Differentiation of glial cells and motor neurons during the formation of neuromuscular junctions in cocultures of rat spinal cord explant and human muscle

Motor axons extending from embryonic rat spinal cord explants form fully mature neuromuscular junctions with cocultured human muscle. This degree of maturation is not observed in muscle innervated by dissociated motor neurons. Glial cells present in the spinal cord explants seem to be, besides remaining interneurons, the major difference between the two culture systems. In light of this observation and the well documented role of glia in neuronal development, it can be hypothesized that differentiated and long-lived neuromuscular junctions form in vitro only if their formation is accompanied by codifferentiation of neuronal and glial cells and if this codifferentiation follows the spatial and temporal pattern observed in vivo. Investigation of this hypothesis necessitates the characterization of neuronal and glial cell development in spinal cord explant-muscle cocultures. No such study has been reported, although these cocultures have been used in numerous studies of neuromuscular junction formation. The aim of this work was therefore to investigate the temporal relationship between neuromuscular junction formation and the differentiation of neuronal and glial cells during the first 3 weeks of coculture, when formation and development of the neuromuscular junction occurs in vitro. The expression of stage-specific markers of neuronal and glial differentiation in these cocultures was characterized by immunocytochemical and biochemical analyses. Differentiation of astrocytes, Schwann cells, and oligodendrocytes proceeded in concert with the differentiation of motor neurons and neuromuscular junction formation. The temporal coincidence between maturation of the neuromuscular junction and lineage progression of neurons and glial cells was similar to that observed in vivo. These findings support the hypothesis that glial cells are a major contributor to maturity of the neuromuscular junction formed in vitro in spinal cord explant-muscle cocultures. J. Comp. Neurol. 438:239-251, 2001. (C) 2001 Wiley-Liss, Inc.