Transient projections from the lateral geniculate to the posteromedial lateral suprasylvian visual cortex in kittens.

The postnatal maturation of the projection from the lateral geniculate nucleus to the posteromedial lateral suprasylvian visual cortex (PMLS) was studied with injections of fluorescent dyes into the PMLS at various postnatal ages. Labeled neurons projecting to the PMLS were present in all laminae of the ipsilateral lateral geniculate on the day of birth. However, there was a conspicuous change in the distribution of labeled geniculo-PMLS neurons by 11 days of age: now very few labeled neurons were present in lamina A, indicating a loss of geniculo-PMLS connections. The loss of connections began at the peripheral margins of lamina A and proceeded through other laminae toward laminae C1-3. By adulthood, labeled geniculo-PMLS neurons were largely confined to laminae C1-3; they were never observed in lamina A or A1 and were rarely observed in lamina C. To determine whether the lateral geniculate neurons survived after their projections to PMLS were lost, injections of fast blue were made at 1 or 2 days postnatally and the animals were allowed long postinjection survival times. Labeled neurons were found in all lateral geniculate laminae, thereby indicating that for many neurons the loss of connections could be attributed to a loss of their axon collaterals rather than to the death of the neurons themselves. After injections of fast blue into the PMLS and diamidino yellow dihydrochloride into area 17 shortly after birth, many double-labeled neurons were present in all laminae, indicating that they have collaterals to both targets. Thus, the survival of many of the geniculo-PMLS neurons contributing to the transient geniculo-PMLS projection seems to be due to sustaining collateral projections to area 17 or other cortical targets.