Development of innervation of skeletal muscle fibers in man: Relation to acetylcholine receptors

Abstract

The aim of this study was to establish the time scale of developmental changes in innervation of skeletal muscle fibers in man. Specimens of thigh and intercostal muscle from 19 embryos and 18 infants were examined with histological methods which enabled the discrimination between fetal (gamma) and adult (epsilon) types of acetylcholine receptors (AChRs). At 8 weeks of development, AChRs were distributed diffusely in the myotube membranes. Following onset of innervation in approximately the ninth week the length of the AChR positive area diminished and reached its shortest size at the sixteenth developmental week. At the sixteenth and eighteenth week some nerve terminals opposed the muscle membrane outside the AChR positive area. Decrease in the number of nerve terminals, strongly suggesting elimination of polyneuronal innervation, started in the sixteenth week and was completed in the twenty-fifth week. This fetal (gamma) type of AChR could no longer be demonstrated after the thirty-first week. The length of the end-plates as determined by the presence of AChRs increased again in the last week before birth and reached a plateau size by the end of the first year after birth. It is concluded that in man the transition from poly- to mononeuronal innervation takes place between the sixteenth and twenty-fifth weeks of development. The evidence available suggests that the retraction of nerve terminals is preceded by loss of AChRs from the muscle membrane facing the terminals. There is no relationship between retraction of nerve terminals and the switch from fetal to adult type of AChR. The size of the presynaptic apparatus changes little after the first year of life.

Human facial muscles: Dimensions, motor endplate distribution, and presence of muscle fibers with multiple motor endplates

“…The measured data of MEP lengths from 18-27 µm (Namba et al, 1968;Wokke et al, 1990;Hesselmans et al, 1993) are in accordance with the evaluated facial MEP lengths of 25-27 µm.…”

“…Former studies by other authors concerning the innervation of human skeletal muscles (Desmedt, 1958;Christensen, 1959;Schwarzacher, 1959;Aquilonius et al, 1984;Carlson, 1986;Wokke et al, 1990;Hesselmans et al, 1993) have shown that focal innervation of muscle fibers with one MEP is the predominant type. We conclude that multifocally innervated fibers would have been found by these authors if present.…”

Antibodies to Acetylcholine Receptor in Parous Women with Myasthenia: Evidence for Immunization by Fetal Antigen

“…Together, these subunits form a cationspecific ion channel that opens when ACh binds to the two sites at the ␣/␦ and either the ␣/␥ or ␣/⑀ interfaces (Unwin, 2000). From about 30 weeks of gestation in humans, the "adult"-specific ⑀ subunit gradually replaces the ␥ subunit (Hesselmans et al, 1993), but the fetal isoform continues to be expressed on rare myoid cells in the adult thymic medulla .…”

Coordination of Lip Muscle Activity by 2-Year-Old Children During Speech and Nonspeech Tasks

“…Speech and nonspeech activities in such a case would be expected to demonstrate similar levels of coupling of upper and lower lip activity, as lip muscle activation patterns would be similar in timing and amplitude. Two-year-old children were chosen for this study, because, at this age: (a) the CNS (e.g., Hesselmans, Jennekens, van den Oord, Veldman, & Vincent, 1993;Simonds & Scheibel, 1989) and oral and facial structures (Moyers & Carlson, 1990) are not yet fully developed, (b) essential investigations of physiologic development of speech motor control are lacking, and (c) most 2-year-olds are able to follow a minimally constrained experimental protocol. Observation of freely behaving children this young presents several challenges.…”