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 microns2 was 11.65 +/- 0.44 with a mean diameter of 12.10 +/- 0.63 microns and a mean center-to-center spacing of 29.8 +/- 0.91 microns. 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.