Intracortical Dentate Fascia Grafts: Mossy Fiber Synapses in the Host Neocortex

Embryonic dentate fascia was grafted into a cavity in the area of the adult rat neocortex which represents the vibrissae (barrel field). We wished to test the possibility of development of connections between the two brain areas which do not have synaptic or tissue contacts in situ The unique characteristics of the giant synaptic boutons of the dentate mossy fibers were used for detection of the dentate synaptic contacts with neocortical neurons at the electron microscopic level. Ultrastructural analysis nine months postgrafting has shown that the bundles of mossy axons eter the host neocortex and develop multiple terminal and en passant contacts with typical characteristics. Neuronal perikarya, large dendritic trunks and fine caliber terminal dendritic branches were used by the mossy fibers as postsynaptic targets, as well as spines of various complexity and configurations. The subsynaptic dendrites seemed to be modified by synapsing giant boutons. Accumulation of cytoplasmic organelles was observed at these sites. Various bumps and protuberances were formed by the subsynaptic dendrite surface. The contents of these appendages were variable; some of them contained organeiles typical of dendroplasm, while others were more spine-like, often with inclusion of ribosomes. It is concluded that mossy fibers growing into the host neocortex can develop typical contacts with inappropriate targets with all the ultrastructural features of functional synapses.