Chapter 2 41 the infundibulum. The lower end of the infundibulum is marked by the insertion point of the sebaceous gland duct, which is a constant trouble area in patients with acne. The middle segment, the isthmus, is the shortened segment of the hair follicle, extending from the attachment of arrector pili muscle (APM) into the entrance of the sebaceous gland duct. At the proximal end, the infundibulum joins the isthmus region of the ORS. The insertion area of APM marks the proximal border of the isthmus, as well as the permanent portion of the hair follicle. The stem cells required for normal hair cycling, including epithelial and melanocyte stem cells, are harbored in the end region of the permanent, non-cycling portion, namely the bulge. The number and abilities of cells in the hair bulb, which are the progeny of adult stem cells, determine the size of hair bulb and generate the HS.7,8 Figure 2 shows a cross-section of the hair follicle. The epithelium forms a cylinder with eight different concentric layers, from the outmost, including the outer root sheath (ORS), the companion layer, the inner root sheath (IRS), and finally the HS. The ORS is directly Figure 1 Anatomy of the hair follicle Sagittal section through a human scalp hair follicle (anagen VI) shows the permanent (above dashed line) and dynamic (below dashed line) portions of the hair follicle image courtesy of Raymond Boissy