Rapid growth of host afferents into fetal thalamic transplants

Fetal cell suspension grafts grow and differentiate when implanted into adult rat CNS areas previously neuron-depleted using an excitotoxin. There is some controversy in the literature concerning the timetable of establishment and possible extent of host-graft connections in these experimental conditions. The present study was undertaken to analyze the development of adult host monoaminergic afferents into a transplant formed by fetal thalamic neurons in the previously excitotoxically lesioned thalamus. It is demonstrated that both norepinephrin- and serotonin-immunoreactive fibers are present in the transplant as soon as 8 days after grafting. At those times, immunoreactive fibers exhibit morphological characteristics typically associated with immature stages. After longer survival time, up to 4 weeks after grafting, immunoreactive fibers are numerous in the transplant and exhibit morphological features comparable to those observed in the adult thalamus. These results demonstrate the rapid ingrowth of some fiber systems of the adult host into the transplant and suggest that grafted fetal cells can be functionally integrated into the host circuitry as soon as a few weeks after grafting.