THE SKIN

INTRODUCTION

The skin and its accessory structures (hair, nails, and glands) make up the integumentary system of the body. Integument means covering, and the skin (weighing 8 to 10 pounds and extending over an area of 22 square feet in an average adult) is the outer covering for the body. It is, however, more than a simple body covering. This complex system of specialized tissues contains glands that secrete several types of fluids, nerves that carry impulses, and blood vessels that aid in the regulation of the body temperature.

The skin has many important functions:

First, as a protective membrane over the entire body, the skin guards the deeper tissues of the body against excessive loss of water, salts, and heat and against invasion of pathogens and their toxins. Secretions from the skin are slightly acidic in nature, which contributes to the skin’s ability to prevent bacterial invasion. Specialized cells (Langerhans cells) react to the presence of antigens and have an immune function.

Second, the skin contains two types of glands that produce important secretions. These glands in the skin are the sebaceous glands and the sweat glands. Sebaceous glands produce sebum, an oily secretion, and sweat glands produce sweat, a watery secretion. Sebum and sweat pass to the outer edges of the skin through ducts and leave the skin through openings, or pores. Sebum lubricates the surface of the skin, and sweat cools the body as it evaporates from the skin surface.

Third, nerve fibers under the skin are receptors for sensations such as pain, temperature, pressure, and touch. Thus, the body’s adjustment to the environment depends on sensory messages relayed to the brain and spinal cord by sensitive nerve endings in the skin.

Fourth, different tissues in the skin maintain body temperature (thermoregulation).Nerve fibers coordinate thermoregulation by carrying messages to the skin from heat centers in the brain that are sensitive to increases and decreases in body temperature.

Impulses from these fibers cause blood vessels to dilate to bring blood to the surface and

cause sweat glands to produce the watery secretion that carries heat away.

ANATOMY OF THE SKIN

Figure A shows three layers of the skin. Label them from the outer surface inward:

Epidermis [1]—a thin, cellular membrane layer; containing keratin

Dermis [2]—dense, fibrous, connective tissue layer; containing collagen

Subcutaneous layer [3]—thick, fat-containing tissue

EPIDERMIS

The epidermis is the outermost, totally cellular layer of the skin. It is composed of squamous epithelium. Epithelium is the covering of both the internal and the external surfaces of the body. Squamous epithelial cells are flat and scale-like. In the outer layer of the skin, these cells are arranged in several layers (strata) to form stratified squamous epithelium. The epidermis lacks blood vessels, lymphatic vessels, and connective tissue (elastic fibers, cartilage, fat) and is therefore dependent on the deeper dermis layer and its rich network of capillaries for nourishment. In fact, oxygen and nutrients seep out of the capillaries in the dermis, pass through tissue fluid, and supply nourishment to the lower layers of the epidermis.

Figure B illustrates the multilayered cells of the epidermis. The deepest layer is called the basal layer [4]. The cells in the basal layer are constantly growing and multiplying and are the source of all the other cells in the epidermis. As the basal layer cells divide, they are pushed upward and away from the blood supply of the dermal layer by a steady stream of younger cells. In their movement toward the most superficial layer of the epidermis, called the stratum corneum [5], the cells flatten, shrink, lose their nuclei, and die, becoming filled with a hard protein material called keratin. The cells are then called keratinocytes, reflecting their composition of keratin. Finally, within 3 to 4 weeks after beginning as a basal cell in the deepest part of the epidermis, the keratinized cell is sloughed off from the surface of the skin. The epidermis is thus constantly renewing itself, with cells dying at the same rate at which they are replaced. are pushed upward and away from the blood supply of the dermal layer by a steady stream of younger cells. In their movement toward the most superficial layer of the epidermis, called the stratum corneum [5], the cells flatten, shrink, lose their nuclei, and die, becoming filled with a hard protein material called keratin. The cells are then called keratinocytes, reflecting their composition of keratin. Finally, within 3 to 4 weeks after beginning as a basal cell in the deepest part of the epidermis, the keratinized cell is sloughed off from the surface of the skin. The epidermis is thus constantly renewing itself, with cells dying at the same rate at which they are replaced.

The basal layer of the epidermis contains special cells called melanocytes [6]. Melanocytes contain a pigment called melanin that is transferred to other epidermal cells and gives color to the skin. The number of melanocytes in all human races is the same, but the amount of melanin within each cell accounts for the color differences among the races. Individuals with darker skin possess more melanin within the melanocytes, not a greater number of melanocytes. Individuals who are incapable of forming melanin are called albinos. Skin and hair are white. In albinos, eye color varies from red to blue to brown, depending on the amount of melanin present in the iris (pigmented portion of the eye). Occulocutaneous albinism affects eyes, skin, and hair, while ocular albinism affects the eyes only.

Melanin production increases with exposure to strong ultraviolet light, and this creates a suntan, which is a protective response. When the melanin cannot absorb all of the ultraviolet rays, the skin becomes sunburned and inflamed (redness, swelling, and pain).

Over a period of years, excessive exposure to sun tends to cause wrinkles, permanent pigmentary changes, and even cancer of the skin. Because dark-skinned people have more melanin, they acquire fewer wrinkles and they are less likely to develop the types of skin cancer that are associated with ultraviolet light exposure.

DERMIS

The dermis, directly below the epidermis, is composed of blood and lymph vessels and nerve fibers, as well as the accessory organs of the skin, which are the hair follicles, sweat glands, and sebaceous glands. To support the elaborate system of nerves, vessels, and glands, the dermis contains connective tissue cells and fibers that account for the extensibility and elasticity of the skin.

The dermis is composed of interwoven elastic and collagen fibers. Collagen (colla = glue) is a fibrous protein material found in bone, cartilage, tendons, and ligaments, as well as in the skin. It is tough and resistant but also flexible. In the infant, collagen is loose and delicate; it becomes harder as the body ages. During pregnancy, overstretching of the skin with weight gain may break the elastic fibers, resulting in linear markings called striae (“stretch marks”) on the woman’s abdomen and elsewhere. Collagen fibers support and protect the blood and nerve networks that pass through the dermis. Collagen diseases affect connective tissues of the body. Examples of these connective tissue collagen disorders are systemic lupus erythematosus and scleroderma.

SUBCUTANEOUS LAYER

The subcutaneous layer (epidermis and dermis are the cutaneous layers) specializes in the formation of fat. Adipocytes (fat cells) are predominant in the subcutaneous layer, and they manufacture and store large quantities of fat. Obviously, fat deposition varies in different areas of the body and among individual people. Functionally, this layer of the skin is important in protection of the deeper tissues of the body, as a heat insulator, and for energy storage.

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