[Differentiation of neurons and synapses of mossy fibers in transplants of fascia dentata developing in the rat neocortex].

Embryonic fascia dentata tissue isolated from the hippocampus was transplanted heterotopically into the neocortex of adult rats. Ultrastructural characteristics of neurons and synapses in transplants were studied nine months later. It has been found that the main types of neurons present in fascia dentata undergo differentiation in the transplants, and a dense neuropile containing various types of synapses is produced. A characteristic feature of the transplanted neurons is the presence of additional microspines on somatic and dendrite surfaces; this appears to be due to a deficiency of external and internal afferents. Gigantic synaptic terminals of granule cell axons (mossy fibers) in transplants possess unique morphological characteristics, which allow their identification in a complex neuropile. Just as in situ, they form two types of contacts: chemical asymmetric contacts with dendrite spines and desmosome-like ones with dendrite surface characteristics. However, accumulations of large vesicles with electron-dense centers can often be observed near the active zones of the synapses, and desmosome-like connections are more prominent. The most important feature is that gigantic synapses in transplants use midsize and small dendrites as postsynaptic targets up to terminal branches, and they contact with spines of the usual shape and size, whereas in situ terminal synaptic contacts of mossy fibers are formed only with gigantic processes of initial segments of the large apical dendrites. Thus, in the absence of normal synaptic targets, mossy fibers can produce contacts having all features of functional synapses, but with atypical postsynaptic structures.