During development of the thalamocortical projection, afferent fibers from the thalamus reach the cortex at a time when their target cells have just been generated but have not yet migrated to their final position. Thalamic axons begin to invade the cortex only shortly before their target layer 4 is formed. The mechanisms responsible for the innervation and termination of thalamic fibers in the cortex are not known. Here we show that the growth of thalamic axons in vitro is influenced by the age of cortical explants. Cortical explants of early embryonic stages were not invaded by thalamic explants, whereas thalamic fibers entered explants from postnatal cortices and terminated properly in their target layer 4 in vitro. Outgrowth assays on cortical cell membranes prepared at different developmental stages revealed that the growth of thalamic axons is selectively influenced by growth- promoting molecules that are up-regulated during development. Moreover, experiments with postnatal cortical membranes isolated from distinct layers revealed that the growth of thalamic axons is selectively reduced on membranes prepared from layer 4. These results provide evidence that membrane-bound molecules in the cortex are involved in both the regulation of thalamic innervation into the cortical layers and their termination in the correct target layer.
The spatiotemporal distribution of neural cell adhesion molecule (N-CAM) in the retinotectal system of adult goldfish was assessed by immunofluorescence using the monoclonal antibody (Mab) D3 against chick N-CAM. In immunoblots with extracts of cell surface membranes of fish brains, Mab D3 recognized a prominent band at 170K and a weak band at 130K (K = 10(3) Mr). N-CAM immunofluorescence on cells was restricted to the marginal growth zones of the retina and the tectum and, in normal fish, to the youngest axons from the new ganglion cells of the peripheral retinal margin. In fish with previously transected optic nerves (ONS), Mab D3 staining was found transiently on all axons from the site of the cut into the retinorecipient layers of the tectum, but disappeared from these axons 450 days after ONS. Growing retinal axons in vitro exhibited N-CAM immunofluorescence throughout their entire extent, including their growth cones. Glial cells cultured from regenerating optic nerves were, however, unlabeled. These data are consistent with the idea that N-CAM is involved in adhesive interactions of growing axons. The temporally regulated expression of N-CAM on the new retinal axons may contribute to the creation of the age-related organization of the axons in the retinotectal pathway of fish.