Intrinsic projections of Cebus-monkey area 17: cell morphology and axon terminals.

Metric features of the axon terminals and cell morphology of intrinsic projections of area 17 were studied in the Cebus apella. Anterograde and retrograde labeled cell appendages were obtained using saturated pellets and iontophoretic injections of biocytin in the operculum. Details of the histological and histochemical procedures have been described elsewhere (Amorim and Picanço-Diniz, 1996). We distinguished three labeled cell types: pyramidal, star pyramidal and stellate cells and three distinct morphologies of axon terminals were found: Ia, Ib and II, located at supragranular layers. Axon terminals of the group I innervate larger extent of striate cortex through longer intermediate segments, and acute branching angles compared to group II. Group II present on average similar characteristics of the smooth neurons axon terminals. The results taken together with the occurrence of only two types of synapses (I and II) from Gray's ultrastructural studies, seem to give an additional support to extend to the Cebus apella the major subdivision of neocortical neuronal morphology that classified them as smooth and spine neurons.