title: Development of brainstem and cerebellar projections to the diencephalon with notes on thalamocortical projections: studies in the North American opossum.
abstract: The North American opossum is born in a very immature state, 12 days after conception, and climbs into an external pouch where it remains attached to a nipple for an extended period of time. We have taken advantage of the opossum's embryology to study the development of brainstem and cerebellar projections to the diencephalon as well as the timing of diencephalic projections to somatosensory motor areas of neocortex. The techniques employed included immunocytochemistry for serotonin, the retrograde and orthograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, and the selective impregnation of degenerating axons. Our results suggest that serotoninergic axons, presumably from the dorsal raphe and superior central nuclei, are present in the diencephalon at birth. Axons from the bulbar reticular formation, the vestibular complex, the trigeminal sensory nuclei, and the dorsal column nuclei reach at least mesencephalic (and probably diencephalic) levels by postnatal day (PND) 3, whereas those from the cerebellar nuclei may not grow into comparable levels until PND 5. The dorsal column and cerebellar nuclei innervate the ventral nuclei of the thalamus by estimated postnatal day (EPND) 17 and all of the diencephalic nuclei supplied in the adult animal by EPND 26. Diencephalic axons enter ventrolateral (face) areas of presumptive somatosensory motor cortex by PND 12, but do not reach dorsomedial (limb) regions until EPND 21. At both ages, diencephalic axons are limited to the cortical subplate and marginal zone; they do not innervate an identifiable internal granular layer until considerably later. Our results suggest that axons from the brainstem and cerebellum grow into the diencephalon early in development, but that they do not influence the cerebral cortex until relatively late. When the results of the present study are compared with those reported previously on the development of ascending spinal (Martin et al., '83) and corticofugal (Martin et al., '80; Cabana and Martin, '85b,c) projections, it appears that specific components of major somatosensory and motor circuits develop according to different timetables.
mesh_terms: D000818:Animals; D001369:Axons / Q000276:immunology / Q000502:physiology; D001933:Brain Stem / Q000254:growth & development* / Q000502:physiology; D002531:Cerebellum / Q000254:growth & development* / Q000502:physiology; D002540:Cerebral Cortex / Q000502:physiology*; D004027:Diencephalon / Q000254:growth & development* / Q000502:physiology; D006735:Horseradish Peroxidase; D009474:Neurons / Q000276:immunology / Q000502:physiology; D009893:Opossums / Q000254:growth & development* / Q000502:physiology; D012701:Serotonin / Q000276:immunology; D009435:Synaptic Transmission; D013788:Thalamus / Q000502:physiology*; D014909:Wheat Germ Agglutinins
chemical_list: D014909:Wheat Germ Agglutinins; D012701:Serotonin; D006735:Horseradish Peroxidase