Development of trochlear motor neurons, superior oblique muscle, and neuromuscular junctions following prevention of cell death by myasthenia gravis immunoglobulin

Paralysis of embryo during the period of naturally occurring motor neuron death produces an increase in the number of surviving neurons while the progressive differentiation and maturation of target muscle is severely retarded. Application of immunoglobulin G from patients with acquired myasthenia gravis to duck embryos during the period of trochlear motor neuron death also prevents this neuron loss but without paralyzing the embryo. The present study was conducted to investigate whether or not the differentiation and maturation of motor neurons, target muscle, and neuromuscular junctions were retarded following prevention of cell loss with myasthenic immunoglobulin. Immunoglobulin concentrates from myasthenics and normal human volunteers were applied daily to the chorioallantoic membrane of duck embryos from day 10 onward. The development of the trochlear neurons and the superior oblique muscle was examined with light and electron microscopy on embryonic days 12, 16 and 20. The motor neurons at the light and electron microscopic level were cytologically indistinguishable between the myasthenic and normal immunoglobulin-treated embryos. Myoblasts fused to form myotubes. which further differentiated into mature myofibers at the same time in both groups. Numerous neuromuscular junctions of normal ultrastructure and nerve fibers with myelin wrappings were observed in both cases. It is concluded that the increased neuron survival following myasthenic immunoglobulin treatment does not accompany retardation in differentiation and maturation of the target muscle which is contrary to the results obtained from studies utilizing neuromuscular blocking agents producing increased cell survival.