What is the structure of skin?

Structure of the Skin

The skin covers the entire exterior of the human body. In an adult, the skin has a surface area of about 1.8 square meters (20.83 square feet). Usually, the skin is only loosely attached to underlying muscle tissue, but where there are no muscles, the skin attaches directly to bone. For example, there are flexion creases where the skin attaches directly to the joints of the fingers.

The skin is sometimes called the cutaneous mem­brane or the integument, and since the skin has several accessory organs, it is also possible to speak of the integu­mentary system. The skin has two regions: the epidermis and the dermis. A subcutaneous layer is found between the skin and any underlying structures, such as muscle.

The skin has two regions: the epidermis and the dermis. A subcutaneous layer lies belovv the skin.

Epidermis

The epidermis (ep"i-der'mis) is the outer and thinner re­gion of the skin. It is made up of stratified squamous epithe­lium divided into several layers; the deepest layer is the stratum basale, and the most superficial layer is the stratum corneum.

Stratum Basale

The basal cells of the stratum basale lie just superior to the dermis and are constantly dividing and producing new cells that rise to the surface of the epidermis in two to four weeks. As the cells push away from the dermis, they move progressively farther away from the blood vessels in the dermis. Since these cells are not being supplied with nutri­ents and oxygen (the epidermis itself lacks blood vessels), they eventually die and are sloughed off.

Langerhans' cells are specialized cells produced in red bone marrow and found in the lower epidermis. These cells phagocytize microbes and then travel to lymphoid or­gans, where they stimulate the immune system to react.

 Melanocytes (mel'ah-no-slts) are another type of specialized cell located in the lower epidermis. Melanocytes produce melanin (mel'ah-nin), the pigment primarily responsible for skin color. Since the number of melanocytes is about the same in all individuals, variation in skin color is due to the amount of melanin produced and its distri­bution. When Caucasians sunbathe, melanocytes produce more melanin in an attempt to protect the skin from the damaging effects of the ultraviolet (UV) radiation in sunlight. The melanin is passed to other epidermal cells, and the result is tanning, or in some people, the formation of patches of melanin called freckles. A hereditary trait characterized by the lack of ability to produce melanin is known as albinism (al'bI-nizm). Individuals with this disorder lack pigment not only in the skin, but also in the hair and eyes. Another pigment, called carotene, is present in epidermal cells and in the dermis and gives the skin of people of Asiatic origin its yellowish coloration. The pinkish color of fair-skinned people is due to the pigment hemo­globin in the red blood cells in the capillaries of the dermis.

Stratum Corneum

As cells are pushed toward the surface of the skin, they be­come flat and hard, forming the tough, uppermost layer of the epidermis, the stratum corneum. Hardening is caused by keratinization, the cellular production of a fibrous, wa­terproofprotein called keratin (ker'ah-tin). Over much of the body, keratinization is minimal, but the palms of the hands and the soles of the feet normally have a particularly thick outer layer of dead, keratinized cells.

The waterproof nature of keratin protects the body from water loss and water gain. The stratum corneum al­lows us to live in a desert or a tropical rain forest without damaging our inner cells.

The stratum corneum also serves as a mechanical barrier against microbe invasion. This function is assisted by the se­cretions of sebaceous glands (discussed in a later section of this chapter), which weaken or kill bacteria on the skin.

The epidermis. the outer region of skin. is made up of stratified squamous epithelium. New cells continually produced in the stratum basale of the epidermis push outward and become the keratinized cells of the stratum corneum. which are sloughed off.

Dermis

The dermis is deeper and thicker than the epidermis and contains both loose and fibrous connective tissue. The upper layer of the dermis has fingerlike projections called dermal papillae. Dermal papillae project into and anchor the epider­mis. In the overlying epidermis, they cause ridges, resulting in spiral and concentric patterns. The function of the epidermal ridges is to increase friction and thus provide a better grip­ping surface. Because they are unique to each person, finger­prints and footprints can be used for identification purposes.

The dermis contains collagenous and elastic fibers.

The collagenous fibers are flexible but offer great resistance to overstretching; they prevent the skin from being torn. The elastic fibers stretch to allow movement of underlying muscles and joints, but they maintain normal skin ten­sion. The dermis also contains blood vessels that nourish the skin. Blood rushes into these vessels when a person blushes and is reduced in them when a person turns cyan­otic or blue. Sometimes, blood flow to a particular area is restricted in bedridden patients, and consequently, they develop decubitus (dik-u'bit-es) ulcers (bedsores) (fig. 5.2). These can be prevented by changing the patient's po­sition frequently and by massaging the skin to stimulate blood flow.

There are also numerous sensory nerve fibers in the der­mis that take nerve impulses to and from the accessory struc­tures of the skin, which are discussed later in this chapter.

The dermis, which Is composed of fibrous connective tissue, lies beneath the epidermis. It contains collagenous and elastic fibers, blood vessels. and nerve fibers.

Subcutaneous Layer

The subcutaneous (sub"ku-ta'ne-us) layer, or hypodermis, lies below the dermis. Subcutaneous injections are per­formed with hypodermic needles.

The subcutaneous layer is composed of loose con­nective tissue, including adipose (fat) tissue. Fat is an energy storage form that can be called upon when necessary. Adipose tissue also helps insulate the body from either gain­ing heat from the outside or losing heat from the inside.

A well-developed subcutaneous layer gives the body a rounded appearance and provides protective padding against external assaults. Excessive development of the sub­cutaneous layer results in obesity.

The subcutaneous layer is made up of loose connective tissue and adipose tissue. vvhich insulates the body from heat and cold.