Transplantation of tectal tissue in rats. II. Distribution of host neurons which project to transplants

Tectal tissue was dissected from fetal rats and transplanted adjacent to the superior colliculus of newborn rats. The recipient animals were then allowed to survive for 6 or more weeks. Subsequent examination revealed that the transplants generally lay over the host inferior colliculus and rostral part of the cerebellum and had substantial fiber connections with the host superior colliculus. To determine which host areas projected to the transplants, horseradish peroxidase (HRP) was injected into the transplants, and the host brain was examined for the presence of retrogradely filled neurons. Labeled cells were found in nearly 50 host areas. Most of these areas are known to project to normal superior colliculus. There was a consistency between one animal and another in the frequency and density of cell label in the various areas. The projection from host cortex (particularly from visual cortical areas) was the densest and most consistent projection. Other areas which commonly projected into the transplants included pretectum, parabigeminal nucleus, superior colliculus, and the brachial region of the inferior colliculus. Sparse and infrequent projections were found from ventral lateral geniculate nucleus, substantia nigra, zona incerta, and catecholaminergic nuclei. No unequivocally labeled retinal ganglion cells were found. The results indicate that the host projection into the transplants is limited to those areas with axons in the vicinity of the host/transplant interconnection. However, the data also suggest that (1) the relative maturity of particular host pathways at the time of transplantation and (2) some form of preferential or absolute affinity expressed between host axons and transplant cells are also factors which influence the pattern of connections formed between host and transplant.