Survival of extraocular muscle in long-term organotypic culture: differential influence of appropriate and inappropriate motoneurons.

The myotrophic effects of a precise matching of motoneuron to target muscle may be particularly apparent during the development of some unique skeletal muscle types, such as the extraocular muscles. To understand how motoneuron-specific factors modulate muscle ontogeny, neonatal extraocular muscle explants were cocultured with either the appropriate midbrain motoneurons or the inappropriate spinal cord motoneurons. Thigh muscle cocultured with spinal motoneurons, which differentiates and survives for several months, served as a control. In short-term cultures (during the first 3 weeks in vitro), neonatal extraocular muscle explants developed myotubes that were immunoreactive for myosin, became innervated, and matured in parallel with hindlimb muscle explants. The origin of the motoneurons (midbrain or spinal cord) did not affect extraocular muscle development during the first 3 weeks in vitro. However, in long-term cultures (after the third week in vitro), extraocular muscle that was innervated by the inappropriate spinal motoneurons failed to survive and degenerated. The failure of extraocular muscle to survive when cocultured with the inappropriate motoneurons did not result from an artifact of in vitro conditions. When extraocular muscle primordia were innervated by midbrain explants, many of which contain oculomotor motoneurons, the cultures thrived for > 60 days. We conclude that the trophic requirements of extraocular muscle in organotypic culture are different from those of skeletal muscle isolated from the hindlimb. The novel oculomotor motoneuron-specific interactions that are essential for the survival of extraocular muscle primordia in vitro may prove to be important for the determination of the unusual properties of extraocular muscle in vivo.