The hairlet follicle in the skin of the wild Norway rat (Rattus norvegicus Berkenhout, 1769). Its morphology and function.

The hairlet follicle of the wild Norway rat (Rattus norvegicus Berkenhout 1769) differs from the other follicles in position and in structural characteristics. As an element of the guard (tylotrich) hair follicle group the hairlet follicles are arranged at the margin of the hair disk. The hairlet follicle, which produces the finest hair fibre, is enclosed by an active unilobular sebaceous gland. The arrector pili muscle is absent, but the follicle does have a supply of lanceolate nerve endings and Ruffini endings. The axons of the lanceolate endings are enclosed sandwich-like by Schwann cells and contact the basal lamina of the outer epithelial root sheath. Ruffini endings run within the band of connective tissue fibres that girdle the follicle at the level of the sebaceous gland. The appropriate stimulus for excitation of the nerve fibres can not be achieved by bending the tiny hairlet fibre. Tissue compression, induced by bending the central guard (tylotrich) hair, which is transformed via the sebaceous gland and the slim outer epithelial root sheath, may exicite the nerves of the hairlet follicle. It can be assumed that the hairlet follicle functions as a mechanoreceptor.