Chapter 5 Objectives
1. Describe the characteristics of the 4 types of cells in the epidermis.
2. Describe the characteristics and order of the cell layers of the epidermis.
3. Describe the regions of the dermis and hypodermis.
4. Describe the structure and functions of hair, hair follicles, and nails.
5. Describe how hair color is determined and what leads to alopecia.
6. Contrast the 3 types of skin cancer.
7. Define the ABCD rule for evaluating skin lesions.
The skin and its derivatives (sweat & oil glands, hair & nails) make up a very complex set of organs that serves a number of functions, mostly protective. Together these components make up the integumentary system (covering). The skin covers the entire body, has a surface area of 1.5-2.0 square meters, and weighs about 4 kg in the average adult. It is estimated that every square centimeter of the skin contains: 70 cm of blood vessels, 55 cm of nerves, 100 sweat glands, 15 oil glands, 230 sensory receptors, 1/2 million cells.
The skin is composed of two distinct regions, the epidermis and the dermis. These two areas are firmly attached to one another along a wavy borderline. The epidermis (epi = upon) composed of epithelial cells, and is the outermost protective shield of the body. The underlying dermis, making up the bulk of the skin, is a tough leathery layer composed of connective tissue proper. Only the dermis is vascularized; nutrients reach the epidermis by diffusing through the tissue. The subcutaneous tissue (just deep to the skin) is known as the hypodermis or superficial fascia. It is made up of loose connective tissue; approximately half of the body's fat stores are located in this region. The hypodermis anchors the skin to the underlying organs and allows the skin to move relatively free. It also acts as a shock absorber and insulates the deeper body tissues from heat loss. The hypodermis is not considered part of the skin.
Structurally, the epidermis is a thick keratinued stratified squamous epithelium consisting of four distinct cell types and five distinct layers.
Cells of the Epidermis
Cells populating the epidermis include: keratinocytes, melanocytes, Merkel cells, and Langerhans cells.
Keratinocytes: The most numerous cells are the keratinocytes which produce keratin, a fibrous protein responsible for protective properties of the epidermis. They arise from the deepest part of the epidermis from cells undergoing almost continuous mitosis. The keratinocytes are organized into 4-5 cell layers depending on body location. By the time the cells reach the surface of the skin, they are dead, scale-like structures. Every 35-45 days a totally new epidermis occurs. In areas of highest friction ( hands, feet) both cell production and keratin formation is accelerated.
Melanocytes: located at the base of the epidermis. Specialized cells that synthesize the pigment melanin. Melanin protects the cell nucleus from the destructive effects of UV radiation. Since all humans have the same relative number of these cells, individual and racial differences in skin coloring are probably due to differences in melanocyte activity.
Langerhans cells: arise from the bone marrow and migrate to the epidermis and other areas of the body containing stratified squamous epithelial tissue. They are macrophages and cooperate with T helper cells to assist in the immune response.
Merkel cells: present in small numbers at the epidermal-dermal junction. Associated with a disc-like ending of a sensory nerve fiber, called a Merkel disc, which functions as a sensory receptor.
Layers of the Epidermis:
In thick skin (palms, fingertips, soles of feet) the epidermis consists of five layers or strata: (from deep to superficial) stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, stratum corneum. Thin skin, which covers the rest of the body has only 4 layers, with the stratum lucidum absent.
Stratum basale: single layer of cuboidal to columnar shaped cells. It is separated from the dermis by the basement membrane. Some cells move toward the surface while others migrate into the dermis and gives rise to sweat and oil glands. Many mitotic cells are seen. About 25% of the cells in this layer are melanocytes.
Stratum spinosum: contains 5-10 rows of cells fitted closely together. The surface of the cells display minute spiney projections. Mitosis occurs here but not as frequently. Langerhans cells are scattered among the keratinocytes. Because cells superficial to this layer do not receive adequate nutrients, they become less viable and finally begin to die.
Stratum granulosum: thin zone consisting of 3-5 layers of flattened cells. Keratinization begins in this third epidermal layer. The plasma membranes of these cells also thicken so that they become more resistant to destruction. Langerhans cells are also found in this layer. At the upper border of this layer, the cells die and lysosomes begin to digest their organelles.
