Fine morphological characteristics and microtopography of the free nerve endings of the human digital skin.

Blocks of osmium-fixed human digital skin, representing a total area of 5 sq mm, were cut serially parallel to the skin surface starting with the epidermis for electron microscopical studies. At intervals, single 1-mu sections were taken and used for recording the microtopography of the receptor organs at various levels. Graphic reconstructions of 28 nerve endings were made. It was found that the digital skin accommodated up to 80 dermal papillae and three to four sweat duct orifices per square millimeter. Each papilla contained one to three fenestrated capillary loops. More than 60% of all papillae contained free nerve endings, sometimes up to five in a single papilla. As a result, up to 100 free nerve endings were counted per square millimeter. Intraepidermal endings were also present, one to five per square millimeter. No endings of any kind were found within the boundary zones between the papillary ridges and in the immediate vicinity of the sweat ducts. The origin of a free ending was marked by the perikaryon of a modified Schwann cell, which constituted the terminal cell of the Schwann sheath. The perikaryon was situated at the base of the dermal papilla, while its distal process provided the cytoplasmic sheath to the axon terminals and their branches. Sometimes two endings merged into a single receptor organ. The terminal Schwann cell frequently served as a rallying point of endings. Three morphological kinds of free endings were observed: open endings, beaded endings, and plain endings. The intraepidermal endings were the derivatives of the plain endings. The free endings in the digital skin had essentially vertical distribution. Each ending covered a minute surface area of skin in a punctate pattern. This is in contrast to the mode of distribution of the free (penicillate) endings of the human hairy skin. Each penicillus covers a large horizontal skin area and overlaps that of their neighbors.