The hair follicle is divided into three segments:  The infundibulum extends from the surface opening of the follicle to the level of the opening of its sebaceous gland. The infundibulum is a part of the pilosebaceous canal, which is used as a route for the discharge of the oily substance sebum.  The isthmus extends from the infundibulum to the level of insertion of the arrector pili muscle.  The inferior segment in the growing follicle (Fig. 15.13) is of nearly uniform diameter except at its base, where it expands to form the bulb. The base of the bulb is invaginated by a tuft of vascularized loose connective tissue called, not surprisingly, a dermal papilla (Plate 47, page 525).
Other cells forming the bulb, including those that surround the connective tissue papilla, are collectively referred to as the matrix, which consists simply of matrix cells.
Matrix cells immediately adjacent to the dermal papilla represent the germinative layer of the follicle. Division and proliferation of these cells accounts for the growth of the hair. Scattered melanocytes are also present in this germinative layer. They contribute melanosomes to the developing hair cells in a manner analogous to that in the stratum germinativum of the epidermis. The dividing matrix cells in the germinative layer differentiate into the keratin-producing cells of the hair and the internal root sheath. The internal root sheath is a multilayered cellular covering that surrounds the deep part of the hair. The internal root sheath has three layers:  The cuticle consists of squamous cells whose outer free surface faces the hair shaft.  Huxley’s layer consists of a single or double layer of flat tened cells that form the middle plate of the internal root sheath.
 Henle’s layer consists of an outer single layer of cuboidal cells. These cells are in direct contact with the outermost part of the hair follicle, which represents a downgrowth of the epidermis and is designated the external root sheath.
A niche of epidermal stem cells that resides within the follicular bulge of the external root sheath provides stem cells for hair growth.
The progression of the external root sheath of the hair follicle upward toward the epidermal surface reveals the insertion site of the arrector pili muscle and the origin of the sebaceous
The role of sebum is not clearly defined. Various investiga-tors have ascribed bacteriostatic, emollient, barrier, and pheromone functions to sebum. Sebum does appear to play a critical role in the development of acne. The amount of sebum secreted increases significantly at puberty in both males and females. Triglycerides contained in sebum are broken down to fatty acids by bacteria on the skin surface, and the free fatty acids liberated may be an irritant in the formation of acne lesions. On histologic examina-tion, acne is characterized by retention of the sebum in the isthmus of the hair follicle, with variable lymphocytic infiltra-tion. In severe cases, dermal abscesses may form in asso-ciation with inflamed hair follicles.