Immunohistochemical and autoradiographic findings suggest that minoxidil is not localized in specific cells of vibrissa, pelage, or scalp follicles.

Immunohistochemistry with a minoxidil antibody suggested that minoxidil-immunoreactivity is associated with the root sheaths, laterally orientated differentiating matrix cells, and dividing epithelial cells of cultured vibrissa follicles of pigmented and albino neonatal mice. The dermal papilla and connective tissue sheath were devoid of minoxidil-immunoreactivity. To verify that minoxodil-immunoreactivity in the follicles was specific, immunostaining was conducted with dissected whisker pads, formalin-fixed "dead" follicles, and sections of spleen, liver and kidney (non-haired organs) cultured with minoxidil. Microscopic examination revealed minoxidil-immunoreactivity in all of these tissues. Follicles and whisker pads cultured with minoxidil, then washed for one h in media were devoid of minoxidil-immunoreactivity. These data suggest that minoxidil-immunoreactivity in cultured vibrissa follicles is probably non-specific. Sections of skin from C3H and CF1 mice which were topically dosed with minoxidil (in vivo) showed no minoxidil-immunoreactivity. Autoradiography demonstrated that tritiated minoxidil was bound in vivo and in vitro only to melanin granules in pigmented follicles of rodent and human tissue. This is probably non-specific binding since melanin is known to accumulate several chemically and pharmacologically unrelated drugs. It is reasonable to conclude that, under the conditions of these experiments, minoxidil is not specifically localized in any cells of whisker, pelage or, scalp follicles.