The following information is regarding the structure and function of the skin, including the appendage structures derived from the skin (hairs and nails) and structures below the surface of the skin (glands and sensory receptors). You will also learn about some of the common diseases of the skin and way the skin changes over the normal life span from infancy to old age.

This maintenance of stable internal conditions is called homeostasis. Homeostasis is a concept that is essential to understanding the functioning of the human body.

As the body's outermost covering, the skin interacts directly with the environment and has several important functions. Amongst these diverse functions are limits on the entry and exit of materials, providing sensory awareness of our surroundings, and the ablity to repair itself upon injury. Clearly the skin is not merely a passive wall around the body.

• Protection and defense. The skin is an effective barrier against mechanical injury and the absorption of dangerous chemicals. It blocks penetration by all but sharp objects. It is the first line of defense against foreign agents, such as bacteria and viruses, which cause disease.
• Sensation. The skin is an early warning system for the body. Specialized sensory organs in the skin continuously monitor the sensations of touch, pressure, pain, warmth, and cold.
• Water balance. The outermost layer of the skin contains a water-resistant protein called keratin. This waterproofing reduces the loss of the body's precious internal water.
• Temperature regulation and excretion of wastes. In humans, the skin plays a direct role in limiting heat loss if the body is too cool and cooling the body if it becomes too warm. To achieve cooling, sweat glands secrete water onto the skin. As it evaporates, this water carries with it heat energy, cools the skin, and lowers body temperature. The sweat glands also secrete salts and small amounts of waste products such as urea and ammonia.
• Synthesis of vitamin D. Exposure to the ultraviolet wavelengths of sunlight causes the skin to produce small quantities of vitamin D which supplement the smount supplied in the diet. This vitamin is essential to the contruction of bones and teeth from the minerals calcium and phosphate.

Functions of the skin as they were described above are accomplished by two relatively thin layers of tissue: an outer epidermis and inner dermis. These are connected by a thin basement membrane which is somewhat anchored into the dermis. The thinner epidermis is constructed from stratified squamous epithelial tissue, while the thicker dermis is a dense connective tissue. The skin ranges in thickness from 0.5 mm over the eyelids to 6mm (about 1/4 in.) or more on areas of the hands and feet that recieve heavy wear and tear.

The Epidermis: a Thin Outer layer.

The epidermis consists up to five different layers, or strata. Of the five we will examine only two layers: the innermost layer called the stratum basale and the outermost layer, called the stratum corneum. The epidermis varies in thickness, and all five strata are present only in thick-skinned areas on the palms and the soles of the feet. The epidermis constantly undergoes growth and renewal. In regions of repeated pressure and friction, production of new cells is stimulated. The skin will formed a callus or corn as a result of pressure from abnormal wearing, so as you know, the epidermis can form thick layers to protect the underlying dermis from abrasion.

New cells are produced by mitoic cell division in the basal stratum. These cells are somewhat columnar and upon division, one daughter cell remains in the basal region, the other migrates towards the surface. This may take up to 56 days. As the new cells are produced, the older cells are pushed upward toward the outer surface of the skin, the cornified stratum.

As these cells move upward, they undergo distinctive changes, the most significant of which is the synthesis of massive amounts of the protein keratin, which eventually fills the cells.The cells that make keratin are called keratinocytes and account for about 95 percent of all the cells in the epidermis. Due to keratinization, change shape and eventually die. These dead squamous cells form the stratum corneum, a layer made of as many as 25 layers or more. In addition to waterproofing, keratin contributes great strength and toughnes to the skin and its appendage structures, the nails and the hair.

The color of the skin, as seen by the appearence of the outer layer of cells is determined by the thickness of the stratum corneum, the underlying blood vessels, and the amount of the brown-black pigment melanin. Melanocytes are a second type of cell found in the epidermis (stratum basalae). They are specialized to produce a dark pigment called melanin. These cells produce and then pass on to other surrounding cells, throughthin cytoplasmic extensions, the pigment. This process is refered to as cytocrine secretion. Although regions of the body may vary in degree of pigmentation, all people have approximatly the same number of melanocytes, despite racial variation in color.

The amount of melanin produced is determined by genetic factors, hormones, and exposure to light. Albinism is a single genetic defect (even though many genes effect pigmentation production) which prevents pigment production. Hormaonal changes during pregnancy can lead to a greater amount of pigmentation to be produced. Increased or decreased blood flow can change the 'pinkness' of the skin, and a decrease in oxygen content can lead to a blueing of the skin (cyanosis). Carotene is a yellowis-orange pigment which is lipid soluble and can be stored in the lipds of the skin. Excessive intake can give the skin an overly yellowish tint.

Melanin protects the DNA of the dividing cells in the basal stratum from damage by ultraviolet wavelegnths of sunlight. Changes in basal cell DNA can lead to skin cancer, and this melanin screen provides some protection. Skin cancer can be a serious consequence of extensive exposure to sunlight.

As with all epithelial tissues, the epidermis contains no blood vessels. Cells in the lower strata can obtain oxygen and nutriants only by diffusion from the dermis, which is well supplied with blood vessels. As the cells reach the upper strata, however, diffusion of nutriants is diminished and the cells die.

The surface of the skin we admire and care for consists of dead cells.Each day millions of dead skin cells are flaked away, to be replaced with new cells from below. Normally it takes about 4 to 6 weeks for cells to move from the basal stratum to the cornified stratum, where they are lost. Some estimates suggest that 80 percent of dust in our homes is actually dead epidermal cells. Excessive flaking of cells produces dandruff.

The Dermis: the Deep, Thick layer

The dermis thicker inner layer. The dermis lies below the epidermis and is substantially thicker than the epidermis. It is a layer of connective tissue that consists of an extracellular matrix with abundant collagen, elastic, and reticular fibers.

The dermis is made of two basic layers: the upper papillary layer and the deeper reticular layer. The reticular layer is primarily collagen and elastic fibers and is responsible for providing the skin with it's strength. Although collagen fibers lie in a multitude of directions within this layer, there is a more specific direction that they may lie, dependent on location. This allows the skin to have a greater flexibility in directions of common 'stretch'. This orientation causes what are commonly called cleavage lines. Damage across these lines such as with excessive stretching or an incision of some sort) will likely scar; damage with the line will produce little if any marking.

The papillary region is called such by a series of projections, papillae, that stretch upward into the epidermis. These help anchor the demis into the epidermis and are most numerous in regions with the greatest amount of wear: the hands and feet. Within the hands and feet, these ridges form elaborate parallel patterns, increasing friction, providing improved gripping power. The papillary region also contains numerous blood vessels to provide the epidermis with nutrients, remove waste, and help regulate body temperature.

The hair follicle is also found within the dermis, as are various glands, smooth muscle cells, sensory receptors and their associated nerves. Vitamin D production occurs in the dermis, stimulated by the sunlight which passes through the melanin protection of the epidermis.

The Hypodermis: the Underlying layer

Beneath the dermis is a layer called Hypodermis or the Subcutaneous layer. While this is not considered part of the integumentary system, it helps support the two layers of the skin by anchoring them to the muscle or bone below. This layer is made of loose connective tissue, including approximatly half the bodies fat (adipose), depending upon gender, age, and heredity. The fibers of the connective tissue is basically continous with the dermis, so no real, defined boundry exists between the two. This fat serves as padding for delicate underlying structures and insulation for retaining body heat. The hypodermis also allows the skin to easily sllide over bones and joints during movement. Medically, the hypodermis is the site for the injection of slow absorbing medications.