Development of fetal retina, tectum, and cortex transplanted to the superior colliculus of adult rats

Earlier studies showed that embryonic retina, cortex, or tectum transplanted adjacent to the superior colliculus of newborn host rats differentiated many of the histological features appropriate for the donor region and developed interconnections with the host nervous system. In the study presented here, the same regions were transplanted to the brain of adult host rats and the development of these transplants was compared to those into newborn hosts. Retina, rostral tectum, or occipital cortex was dissected from donor rat embryos on gestational day 14 or 15. A portion of cortex was aspirated in 2‐;menth‐old host rats to expose the right superior colliculus, and one of the donor tissues was placed adjacent to the colliculus in each host. Two to 4 months after transplantation, transplant histology and neuronal interconnections between the transplant and host nervous system were studied by using Nissl and neurofibrillar stains and ³H‐proline and HRP tract tracing techniques. Four main points can be drawn from these results. First, 80% of the transplants survived in adult hosts –a percentage comparable to that found in newborn hosts. Second, each of the types of tissues transplanted differentiated histological characteristics appropriate for its site of origin, although the degree of differentiation was always much less than in transplants to newborns. Third, the transplants developed only relatively local projections into the host cortex and superior colliculus. This contrasts with the extensive projections found from the transplants into the brain of newborn hosts. Fourth, no definitive projections from the host retina or brain were identified to any of the transplants into adults, whereas both cortical and tectal transplants into newborns received projections from the host.