Tectal tissue grafted to the midbrain of newborn rats: effect of donor age on the survival, growth and connectivity of transplants.

Tectal tissue was removed from rats at embryonic ages (E) E15, E18, E20 and postnatal day 0 (P0) and grafted onto the midbrain of newborn host rats. Six to 24 weeks after transplantation we examined 1) the growth characteristics of the grafts, 2) their morphology and 3) the pattern of retinal innervation of the grafted tissue. Graft survival was markedly affected by donor age. Transplants from E15 and E18 donors showed a survival rate of 90% which decreased to 35% when tissue was taken from E20 animals. Only one graft could be definitively identified in the P0 group. The ultimate volume of the graft was inversely related to donor age; grafts taken from E15 donors grew in size and produced the largest grafts, whereas E20 grafts showed a reduction in tissue volume from the time of implantation. Host retinal input was found in surviving grafts from all fetal donor ages (E15-E20). This input was always restricted to localized areas in the grafts containing high AChE activity; these areas are believed to contain presumptive superior collicular cells from the superficial layers. Thus, in tissue taken from fetal rats, it appears that altering the donor age does not affect the selectivity with which host retinal axons grow into and innervate specific areas within tectal grafts.