Conditions for survival of raphe nuclei transplants in the IVth ventricle of the adult rat

By studying a neuronal population challenged with a novel context in which to develop i t may be possible to determine which features are specif ical ly programmed into the developmental scheme for that population versus those features which are imposed by the environment. Such an approach was used to study the development of retina. Retinae, when st i l l a simple pseudostrat i f ied neuroepithelium, were excised from rat fetuses and placed in the brain of newborn rats. One month af ter transplantation these retinae had many of the histological characteristics of normal retina. Electron microscopy, Golgi staining and transmit ter immunohistochemistry revealed a relat ively normal morphology for each neuronal type and for the various sub-types studied. The most notable deviation from normal observed in these transplants is Mi'dler cells expressing a GFAP-Iike immunoreact iv i ty. This is not seen in normal retina but is typical of astrocytes in the brain. The connections made by the retinal transplants with the host brain are also appropriate for retina. Projections have been identi f ied to many of the visual nuclei and no projections have been found to nuclei which are not normally ret inorecipient. Neither the lateral i ty of the projection nor the pattern of terminat ion within the nuclei seem as r igidly respected wi th the transplants as with normal ret inal projections. Fetal ret ina transplanted in the same manner to the brain of adult rats fai led to d i f ferent iate as completely as in newborn hosts, and only meager connections were found from these transplants into the host brain.