MORPHOLOGICAL OBSERVATION ON THE DEVELOPMENT OF NEUROMUSCULAR JUNCTIONS IN FETAL MOUSE SPINAL CORD AND MUSCLE CO-CULTURE / 解剖学报

ABSTRACT

Neuromuscular junctions were formed in vitro between spatially separated explants of the fetal mouse spinal cord and the skeletal muscle. The interdependence between nerve and muscle and the changes of pre- and postsynaptic structures were observed during the formation of junctions in living and stained specimens. On the 7th day in vitro, the myotube was innervated by the neurite to form a small bulblike ending, and the cholinesterase activity was not observed at the contact region between nerve and myotube. On the 14th day in vitro, the nerve fiber sent out its branches, and the myofibers were innervated by these branches. Except some bulblike endings, some simple arborized nerve endings were also observed on the myofibers. At this stage, the cholinesterase activity began to appear at the contact region between the ending and the myofiber, and the multiple innervation of myofiber also occurred. Sometimes there were more than one nerve-muscle contact on a single myofiber, and sometimes a myofiber was innervated by several axons to form a "mixed-contact" at same place. The cholinesterase activity can only be examined in one of the multiple contacts but it is absent in the "mixed-contact". On the 21st day, both the number and the cholinesterase activity of the endplate were increased markedly. After 30 days in culture, there were intense cholinesterase activities on some endplates which were similar to young endplates in vivo, these endplates were innervated separately by single axons. When the spinal cord and muscle were co-cultured, the differentiation of muscle fiber depended on the innervation of motor nerve, and not on sensory nerve. On the other hand, the growth and maintenance of the neuron is closely related to the presence of the muscle fiber.