The early development of major projections from caudal levels of the spinal cord to the brainstem and cerebellum in the gray short-tailed Brazilian opossum, Monodelphis domestica.

The Brazilian short-tailed opossum, Monodelphis domestica, is born 14-15 days after copulation and is available for experimentation at stages of development corresponding to those which occur in utero in placental mammals. In the present study, we took advantage of the opossum's embryology to study the development of projections from caudal levels of the spinal cord to the brainstem and cerebellum using axonal tracing methods. In all cases, a 2-3 day survival time was used for axonal transport. When injections of Fast blue (FB) were made into caudal levels of the thoracic cord at postnatal day (PD) 1 or 2, axonal labeling could not be identified at supraspinal levels. When injections were made at PD3, however, labeled axons were found in the fasciculus gracilis at caudal medullary levels, within the ventrolateral medulla and pons, within an incipient inferior cerebellar peduncle, and within the cerebellar anlage. The dorsal root origin of at least some of the axons within the fasciculus gracilis was evidenced by the transganglionic transport of cholera toxin conjugated to horseradish peroxidase from the hindlimbs. After FB injections at PD7, a few labeled axons could be traced from the fasciculus gracilis into the nucleus gracilis and from the ventrolateral pathway to the inferior olive. Generally comparable results were obtained using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). In cases injected with FB at PD9, the pattern of brainstem labeling was adult-like. Although labeled axons were present within the cerebellum of animals injected with FB on PD3, they were limited to the marginal zone. Axonal labeling was present within an identifiable internal granular layer in cases injected with either FB or WGA-HRP at PD16, and it appeared to be limited to specific bands which foreshadowed those seen at later stages of development and in the adult animal. In some cases, labeled axons were present within the molecular layer where they were not seen in the adult animal. Our results provide a timetable for the normal development of projections from caudal levels of the spinal cord to the brainstem and cerebellum in Monodelphis and show that such development occurs postnatally rather than prenatally, as in placental mammals.