Transplantation of fetal lateral geniculate nucleus to the occipital cortex: connectivity with host's area 17.

The developing lateral geniculate complex was excised from fetal albino rats at 18 days of gestation and implanted into the occipital cortex of host animals at 5 days of postnatal age. Groups of host animals were sacrificed at 10, 20 and 30 days following this procedure. The transplant tissue of selected animals was stereotaxically lesioned 2 days prior to scheduled sacrifice and their brains subjected to either Fink-Heimer or electron microscopic analysis of the distribution and density of degenerating efferents from the transplant. The remaining animals were analysed by means of Bodian, Golgi-Cox or electron microscopic techniques. Transplanted neurons displayed typical dendritic branching patterns of geniculate relay neurons by 20 days following implantation. Intrinsic neurons, characterized by a small ovoid soma and two main stem dendrites, only became evident in transplant tissue by 30 days and were much reduced in number. Synapses developed by 10 days and rapidly increased in number by 20 and 30 days. Most complexes were simple axo-dendritic, asymmetric junctions. Multiple serial and reciprocal complexes, as well as the characteristic glomerular complex, failed to appear. Analysis of Bodian stained material revealed a dense network of fibers coursing about the transplant. Distinct bundles of these fibers were observed extending from the medial edge of the transplant into area 17 by 20 days following implantation. A Fink-Heimer analysis of animals whose transplants were stereotaxically lesioned revealed degeneration in Layers II-VI of the primary visual cortex but the majority of these fibers terminated within the lateral two-thirds of Layer IV. Few degenerated fibers could be found in the underlying white matter indicating that efferents from the transplant found their way to their "correct" target zone by growing through a complex neuropil which provided minimal physical substrates to guide such growth. Most of the contacts formed by these fibers were simple junctions along the shafts of dendrites with a wide range in diameter. It is concluded that the nearby host visual neurons, which are the correct target cells for the afferents arising in the transplant, induced a directed growth of these fibers.