to define myotubes contacted by a single motoneuron was anterograde labelling of theneuron, followed by electron microscopic detection of labelled terminals and their postsynaptictargets. We find that prenatal motor units are inhomogeneous with respect to theirprimary/secondary myotube composition. Most individual motoneurons show many permutationsof contact with primary myotubes, secondary myotubes, and undifferentiated cells, includingsingle nerve terminals which contact both primary and secondary myotubes. Our results areinterpreted in terms of changes to the composition of both the muscle and of the motor unitsduring the final 5 days of gestation. We demonstrate that motoneurons necessarily make theirinitial contacts on primary myotubes, but that these are surprisingly sparse. As secondarymyotubes appear and become innervated, motor units are at first all similar and all heterogeneous.However, primary myotubes are represented more often in motor units than in the muscle as awhole. This probably reflects the relative densities of poly‐innervation of primary vs. secondarymyotubes. By embryonic day 20, motor units have become divergent in composition, with somedominated by primary myotubes and others by secondaries. We propose that motoneurons initiallyestablish contacts at random on either myotube type, but then begin to express preference for onetype or the other and reorganise their periphery. Refining of motor unit composition towardshomogeneity in the postnatal period probably involves other elements, such as mutability ofmuscle fibre and/or motoneuron characteristics as a function of usage and muscle position,perhaps influenced by sensory feedback mechanisms. © 1996 Wiley‐Liss, Inc.