Topography of nerve terminals in Merkel nerve endings in mammals

Merkel nerve endings are found in the skill as well as oral and anal mucosa of all mammals. In the two types of glabrous skin, individual or groups of Merkel nerve endings are found in the basal layer of the epidermal ridges close to the excretory ducts of sweat glands or in the basal layer of the epithelial pegs, respectively Here, Merkel cells are oval in shape and their long axis runs parallel to the surface. The nerve terminals have discoid form and are in close contact with the basal part of the Merkel cells. In the hairy skin, Merkel cells are found in the epithelial thickenings between hair follicles ("Tastscheiben") as well as in the basal layer of all types of hair follicles. While the "Tastscheiben" - structure is mainly identical to the arrangement in the glabrous skin, in sinus hair follicles Merkel cells (up to several thousand) form a cuff around the thicker part of the hair follicle below the sebaceous gland. The discoid nerve terminals are located on the side of the Merkel cell opposite to the basal lamina. Their longitudinal axis runs parallel to the hair shaft. In the anal mucosa the epithelium forms similar thickenings as seen in the epithelial pegs with the same arrangement of the nerve terminals. In the oral mucosa Merkel nerve endings are found mainly in the masticatory mucose of the gingiva and hard palate. In most species, the arrangement is comparable to "Tastscheiben". However, in the monkey papilla incisiva we found Merkel cells arranged with their longitudinal axis perpendicular to the surface and the nerve terminal facing their labial side. In conclusion, the location of the nerve terminal obviously depends on the direction of the main mechanical stimulus in the way that the Merkel cell is placed between the mechanical stimulus and the nerve terminal.