Innervation in cultures of fetal rodent skeletal muscle by organotypic explants of spinal cord from different animals

Summary

Functional neuromuscular junctions formin vitro between spatially separated explants of fetal mammalian spinal cord and fetal skeletal muscle, even across species lines (rat and mouse). Differentiation and innervation occur when the muscle explant is oriented toward the ventral edge of the spinal cord cross-section, in the path of ventral-root nerve fibers. Arrival of these neurites enhances muscle development. This trophic influence is particularly apparent when cortisone is included in the nutrient fluid. Cross-striations begin to form toward the end of the first week of coupling, and acetylcholinesterase-positive loci appear by three weeks. In cultures maintained for 5–11 weeks, the more differentiated motor endplate structures show characteristic subneural infoldings, increased soleplate sarcoplasm, and terminal Schwann cells. Myelinated ventral-root fibers can be seen to bridge the gap between the cord and muscle explants, and to arborize and terminate on muscle fibers. Selective stimulation of ventral cord or ventral root can evoke widespread synchronized contractions of large numbers of fibers in the muscle expiant, demonstrating abundant formation of functional neuromuscular junctions between the coupled tissues.