Observations on postnatal neurogenesis in the superior colliculus and the pretectum in the opossum

Postnatal neurogenesis has been detected in the superior colliculus (CS) and caudal pretectum of the opossum in the period ranging from 2 to 13 days ( PND2 to PND13 ) of life in the pouch. Examination of the pattern of labeling in specimens exposed to a pulse of tritiated thymidine ( [3H]T) in PND4 or PND7 and allowed 1.5 h survival reveals that postnatal cell proliferation for the CS is virtually confined to the ventricular zone with no evidence for in situ[3H]T uptake in the collicular plate. Semi-quantitative analysis in long survival animals shows that postnatal neurogenesis peaks later in the CS ( PND7 ) than in the caudal pretectum ( PND4 ) and also persists longer in the former than in the latter. Comparisons of the numerical density of heavily labeled neurons suggest the occurrence of ventro-dorsal and rostro-caudal gradients of neurogenesis in the CS. Separate analysis of superficial, intermediate and deep layers shows, in addition, a combined rostrolateral-to-caudomedial gradient of neurogenesis in the superficial layers. Comparisons of the time schedules of neurogenesis for the superficial layers and of the deployment of optic fibers suggest that migration of neurons to their eventual destination is completed at or after the arrival of afferents.