Comparison of perinatal and adult multi-innervation in human laryngeal muscle fibers

The innervation of human laryngeal myofibers appears distinct from that of skeletal myofibers, because some of them exhibit multiple neuromuscular jun ctions. We attempted to understand the significance of the multi-innervatio n phenomenon by comparing intrinsic laryngeal muscles obtained from autopsi es of a fetus, a stillbirth, and a 7-month-old infant to muscles from adult s. In longitudinal sections (40 to 60 mu m thick) the cholinesterase sites and the nerve terminals were stained simultaneously for light microscopy. M ulti-innervation reached no more than 23% at 7 months, and this percentage decreased notably with age. Until 7 months, end plates were innervated eith er by single or by multiple axons (unineuronal and polyneuronal innervation ), whereas in adults, the pattern was exclusively unineuronal. Moreover, th e structure of neuromuscular junctions in the young was limited to 1 spot o f acetylcholinesterase, while in adults the end plate zones increased in si ze and complex axon terminals were observed. These observations are consist ent with a delayed maturation of human laryngeal innervation compared to th at of skeletal muscles.