The vestibular primary afferents and the vestibulospinal projections in the developing and adult opossum, Monodelphis domestica.

The gray short-tailed opossum Monodelphis domestica is born in a very immature state (eyes and ears closed, budlike hindlimbs, etc.) 14 days post-coitum, but it can locomote with its forelimbs from the mother's genital aperture to a nipple to which it attaches. The forelimb movements allowing this behavior may be the expression of central pattern generators in the spinal cord, but sensory clues must guide it. One such system may be the vestibular system, which senses linear and angular acceleration and has a strong influence on posture and balance at rest and during locomotion in adult animals. Using the neuronal tracer DiI, we have looked at the vestibular primary afferents and the vestibular nuclear projections to the cervical spinal cord in newborn and postnatal opossums, as well as in the adult animal. The projections in the adult opossum conform to those described for other mammals. Fibers of the vestibular portion of the eighth nerve distribute to all four vestibular nuclei and toward the cerebellar primordium on the day of birth. In addition, some of the fibers project to the contralateral vestibular ganglion, a projection that is not found in the adult opossum. Projections from the lateral, medial and inferior vestibular nuclei to the cervical cord are also present in the newborn. Although we cannot exclude the possibility that chemical and/or tactile guidance is used for directing the movements of the newborn opossum, our results support the hypothesis that the vestibular system may be directly involved in the control of these movements. However, not all components of the system are equally developed at birth, and the circuit from the utricle to the lateral vestibular nucleus and from the latter to the cervical cord may be better formed than that from the semicircular canals to the medial and inferior vestibular nuclei to the cervical cord.