The development of rubrospinal, cerebellorubral, and corticorubral connections in the North American opossum. Evidence for asynchronism.

We have employed axonal transport and degeneration techniques to study the development of major rubral connections in the North American opossum. Opposums were chosen for study because they are born 12 d after conception and have a protracted postnatal development. Our results suggest that: The red nucleus innervates the spinal cord early in development, well before the somatic motor-sensory cortex (Cabana and Martin, 1984); the red nucleus projects to the spinal cord before it receives substantial projections from the cerebellum or cerebral cortex; and projections from the cerebellum reach the red nucleus significantly earlier than those from the cerebral cortex.