The Integumentary System
The integumentary system encloses the body, protecting it from, as well as allowing its interactions with, the external environment. Physician specialists who treat conditions of the SKIN, HAIR, and NAILS are dermatologists. This section, “The Integumentary System,” presents an overview of the structures and functions of the integumentary system, a discussion of dermatological health and disorders, and entries about the health conditions that can affect the skin, hair, and nails.
Functions of the Integumentary System
The integument, from the Latin word for “cloak,” covers the body. Its structures - the SKIN, HAIR, and NAILS - form the image the body presents to the outside world. Its functions—protective barrier, tactile perception, temperature regulation, IMMUNE RESPONSE—enable the body to survive in that outside world.
The foundation of the integumentary system is the skin, which, as the body’s largest organ, accounts for 15 percent of the body’s weight. It sheaths the body in protective insulation from scalp to sole, coating every stretch and fold between. The skin’s three layers—epidermis, dermis, and subcutaneous layer—form the interface between the body’s internal and external environments. The endless exchange of information is so wearing that the skin completely replaces its outermost layer, the epidermis (about 36,000 square inches of surface area), every three to four weeks. What looks remarkably the same from day to day is, in reality, always changing.
In the beginning
Attesting to the skin’s importance for survival and function, the skin and the BRAIN are the first two distinctive organs to emerge during embryonic development. The cells of each arise from the primitive neural crest, also called the neuroectoderm. By three weeks of gestational age the neural crest differentiates. The cells that migrate inward become NERVE cells, forming the brain and SPINAL CORD. The cells that migrate outward become the two major cell types of cells that form the skin: keratinocytes and melanocytes.
By seven weeks gestational age the skin develops hair follicles that, six weeks later, begin to cover the head with hair. At 20 weeks gestational age that hair coat, called lanugo, has spread to cover the entire body. Some babies, especially those born prematurely, still sport this coat at birth, which is often disconcerting to anxious parents but quickly falls away. The formation and function of the sebaceous glands parallels that of the hair follicles. As hair begins to sprout, the sebaceous glands secrete a thick, ointmentlike precursor to sebum, called vernix, that covers the skin’s surface. Vernix establishes a waterproof barrier that protects the skin as the FETUS floats in AMNIOTIC FLUID. Also by 20 weeks the skin contains SWEAT GLANDS, eyelashes and eyebrows, fingernails and toenails, and the unique surface ridges on the fingertips that will become fingerprints.
Renewal and protection: the epidermis
The primary cells of the skin’s surface layer, the epidermis, are melanocytes, which produce the pigment melanin, and keratinocytes, which produce the fibrous protein keratin. Both types of cells arise from the base level of the epidermis, the stratum germinativum (“birth”), also called the basal level or the Malpighian level. As the Latin name implies, this level germinates, or originates, cells. Melanocytes remain in the stratum germinativum for all of their existence. Keratinocytes migrate upward to the stratum corneum (“horny”), the cornified or hardened surface level of the epidermis.
Melanocytes produce melanin, the pigment that gives color to the skin, hair, and eyes (iris). This color has a purpose: melanin is the skin’s primary protection against the sun’s ultraviolet rays. It absorbs ultraviolet lightwaves, preventing them from causing damage to cell structure and function. Melanocytes store the enzyme tyrosinase, which other cells in the body produce, and acquire the amino acid tyrosine from the circulating blood. The tyrosinase catalyzes a series of chemical actions that convert tyrosine to two tones of pigment: eumelanin (black-brown pigment) and pheomelanin (yellow-brown pigment). The melanocytes package these pigments into granules collectively called melanin.
Melanocytes also pigment cells in other organs. When the neural crest differentiates early in embryonic development, some melanocytes migrate with the cells that form the structures of the brain. A dense population of melanocytes settles in a structure in the midbrain, the substantia nigra (a name that means “black substance”). The melanocytes pigment specialized cells in the substantia nigra that produce DOPAMINE, a brain NEUROTRANSMITTER essential to neuromuscular function. When melanocytes in the skin slow melanin production, the result may be depigmentation disorders such as VITILIGO or white hair. Though often distressing, these consequences are not serious threats to health and life. When melanocytes in the substantia nigra stop producing melanin, however, the substantia nigra stops producing dopamine, and the body stops moving, a degenerative condition called PARKINSON’S DISEASE.
Keratinocytes produce two proteins: keratin, a fibrous substance, and cytokine, an IMMUNE RESPONSE mediator. After they mature in the stratum germinativum, keratinocytes pick up melanin granules and begin their migration to the surface. The three and a half week journey to the stratus corneum is a final rite of passage that literally squeezes the life from the keratinocytes. During this passage, the keratinocytes undergo denuclearization, a process of nucleus deterioration that gradually diminishes cell function. The upswell of continuous movement that carries the keratinocytes along compresses their remaining cell structure. By the time the keratinocytes break through to the surface, they are flat, brittle, and lifeless husks of keratin. They layer tightly against one another, forming the tough covering the skin presents to the outside world. The friction of interacting with the external environment brushes them loose and they fall away, a process called exfoliation. Keratinocytes that funnel through the hair follicles become hair shafts.
WHY A WOUND THAT FAILS TO HEAL MAY SUGGEST SKIN CANCER
The epidermis completes a total turnover of cells every 24 to 28 days, meaning that this outer layer of the SKIN is fresh and new about once a month. Any skin wound that takes longer than three or four weeks to heal represents an area of cells that is growing at its own pace, distinct from the other skin cells—in short, a CANCER.