Anatomical and behavioral correlates of a xenograft-mediated pupillary reflex

Retinae from mouse embryos were transplanted to the midbrain of neonatal rats which were unilaterally enucleated at the time of transplantation. At maturity the host's remaining optic nerve was cut and the skull over the transplant was removed. Illumination of the transplant caused pupilloconstriction of the host eye, a response abolished by removal of the transplant or damage to the olivary pretectal nucleus which is the pupilloconstriction center of the brain stem. The degree of constriction correlated with the intensity of transplant illumination. Using mouse-specific monoclonal antibodies, we demonstrated that the grafts projected only to areas of the brain stem normally innervated by the eye. The density of label in the olivary pretectal nucleus did not, however, correlate with the briskness of the graft-mediated pupillary reflex. In summary, this study shows that ectopic neural xenografts are capable of making specific connections which can drive an appropriate response to a natural stimulus in the host animal. While important in showing the ability of transplants to make specific connections, this study also provides a simple assay system for examining conditions which may influence the efficacy of host innervation by the transplant.