Porth's Essentials of Pathophysiology, 4e

1147

Structure and Function of the Integumentum

C h a p t e r 4 5

the secretion of apocrine glands is oily. In many animals, apocrine secretions give rise to distinctive odors that enable animals to mark their territory and attract a mate. In humans, apocrine secretions are sterile and odorless until mixed with the bacteria on the skin surface; they then produce what is commonly known as “body odor.” The sebaceous glands are located over the entire skin surface except for the palms, soles, and sides of the feet. They are part of what is called the pilosebaceous unit , with their ducts opening into the upper third of the hair follicle. They secrete sebum , a waxlike mixture of triglycerides, cholesterol, and cellular debris. Sebum lubricates the hair and skin, prevents undue evapora- tion of moisture from the stratum corneum during cold weather, and helps to conserve body heat. Sebum production is an example of holocrine secre- tion. The cytoplasm of the glandular epithelial cell produces and becomes filled with the fatty product. Eventually, the cell’s plasma membrane ruptures, and the cell dies. Both the secretory product and the cell frag- ments, which together constitute sebum, are then dis- charged from the gland into the hair follicle. New cells are produced by mitosis of the basal cells in the gland. Sebum production is under the control of genetic and hormonal influences. Sebaceous glands are relatively small and inactive until individuals approach adoles- cence. The glands then enlarge, stimulated by the rise in sex hormones. Gland size directly influences the amount of sebum produced, and the level of androgens influ- ences gland size. The sebaceous glands are the structures that become inflamed in acne (see Chapter 46). Hair and Hair Follicles Hair is a filamentous, keratinized structure that consists of the hair follicle, sebaceous gland, hair muscle (arrector pili), and, in some instances, an apocrine gland (Fig. 45-6). Most hair follicles are associated with sebaceous glands,

and these structures combine to form the pilosebaceous unit. The arrector pili muscle, located under the sebaceous gland, provides a thermoregulatory function by contract- ing to cause goose bumps, thereby reducing the skin sur- face area that is available for the dissipation of body heat. Hair color is attributable to the melanin pigments that the hair contains. Variations mainly reflect the quantity and ratio of the black to dark brown pigment eumelanin and the reddish-brown pigment pheomelanin. The hair follicle is divided into three parts: the infun- dibulum, which extends from the surface opening to the level of the opening for the sebaceous gland; the isth- mus, which extends from the infundibulum to the level of the arrector pili muscle; and the expanded inferior hair bulb, which is indented to conform to the shape of the dermal papilla occupying it. The dermal papilla contains a rich supply of nutrients and oxygen for the cells in the hair follicle. Growth of the hair is centered in the bulb (i.e., base) of the hair follicle, and the hair undergoes changes as it is pushed outward. Hair growth goes through three cyclic phases: the anagen (growth), catagen (atrophy), and telogen (resting) phases. Anagen hair has long inner roots and outer root sheaths, is deeply rooted in the der- mis, is difficult to detach, and does not come out with regular brushing. Anagen follicles are actively replicating and therefore especially susceptible to nutritional defi- ciencies and metabolic insults. The catagen phase repre- sents degeneration of the root structure and migration of dermal papillae and the follicular unit toward the more superficial layers of the dermis. Catagen hair usually represents approximately 1% of all scalp hairs. Telogen hair has short, white, club-shaped roots. With forma- tion of new anagen hair below the root, the developing follicle will eventually replace the telogen hair, leading to the shedding of approximately 50 to 100 hairs a day. Nails Nails are hardened keratinized plates that protect the fin- gers and toes and enhance dexterity. The nails grow out from a curved transverse groove called the nail groove. The floor of this groove, called the nail matrix, is the ger- minal region of the nail plate (Fig. 45-7). The underlying epidermis, attached to the nail plate, is called the nail bed.

Hair shaft

Arrector pili muscle

Epidermis

Sebaceous gland Keratinized cells

Nail groove

Eponychium

Lunula Nail matrix

Nail bed

Nail plate

Dermis

Hair follicle Dermal papilla Dermal blood vessels

FIGURE 45-6. Parts of a hair follicle.

FIGURE 45-7. Parts of a fingernail.