The origin of brain stem-spinal projections at different stages of development in the North American opossum.

The origin of brain stem neurons giving rise to axons innervating the thoracic cord has been determined in a developmental series of pouch-young opossums. The first spinal axons identified (17 days after conception, postnatal day 5) arise from the medullary and pontine reticular formation, certain raphe nuclei, the vestibular nuclei and the coeruleus complex. The contribution of the red nucleus before postnatal day 10 is not certain. The brain stem sensory relay nuclei and the hypothalamus do not project spinalwards until much later in development.