Three-dimensional comparative study of Merkel cell distribution in various body sites of guinea pigs and rats.

The hair cycling pattern differs between humans and mice: for the former it is mosaic, while the latter show a wave pattern. We focused on the presence or absence of synchronization of the hair cycle based on the distribution of Merkel cells. The three-dimensional distribution and number of Merkel cells in the skin at various sites were determined in guinea pigs and rats. Using ethylenediaminetetraacetic acid, various types of hair follicles were obtained from the facial, abdominal, nipple, back, tail, perianal and plantar skin. Epidermal sheets containing numerous hair follicles were immunostained using monoclonal murine antibody CK20 which is generally accepted to be a specific immunocytochemical marker for Merkel cells of various species. Merkel cells were absent in the trunk skin of guinea pigs and rats, in complete contrast to the situation in humans. In vibrissa and other hair follicles within specialized sensitive areas, Merkel cells were found in varying numbers. Some differences in their distribution and number were observed between guinea pigs and rats. The absence of Merkel cells in the hair follicles of the trunk skin of rats and guinea pigs implies that Merkel cells are not essential for the induction and promotion of hair growth but have some different functional significance in hair follicles.