Cross‐species septohippocampal transplants: Ultrastructure of Thy‐1.2‐labeled donor fibers into the dentate gyrus

A naturally occurring species‐specific membrane marker was used to identify unambiguously transplanted septal cells and their fibers which have grown into host tissue. Cell suspensions of the septum/basal forebrain region of C57B1/6 mouse embryos were transplanted into the dentate gyrus of Sprague‐Dawley rats that had received a fornix lesion. The membranes of the mouse contained Thy‐1.2, while the membranes of the rat contained Thy‐1.1. An antibody to Thy‐1.2 clearly identified the donor tissue and did not react with the Thy‐1.1 of the host's membranes. The ultrastructure of the immunoreactively labeled tissue confirmed previous biochemical findings on the distribution of Thy‐1 and showed Thy‐1.2 immunoreactivity on axons and dendrites, microtubules, some mitochondrial membranes, and the surface membranes of cell bodies. Within the transplant, a few glial profiles showed immunoreactive fibrils, but most glial profiles within the transplant and all glial profiles outside the transplant were not immunoreactive. Astrocyte fibers enclosed the outgrowing labeled fibers to form fascicles, but did not penetrate the fascicle. There was no other distinctive association of astrocytic profiles with immunoreactive fibers. Dendrites grew for long distances into the host's molecular layer. Many immunoreactive dendritic profiles formed synapses with unlabeled terminal profiles from the host. The host synapses on the long dendrites of the transplanted neurons may form an important source of input for the initiation of physiological activity in the new circuits established by the transplant. A few labeled (donor) synaptic terminals were observed in the molecular layer, but Thy‐1.2‐labeled dendritic profiles were much more prominent than labeled axonal profiles.