[Subcortical projections of the visual cortex in the adult albino rat and during postnatal development]

We have investigated the subcortical projections of the rat striate cortex by using the silver-degeneration method and the HRP-technique too. Cortical lesions were made in 60 young animals (1, 4, 5, 6, 7, 10 and 14 days old) and in 6 adult rats. The terminal regions of projection occurred only ipsilateral to the lesions. After passing the internal capsule the degenerating pathway divides into 2 bundles. In the dorsal thalamus one of them runs in caudal direction. The other bundle turns ventrally, reaches the cerebral peduncle and terminates in the pons. The first fibre bundle terminates in the following structures: Nc. reticularis thalami, Nc. lateralis thalami, Nc. lateralis posterior thalami, Corpus geniculatum laterale, pars dorsalis (Cgld), Corpus geniculatum laterale, pars ventralis (Cglv), Nc. praetectalis anterior et posterior and Colliculus superior. The fibers of the second bundle innervate the Nc. lateralis pontis. Fibers from this bundle terminate probably in the Cglv and in the Zona incerta too. By using the HRP-technique it could be demonstrated that the axons terminating in the Cgld originate in layer VI of the area 17. In contrast, the projection to Cglv, Nc. lateralis posterior, Colliculus superior and Nc. lateralis pontis originates from pyramidal cells in layer V. The development of the projection in young animals indicates: Like in adults rats, terminal degeneration is present in all subcortical projection regions at postnatal day (PD) 10. At PD 4-7 we can observe heavily degenerating axons but the terminal degeneration is different. It is remarkable in the "visual" part of the reticular nucleus and iln the Cgld (decreasing from inside to outside). Only a weak terminal degeneration is visible in the pretectal region and in the superior colliculus. At PD 1 the trajectory of degenerating fibres is clearly visible. Signs of terminal degeneration can only be found in the reticular nucleus. It is discussed whether the date of generation of the cortical neurons and the time of the differentiation of the cortical layers is of importance for the time of innervation of the subcortical projection fields. The question when the axons arrive at their terminal region and form there synaptic contacts has not yet been exactly answered. To solve this problem electronmicroscopic investigations are necessary.