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.

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The growth associated protein B-50 (GAP-43) is demonstrated in human intramuscular nerves and neuromuscular junctions by light microscopy and electron microscopy. Nearly all fetal endplates are shown to be immunoreactive for B-50. The percentage of B-50 positive endplates decreases significantly during the peri- and neonatal period, but in children and adults a low percentage of B-50 positive endplates remains present. These data indicate that also in the human peripheral nervous system the expression of B-50 is developmentally regulated.

A young boy is described who, since early infancy, suffered from weakness of predominantly proximal limb muscles. Electromyography revealed impairment of neuromuscular transmission. There were no antibodies against acetylcholine receptors. The amplitude of miniature end-plate potentials was reduced. Neuromuscular junctions showed somewhat coarse postsynaptic junctional folds and degeneration products in the synaptic clefts and in postsynaptic areas. Using qualitative and semiquantitative histochemical methods, at light- and electronmicroscopical levels, acetylcholinesterase (AChE) activity and acetylcholine receptors (AChRs) were demonstrated to be deficient. This patient demonstrates that the clinical picture of congenital myasthenia may closely resemble a congenital myopathy. The findings provide evidence that AChR deficiency offers protection from some of the effects of AChE deficiency. The clinical features of AChE deficiency depend, not only on the level of residual enzyme activity, but also on the degree of AChR reduction, if present.

Underlying the Lambert-Eaton myasthenic syndrome (LEMS) is a decrease in the release of the neurotransmitter acetylcholine. Only few reports on light and transmission electron microscopical observations of motor endplates in LEMS are available and changes reported so far differ from those found in experimental blocking of acetylcholine release. We performed a quantitative study on intercostal muscle biopsies of five patients. The main light microscopical finding was an enlargement of the area of contact between nerve and muscle, which was interpreted as a compensatory phenomenon. At the ultrastructural level we found the mean postsynaptic area and membrane length of the neuromuscular junctions to be decreased, putatively due to small postsynaptic regions of newly created neuromuscular junctions. The secondary changes in LEMS endplates as seen in this study are similar to those reported in experimental blocking of acetylcholine release.