Connections made by transplants to the cerebral cortex of rat brains damaged in utero.

Homografts consisting of pieces of neocortex or dissociated cortical cells were transplanted from fetal rats into the cortex of newborn, microcephalic hosts. The cortex of the hosts lacked cells of the superficial layers, as a result of prenatal administration of a cytotoxic drug methylazoxymethanol. Grafts exhibited an internal organization with a tendency to form a molecular layer, and alternating cell and fiber zones, although these were not consistently oriented with respect to the host cortex. Both pyramidal and nonpyramidal cells survived. Some grafts were shown to receive callosal connections. Axonal outgrowth from transplanted neurons to several host brain areas was demonstrated with retrograde tracers. Outgrowth occurred not only to the contralateral cortex, in which the host's own callosal projection was deficient, but also to the thalamus and spinal cord, in which the host projection was intact. Thus, grafted fetal cortex is capable of making connections in the methylazoxymethanol-damaged host, but the pattern of connections made is not influenced by host deficiencies.