White Matter Tracts Visualized by Parvalbumin in Nonhuman Primates

A well-developed white matter (WM) is one of the characteristics of the primate brain. WM compartments (“tracts” or “bundles”) are easily discernible by myelin or neuro filament stains, anterograde tracer injections in nonhuman primates (NHP), and, more recently, diffusion MRI. Relatively overlooked is the fact that several corticofugal and thalamocortical compartments and tracts can be visualized by immunohistochemistry (IHC) for calcium-binding proteins. Since this technique can be easily carried out on post mortem tissues, IHC for calcium-binding proteins is potentially an important bridge for comparisons between NHP and human tissues. This chapter attempts a brief overview of three WM tracts visualized by the calcium-binding protein parvalbumin (PV), as well as a description of the probable origin of the two corticofugal tracts; namely, from PV+ pyramidal cells. Furthermore, the complex, intertwining trajectory of callosal axons is illustrated by single axon reconstruction of five small groups of parietal cortical axons, anterogradely labeled by biotinylated dextran amine.