Morphometric analysis of intrinsic axon terminals of Cebus monkey area 17

A morphological study of intrinsic projection in area 17 of Cebus monkey was conducted after iontophoretic injection of biocytin. Thirty axon terminals located in supragranular layers were qualitatively and quantitatively analyzed using 3-d automatic microscopy. Three types of axon terminals could be identified: Ia, Ib and II. Group I was characterized by a sparse and/or long-distance branch pattern, while type II presented compact and localized arborization. Ia axon terminals formed "clusters" and "terminaux" boutons while Ib did not. On average, group II axon terminals tended to present straight or obtuse branching angles and a much more ramified pattern, and occupy a smaller cortical territory with shorter intermediate segments and higher density of synaptic potential sites than group I. The common characteristics of group I included innervation of larger cortical territories, longer intermediate segments, acute branching angles and lower synaptic density compared to group II. The results are compatible with the major subdivisions of neocortical neuronal morphology that classifies them as smooth and spine neurons. Smooth neurons may be related to axon terminals of group II while spine neurons may be related to group I.