The early development of subcortical projections to presumptive somatic sensory-motor areas of neocortex in the North American opossum

Summary We have studied the early development of subcortical projections to presumptive somatic sensory-motor areas of neocortex in the North American opossumDidelphis virginiana. The opossum is born in a very immature state, 12–13 days after conception, and climbs into an external pouch where it is available for experimental manipulation. Using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, we have obtained evidence that axons from the dorsal raphe and superior central nuclei, the substantia nigra, the locus coeruleus and the parabrachial nuclei reach presumptive somatic sensory-motor areas of neocortex by at least postnatal day (PND) 10. Axons showing serotonin-like immunoreactivity, presumably from the dorsal raphe and/or superior central nuclei, and axons containing tyrosine hydroxylase immunoreactivity, presumably from the substantia nigra and/or locus coeruleus, are present in the same areas at birth or shortly thereafter. Thalamic axons do not grow into comparable areas of neocortex until after PND 10. Such axons reach the subplate region of ventrolateral neocortex first and then proceed dorsomedially; by estimated PD (EPND) 21, they are present in presumptive layers I, V and VI, but they do not innervate an identified layer IV until EPND 48. The developmental sequences suggested by our study are compared with those reported for other species and are discussed in light of their importance in the formation of major sensory and motor circuits.Abbreviations qq cerebral aqueduct - ca anterior commissure - Cb cerebellum - CcD dorsal cochlear nucleus - Cd caudate nucleus - CeS superior central nucleus - CI interior colliculus - Coe locus coeruleus - CP cortical plate - CS superior colliculus - CxA cortex ammonis - DB nucleus of the diagonal band - Dien diencephalon - Dor dorsal - EP ependyma - Fac facial nucleus - GLD dorsal lateral geniculate nucleus - GM medial geniculate body - HVM ventromedial hypothalamic nucleus - IC internal capsule - Lat lateral - Med medial - ML medial lemniscus - mlf medial longitudinal fasciculus - OSL superior olivary nucleus - PB parabrachial complex - ped cerebral peduncle - PFP parafascicular nuclei - Pu putamen - RaD dorsal raphe nucleus - rfl fasciculus retroflexus - RN red nucleus - SN substantia nigra - SNc substantia nigra, pars compacta - SNr substantia nigra, pars reticulata - STh subthalamic nucleus - Tect tectum - TgV ventral tegmental area - TrMo motor trigeminal nucleus - xVB ventrobasal nucleus of thalamus