Stratum lucidum (clear layer): translucent band just above the S. granulosum. Consists of a few rows of flattened dead keratinocytes with indistinct boundaries. Present only in thick skin.
Stratum corneum: outermost layer; a broad zone 20-30 cell layers thick. Accounts for about 3/4 of the epidermal thickness. The shingle-like dead cells are remnants, completely filled with keratin fibrils, and are referred to as cornified or horny cells. Keratin provides a durable abrasion resistant and water-repellent "overcoat" protecting deeper cells from the environment.
The second major skin region, is a strong but flexible connective tissue layer. The cell types found in the dermis are fibroblasts, macrophages, and occasional mast cells and white blood cells. Its gel-like matrix is heavily embedded with collagen, elastin, and reticular fibers. The dermis is your "hide" and is richly supplied with nerve fibers, blood vessels, and lymphatic vessels. The major portions of hair follicles, as well as oil and sweat glands, reside in the dermis, but are derived from epidermal tissue. The dermis varies in thickness and it has two major layers: papillary and reticular.
Papillary: Thin superficial layer of loose connective tissue fibers. Forms a loosely woven mat that is heavily invested with blood vessels. Its superior surface is thrown into nipple-like projections called dermal papillae, that indent the epidermis above. Many dermal papillae contain capillary loops; others house free nerve endings and touch receptors (Meissner's corpuscles). On the ventral aspect of the hands and feet, the papillae are arranged in definite patterns that are reflected in the conspicuous looped and whorled ridges which enhance the gripping ability of the fingers and feet.
Reticular: accounts for about 80% of the dermis and is a typical dense irregular connective tissue. It contains bundles of interlocking collagen fibers that run in various planes parallel to the skin surface. The fibers interlace in a netlike manner with the spaces between the fibers occupied by a small amount of adipose tissue, hair follicles, nerves, oil glands and ducts of sweat glands. The connective tissue fibers of the dermis give skin its strength and resiliency. Collagen binds water, thus helping to maintain the hydration of the skin. The reticular region is attached to underlying organs (bones, muscles) by the subcutaneous layer.
Skin Color: Three pigments contribute to skin color: melanin, carotene, and hemoglobin. Melanin ranges in color from yellow to brown to black. Racial differences in skin coloring reflect the relative kind and amount of melanin made. Freckles and pigmented moles are local accumulations of melanin. Melanocytes are stimulated to greater activity when exposed to sunlight. Prolonged sun exposure causes a substantial melanin buildup, which helps protect viable skin cells from UV radiation. Despite melanin's protective effects, excessive sun exposure eventually damages the skin.Excessive sun exposure causes clumping of the elastin fibers, leading to "leathery skin", temporarily depresses the immune system, and can alter the DNA of skin cells leading to cancer. The yellowish tinge of skin of some Asian peoples is due to variations in melanin. The pinkish hue of fair skin reflects the crimson color of oxygenated hemoglobin in the red cells circulating through the dermal capillaries. Since fair skin contains only small amounts of melanin, the epidermis is quite transparent and allows hemoglobin's rosy color to show through.
Appendages of the Skin: Skin appendages include hairs and hair follicles, nails, sweat glands, and sebaceous glands. Each is derived from the epidermis and has a unique role in maintaining body homeostasis.
Hairs and Hair Follicles: Distributed over nearly the entire body surface and has some minor protective functions, such as guarding the head against physical trauma, heat loss, and sunlight. Eyelashes shield the eyes and nose hairs help keep dust and other foreign particles out of the respiratory tract.
Structure: Flexible strand-like structure produced by the hair follicle. Consists largely of fused keratinized cells. Chief regions of a hair are: shaft and root. Shape of the shaft determines the shape of the hair. A hair has 3 concentric layers of keratinized cells:
central core -- medulla
cortex -- bulky layer -- several layers of flattened cells
cuticle -- single layer of overlapping cells, most heavily keratinized, provides strength, subject to most abrasion.
