Morphological and functional studies of rat tectal transplants.

Donor tectal tissue from fetal rats was transplanted to the tectal region of intact host newborn rats. Histological and ultrastructural characteristics of mature tectal grafts were examined with special emphasis placed on glial elements and the interface between transplant and host brain. Most transplants survived, proliferated, and differentiated to mature neurons and glial cells. Mature transplants were found to contain small and medium sized neurons which formed clusters along the transplant border. Neurites traversed the interface between transplant and host brain. There was an absence of phagocytosis, inflammatory reactions and chromatolytic neurons throughout the transplant. Classical staining techniques and GFAP immunoreactive stains revealed only mild focal astrogliosis confined mostly to the subpial and perivascular regions of the transplant-host interface. Ultrastructural examination of the transplant revealed numerous axodendritic synapses with aggregates of synaptic vesicles and well formed postsynaptic membranes. The absence of a leptomeningeal membrane at several points along the transplant-host interface may allow neurites from both sides to establish functional synaptic contacts. Single unit recording combined with anterograde WGA-HRP neuronal tract tracing was used to determine the presence and extent of host retinal innervation of tectal transplants.