5.4 Accessory Skin Structures

Nails

- A nail is a thin plate consisting of layers of dead stratum corneum cells that contain a very hard type of keratin. Nails are located on the distal ends of the digits (fingers and toes). A nail consists of the proximal nail root and the distal nail body. - The nail root is covered by skin, and the nail body is the visible portion of the nail. The lateral and proximal edges of the nail are covered by skin called the nail fold, and the edges are held in place by the nail groove. The stratum corneum of the nail fold grows onto the nail body as the cuticle, or eponychium. Beneath the free edge of the nail body is the hypo- nychium, a thickened region of the stratum corneum. The nail root extends distally from the nail matrix. The nail also attaches to the underlying nail bed, which is located between the nail matrix and the hyponychium. The nail matrix and bed are composed of epithelial tissue, with a stratum basale that gives rise to the cells that form the nail. The nail matrix is thicker than the nail bed and produces nearly all of the nail. The nail bed is visible through the clear nail and appears pink because of blood vessels in the dermis. A small part of the nail matrix, the lunula, is seen through the nail body as a whitish, crescent- shaped area at the base of the nail. The lunula, seen best on the thumb, appears white because the blood vessels do not show through the thicker nail matrix. - As the nail forms in the nail matrix and bed, it slides over the nail bed toward the distal end of the digit. Nails grow at an average rate of 0.5-1.2 mm per day, and fingernails grow more rapidly than toenails. Unlike hair, they grow continuously throughout life and do not have a resting phase.

Hair

- The presence of hair is one of the characteristics of all mammals; if the hair is dense and covers most of the body surface, it is called fur. In humans, hair is found everywhere on the skin except the palms, the soles, the lips, the nipples, parts of the external genitalia, and the distal segments of the fingers and toes. - By the fifth or sixth month of fetal development, delicate, unpigmented hair called lanugo has developed and covered the fetus. Near the time of birth, terminal hairs, which are long, coarse, and pigmented, replace the lanugo of the scalp, eyelids, and eyebrows. Vellus hairs, which are short, fine, and usually unpigmented, replace the lanugo on the rest of the body. At puberty, terminal hair, especially in the pubic and axillary regions, replaces much of the vellus hair. The hair of the chest, legs, and arms is approximately 90% terminal hair in males and approximately 35% in females. In males, terminal hairs replace the vellus hairs of the face to form the beard. The beard, pubic, and axillary hair are signs of sexual maturity. In addition, pubic and axillary hair may function as wicks for dispersing odors produced by secretions from specialized glands in the pubic and axillary regions. It also has been suggested that pubic hair protects against abrasion during intercourse and axillary hair reduces friction when the arms move. [Hair structure] - A hair is divided into the shaft, which protrudes above the surface of the skin, and the root, located below the surface. The base of the root is expanded to form the hair bulb. Most of the root and the shaft are composed of columns of dead, keratinized epithelial cells arranged in three concentric layers: the medulla, the cortex, and the cuticle. The medulla is the central axis of the hair; it consists of two or three layers of cells containing soft keratin. The cortex forms the bulk of the hair; it consists of cells containing hard keratin. The cortex is covered by the cuticle, a single layer of cells that contain hard keratin. The edges of the cuticle cells overlap like shingles on a roof. - The hair follicle is a tubelike invagination of the epidermis that extends into the dermis from which hair develops. A hair follicle consists of a dermal root sheath and an epithelial root sheath. The dermal root sheath is the portion of the dermis that surrounds the epithelial root sheath. The epithelial root sheath is divided into external and internal parts. At the opening of the follicle, the external epithelial root sheath has all the strata found in thin skin. Deeper in the hair follicle, the number of cells decreases until at the hair bulb only the stratum basale is present. This has important consequences for skin repair. If the epidermis and the superficial part of the dermis are damaged, the undamaged part of the hair follicle that lies deep in the dermis can be a source of new epithelium. The internal epithelial root sheath has raised edges that mesh closely with the raised edges of the hair cuticle and hold the hair in place. When a hair is pulled out, the internal epithelial root sheath usually comes out as well and is plainly visible as whitish tissue around the root of the hair. - The hair bulb is an expanded knob at the base of the hair root. Inside the hair bulb is a mass of undifferentiated epithelial cells, the matrix, which produces the hair and the internal epithelial root sheath. The dermis of the skin projects into the hair bulb as a hair papilla; it contains blood vessels that provide nourishment to the cells of the matrix. [Hair Growth] - Hair is produced in cycles that involve a growth stage and a resting stage. During the growth stage, hair is formed by matrix cells that differentiate, become keratinized, and die. The hair grows longer as cells are added at the base of the hair root. Eventually, hair growth stops; the hair follicle shortens and holds the hair in place. A resting period follows, after which a new cycle begins and a new hair replaces the old hair, which falls out of the hair follicle. Thus, losing a hair normally means that the hair is being replaced. The length of each stage depends on the hair—eyelashes grow for approximately 30 days and rest for 105 days, whereas scalp hairs grow for 3 years and rest for 1-2 years. At any given time, an estimated 90% of the scalp hairs are in the growing stage, and loss of approximately 100 scalp hairs per day is normal. - The most common kind of permanent hair loss is "pattern baldness." Hair follicles shrink and revert to producing vellus hair, which is very short, transparent, and for practical purposes invisible. Eventually, hair production in these smaller follicles may completely cease. Although baldness is more common and more pronounced in certain men, it can also occur in women. Genetic factors and the hormone testosterone are involved in causing pattern baldness. - The average rate of hair growth is approximately 0.3 mm per day, although hairs grow at different rates, even in the same approximate location. Cutting, shaving, or plucking hair does not alter the growth rate or the character of the hair, but hair can feel coarse and bristly shortly after shaving because the short hairs are less flexible. Maximum hair length is determined by the rate of hair growth and the length of the growing phase. For example, scalp hair can become very long, but eyelashes are short. [Hair Color] Melanocytes within the hair bulb matrix produce melanin and pass it to keratinocytes in the hair cortex and medulla. As with the skin, varying amounts and types of melanin cause different shades of hair color. Blonde hair has little black-brown melanin, whereas jet black hair has the most. Intermediate amounts of melanin account for different shades of brown. Red hair is caused by varying amounts of a red type of melanin. Hair sometimes contains both black-brown and red melanin. With age, the amount of melanin in hair can decrease, causing hair color to fade or become white (i.e., no melanin). Gray hair is usually a mixture of unfaded, faded, and white hairs. Hair color is controlled by several genes, and dark hair color is not necessarily dominant over light. [Muscles] Associated with each hair follicle are smooth muscle cells called the arrector pili, which extend from the dermal root sheath of the hair follicle to the papillary layer of the dermis. Normally, the hair follicle and the hair inside it are at an oblique angle to the surface of the skin. When the arrector pili muscles contract, however, they pull the follicle into a more per- pendicular position, causing the hair to "stand on end." Movement of the hair follicles produces raised areas called "goose bumps."