Hair pigment is made by melanocytes at the base of the hair follicle. Different proportions of melanin lead to different colors (yellow, black, brown). They combine to produce all varieties of hair color. Gray or white hair results from a decrease in melanin production and the replacement of melanin by air bubbles in the hair shaft.
Extends from the epidermal surface to the dermis. The deep end of the follicle is expanded forming a bulb. A knot of sensory nerve endings (root hair plexus) wraps around the bulb. Hair acts as sensitive touch receptors. Papilla is a nipple-like bit of tissue containing a knot of capillaries which protrudes into the hair bulb and supplies it with nutrients. The epithelial cells of the hair matrix (bulb) are actively mitotic germinal centers which produce the hair. As new hair cells are produced the older parts are pushed up. The cells fuse and are increasingly keratinized and they die. The hair shaft is made almost entirely of keratin.
Structures associated with hair follicles: Nerve endings, Sebaceous glands, Smooth muscle structures -- arrector pili
Most hair follicles approach the skin surface at a slight angle. The arrector pili attach to the follicle so that their contraction pulls the hair into an upright position, dimpling the skin surface and producing "goose bumps." The arrector pili are regulated by the nenous system.
Distribution: Millions of hairs are scattered over nearly all the body. Only the lips, nipples, parts of the external genitalia, and the thick skin areas of the palms and soles lack hair.
Various sizes and shapes of hair:
vellus - pale, fine hair - body hair of children and adults
terminal - coarser, often longer hair of the eyebrows & scalp
Hair growth and density are influenced by many factors including nutrition and honnones. Rate of growth varies with sex and age. Averages 2mm/week. Each follicle goes through growth cycles (active growth & rest). Life span varies. The follicles of the scalp remain active for years before becoming inactive for a few months. The eyebrows remain active for only 3-4 months.
Thinning and Baldness:
Hair growth is fastest from the teen years to the 40's and then growth slows. Alopecia (baldness) - process begins at the anterior hairline and progresses posteriorly. Course hairs are replaced by vellus hairs. Male pattern baldness is genetically determined. Hair thinning is induced by a number of factors that lengthens the follicular resting phase. Examples of stressors include acutely high fever, surgery, severe emotional trauma, certain drugs, protein-deficient diets and some antidepressants.
Scale-like modifications of the epidermis that forms a clear protective covering on the dorsum of the distal part of a finger or toe. Each nail has a free edge, a body, and a root embedded in the skin. The thickened proximal part of the nail bed - nail matrix - is responsible for nail growth. Nail cells are heavily keratinized. Nail body slides distally over the nail bed during growth.
More than 2.5 million sweat glands are distributed over the entire body. Two types of sweat glands: eccrine and apocrine. Eccrine - more numerous especially on the palms, soles and forehead. They secrete a hypotonic solution (sweat) derived from blood plasma. It is 99% water with some other substances in it. Sweating is regulated by the sympathetic nervous system. Assists in thermoregulation. Apocrine - largely confined to the axillary and anogenital regions. They are larger in size. The secretions are the same as eccrine with the addition of fatty substances and proteins which make the secretions viscous. The secretions are odorless until bacterial decomposition occurs. Precise function unknown. Activated by the sympathetic nervous system during pain or stress.
Ceruminous Glands - secrete ear wax.
Mammary Glands - secrete milk.
Sebaceous (oil) Glands: Simple glands found all over the body except on the palms and soles. They are small on the body trunk and limbs and larger on the face, neck, and upper chest. Secrete an oily secretion called sebum. The sebum is usually ducted into a hair follicle. Sebum softens and lubricates the hair and skin and it has a bactericidal action. Acne is an active inflammation of the glands accompanied by "pimples." Acne is usually caused by bacteria, especially staphylococcus.
Homeostatic Imbalances: Because of the skin's complexity and extensiveness, it can develop more than 1000 different conditions and ailments. The most common skin disorders result from bacterial, viral, or yeast infections.
Most are benign and do not spread. Some are malignant and invasive. The most important risk factor is over exposure to U.V. radiation.
A: asymmetry - 2 sides do not match
B: irregular border - not smooth
C: color - pigmented spot
D: diameter - larger than 6mm
Therapy is wide surgical excision and chemotherapy.