Development of the facial and hypoglossal motor nuclei in the neonatal Brazilian opossum brain

The development of the facial and hypoglossal motor nuclei were examined in the neonatal Brazilian opossum (Monodelphis domestica), a marsupial in whi ch postnatal central nervous system development has been well characterized . In this study, we utilized postnatal injection of the retrograde tracer c holera toxin subunit B (CtB) to characterize the formation of the facial an d hypoglossal motor nuclei in the developing neonatal opossum brainstem. In jections of CtB were made into the cheek/lip region or tongue of opossum pu ps to retrogradely label the facial or hypoglossal motor nuclei, respective ly. Following a 2 h survival time, facial motoneurons in newborn opossum pu ps (1 PN) exhibited CtB labeling, with their cell bodies localized near the developing cranial abducens nucleus. At 3 and 5 PN, following a 48 h survi val time, CtB-labeled facial motoneurons were observed in and migrating to the region of the adult facial motor nucleus in the rostral medulla. Betwee n 7 and 10 PN, almost all facial motoneurons had migrated to their destinat ion within the facial motor nucleus. Hypoglossal motoneurons also exhibited CtB labeling from 1 PN; however, their cell bodies were localized within t he hypoglossal motor nucleus at the earliest age examined. Double label stu dies, to examine guidance of facial motoneurons during migration, demonstra ted that CtB-labeled facial motoneurons are in close proximity to vimentin- like immunostained radial glial fibers during migration. These results sugg est: (I) migration of facial motoneurons to the facial motor nucleus is a p ostnatal event, (2) efferent projections from facial and hypoglossal motone urons project into the peripheral region of their target muscles from the d ay of birth, and (3) facial motoneurons migrate to their destination in the brainstem thereafter, in close association with radial glial fibers. (C) 1 999 Elsevier Science B.V. All rights reserved.