The sensory innervation of primate facial skin. I. Hairy skin

The present study analyzes the variety of sensory nerve terminals present in the hairy skin of the monkey face. In addition to vellus hairs, guard hairs and sinus hairs, a unique type of sinus hair has been identified in the skin of the lip designated a hemisinus hair. Hemisinus hairs have a smaller blood sinus as contrasted to sinus hairs in that the sinus does not extend to the bulb of the hair follicle.

Each type of hair of the face and the lip has its own distinctive pattern of innervation utilizing 5 identifiable unique nerve terminals: free nerve endings, Merkel, lanceolate, Ruffini, and finally, scattered corpuscular receptors at least in some sinus hairs. Hemisinus and guard hairs lack corpuscular receptors and thus can have 4 different terminals, although Merkel terminals are not consistently present in guard hairs. Vellus hairs have only 3 types of receptors: lanceolate, Ruffini and free nerve endings.

Free nerve endings (FNE's) have been found in the connective tissue capsule of primate sinus and hemisinus hairs in the angle between the hair shaft and sebaceous gland and in the same site in guard and vellus hairs. Small diameter myelinated fibers branch and end blindly in the connective tissue or can be intimately associated with the sebaceous gland. FNE's are characterized by the presence of numerous mitochondria, occasional electron opaque lipoidal inclusions, granules of glycogen, a variable population of small vesicles, a tendency to a 1:1 relationship between Schwann cell and enveloped axon. The Schwann cell investment is often deficient with the axolemma directly abutting its basal lamina.

Merkel cells and associated axons are present in rete ridge collars surrounding sinus and hemisinus hairs and in the external root sheath of these two types of sinus hairs. Hemisinus hairs have scant Merkel cells as compared to sinus hairs.

Lanceolate terminals are arranged longitudinally with respect to the axis of the hair and abut the basal lamina of the external root sheath. Guard hairs have a complete palisade of 2–3 dozen lanceolate terminals; however, vellus hairs may have scant or no lanceolate terminals.

Ruffini terminals can be identified on all 4 hair types. Some vellus hairs lack Ruffini terminals, whereas all sinus and hemisinus hairs and most guard hairs have Ruffini terminals. The ultrastructural as well as light microscopic criteria for the identification of each of these receptors has been described and discussed.