Double bouquet cell axons in the human temporal neocortex: relationship to bundles of myelinated axons and colocalization of calretinin and calbindin D-28k immunoreactivities

We have examined the distribution of double bouquet cell axons, immunocytochemically stained for the calcium-binding proteins calretinin and calbindin D-28k in the human temporal neocortex, in relation to bundles of myelinated axons (originating from pyramidal cells) and the colocalization of these calcium-binding proteins. The large number and regularity of distribution of double bouquet cell axons was clearly visualized in tangential Sections from cortical layers III–V. In these sections, we estimated that the mean number±standard deviation of double bouquet cell axons per 10 000 μm 2 was 11.65±0.44 with a mean diameter of 12.10±0.63 μm and a mean center-to-center spacing of 29.8±0.91 μm. These values are very similar to those previously reported in the monkey neocortex. The distribution of double bouquet cell axons was closely related to bundles of myelinated axons; there was overlapping with basically a one-to-one correspondence. Finally, double-label immunofluorescence experiments revealed that the vast majority of double bouquet cell axons immunoreactive for calbindin were also stained for calretinin. Since relatively few cell somata were double-labeled in the human temporal cortex, we concluded that double bouquet cells may represent a significant subpopulation of neurons that colocalize these calcium-binding proteins.