Glands

The major glands of the skin are the sebaceous glands and the sweat glands (figure 5.7). [Sebaceous Glands] Sebaceous glands, located in the dermis, are simple or compound alveolar glands that produce sebum, an oily, white substance rich in lipids. Because sebum is released by the lysis and death of secretory cells, sebaceous glands are classified as holocrine glands. Most sebaceous glands are connected by a duct to the upper part of the hair follicles, from which the sebum oils the hair and the skin surface. This prevents drying and protects against some bacteria. A few sebaceous glands located in the lips, the eyelids (meibomian glands), and the genitalia are not associated with hairs but open directly onto the skin surface. [Sweat Glands] - There are two types of sweat, or sudoriferous glands: eccrine glands and apocrine glands. At one time, physiologists believed that secretions were released in a merocrine fashion from eccrine glands and in an apocrine fashion from apocrine glands. But we now know that apocrine sweat glands also release some of their secretions in a merocrine fashion, and possibly some in a holocrine fashion. Traditionally, they are still referred to as apocrine sweat glands. - Eccrine sweat glands (sometimes called merocrine sweat glands) are the most common type of sweat gland. They are simple, coiled, tubular glands that open directly onto the surface of the skin through sweat pores. Eccrine sweat glands can be divided into two parts: the deep, coiled portion, which is located mostly in the dermis, and the duct, which passes to the skin surface. The coiled part of the gland produces an isotonic fluid that is mostly water but also contains some salts (mainly sodium chloride) and small amounts of ammonia, urea, uric acid, and lactic acid. As this fluid moves through the duct, sodium chloride moves by active trans- port from the duct back into the body, thereby conserving salts. The resulting hyposmotic fluid that leaves the duct is called sweat. When the body temperature starts to rise above normal, the sweat glands produce sweat, which evaporates and cools the body. Sweat also can be released in the palms, soles, and axillae as a result of emotional stress. Emotional sweating is used in lie detector (polygraph) tests because sweat gland activity may increase when a person tells a lie. Such tests can detect the sweat produced, even in small amounts, because the salt solution con- ducts electricity and lowers the electrical resistance of the skin. - Eccrine sweat glands are most numerous in the palms of the hands and soles of the feet but are absent from the margin of the lips, the labia minora, and the tips of the penis and clitoris. - Apocrine sweat glands are simple, coiled, tubular glands that usually open into hair follicles superficial to the opening of the sebaceous glands. Apocrine sweat glands are found in the axillae and genitalia (scrotum and labia majora) and around the anus. They do not help regulate temperature in humans. Apocrine sweat glands become active at puberty as a result of sex hormones. Their secretions contain organic substances, such as 3-methyl-2- hexenoic acid, that are essentially odorless when first released but are quickly metabolized by bacteria to cause what is commonly known as body odor. Many mammals use scent as a means of communication, and physiologists have suggested that the activity of apocrine sweat glands may signal sexual maturity. [Other Glands] - Other skin glands are the ceruminous glands and the mammary glands. The ceruminous glands are modified eccrine sweat glands located in the ear canal (external auditory canal). Cerumen, or earwax, is composed of the combined secretions of ceruminous glands and sebaceous glands. Cerumen and hairs in the ear canal protect the tympanic membrane by preventing the entry of dirt and small insects. However, an accumulation of cerumen can block the ear canal and make hearing more difficult. - The mammary glands are modified apocrine sweat glands located in the breasts. They produce milk. The structure and regulation of mammary glands are discussed in chapters 28 and 29.