FIGURE 15.13 • Hair follicle and other skin appendages. a. Diagram showing a hair follicle. Note the cell layers that form the hair shaft and the surrounding external and internal root sheaths. The sebaceous gland consists of the secretory portion and a short duct that empties into the infundibulum. The arrector pili muscle accompanies the sebaceous gland; its contraction assists in gland secretion and discharge into the infundibulum. The apocrine gland also empties into the infundibulum of the hair follicle. Note that eccrine sweat glands are independent structures and are not associated directly with the hair follicle. b. Photomicrograph of H&E–stained section of thin skin from human scalp. The growing end of a hair follicle consists of an expanded hair bulb (HB) of epithelial cells that is invaginated by a papilla of connective tissue. The epithelial cells form the unspecialized matrix surrounding the papilla; as the cells leave the matrix, they form cell layers that differentiate into the shaft of the hair and the inner and outer root sheaths of the hair follicle (HF ). Note that several oblique and longitudinal sections of the hair follicles are embedded in the adipose tissue (AT ) of the hypodermis. Some of them reveal a section of the hair. Sebaceous glands (SG) are visible in conjunction with the upper part of the hair follicle. 60.
duct and gland from the wall of the follicular canal (see reside in this area indefinitely and undergo self-renewal or Fig 15.13). In this general region resides an aggregate of rela-differentiation into specific cell lineages. Under normal contively undifferentiated epithelial cells called the follicular ditions, ES cells are responsible for providing stem cells for bulge. Recent studies identified the follicular bulge as a the growth of hair follicles (the internal root sheath, cortex, niche of epidermal stem (ES) cells. The ES cells can and medulla) as well as sebaceous glands. The ES cells that  FOLDER 15.4
Functional Considerations: The Role of Sebum normally reside in the follicular bulge do not contribute to the population of the basal stem cells of the epidermis. However, when the epidermis is injured or lost (such as in extensive skin burns and superficial skin wounds), the ES cells become reprogrammed, migrate toward the wound surface from their follicular niches, and participate in the initial resurfacing of the wound.
Hairs are composed of keratinized cells that develop from hair follicles.
Keratinization of the hair and internal root sheath occurs shortly after the cells leave the matrix in a region called the keratogenous zone. By the time the hair emerges from the follicle, it is entirely keratinized as hard keratin. The internal root sheath, consisting of soft keratin, does not emerge from the follicle with the hair but is broken down at about the isthmus level where sebaceous secretions enter the follicle. A thick basal lamina, called the glassy membrane, separates the hair follicle from the dermis. Surrounding the follicle is a dense irregular connective tissue sheath containing follicular bulge. The arrector pili muscle is attached to follicular bulge which, as was indicated above, also serves as a epidermal stem cell niche.
Hairs are elongated filamentous structures that project from the hair follicles. They also consist of three layers (see Fig. 15.13):  The medulla forms the central part of the shaft and con tains large vacuolated cells. The medulla is present only in thick hairs.  The cortex is located peripherally to the medulla and con tains cuboidal cells. These cells undergo differentiation into keratin-filled cells.  The cuticle of the hair shaft contains squamous cells that form the outermost layer of the hair. In addition, the hair shaft contains melanin pigment produced by melanocytes present in the germinative layer of the hair bulb.