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Integumentary System

Basic Skin Functions
Structure of the Skin
Appendages of the Skin
Homeostatic Imbalances of Skin
Developmental Aspects of Skin

Basic Skin Functions

Skin functions include protection of the deeper tissue from chemicals, bacteria, bumps, and drying; regulation of body temperature through radiation and sweating; and synthesis of defensive proteins and vitamin D. The cutaneous sensory receptors are located in the skin.

Structure of the Skin

1. Epidermis

   The epidermis, the more superficial part of the skin, is formed of stratified squamous epithelium which contains keratin and lacks blood vessels. Cells at its surface are dead and continually flake off. They are replaced by division of cells in the basal layer. As the cells move away from the basal layer, they accumulate keratin and die. Melanin, a pigment produced by melanocytes, protects the nuclei of epithelial cells from damaging rays of the sun (Figure 4.1).

2. Dermis

   The dermis is composed of dense connective tissue. It is the site of blood vessels, nerves, and epidermal appendages. It has two regions, the papillary and reticular layers. The papillary layer has ridges, which produce fingerprints.

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Appendages of the Skin

Skin appendages are formed from the epidermis but reside in the dermis.

1. Sebaceous glands produce an oily product (sebum), usually connected by a duct into a hair follicle. Sebum keeps the skin and hair soft and contains bacteria-killing chemicals (Figure 4.2).

2. Sweat (sudoriferous) glands, under the control of the nervous system, produce sweat, which is connected by ducts to the epithelial surface. These glands are part of the body's heat-regulating apparatus.

3. A hair is primarily dead keratinized cells and is produced by the hair bulb. The root is enclosed in a sheath, the hair follicle (Figure 4.3).

4. Nails are hornlike derivatives of the epidermis. Like hair, nails are primarily dead keratinized cells (Figure 4.4).

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Homeostatic Imbalances of the Skin

1. Infections and Allergies
   

   The most common skin disorders result from bacterial, viral, or fungal infections or allergies. For example, athlete's foot results from a fungal infection. Contact dermatitis is caused by exposure of the sun the to chemicals (e.g., those in poison ivy) that provoke allergic responses in sensitive individuals.

2. Burns
   

   Burns result in loss of body fluids and invasion of bacteria and represent a major threat to the body. The severity (depth) of burns is described as first-degree (epidermal damage only), second-degree (epidermal and some dermal injury), and third-degree (epidermis and dermis totally destroyed). Third-degree burns require skin grafts.

3. Skin Cancer
   

   The most common cause of skin cancer is exposure to ultraviolet radiation. Cure of basal cell and squamous cell carcinoma is complete if they are removed before . melanoma, a cancer of melanocytes, is still fairly rare but is fatal in about half the cases.

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   Developmental Aspects of Skin

   1. The skin is thick, resilient, and well hydrated in youth but loses elasticity and thins as aging occurs. Skin cancer is a major threat to skin exposed to excessive sunlight.

   2. Balding and/or graying occurs with aging. Both are genetically determined, but other factors (drugs, emotional stress, and so on) can result in either.

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