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DERMATOLOGY STRUCTURE AND FUNCTION Purpose of the skin The skin covers the entire body and provides protection against injuries and infection and aids in the regulation of body temperature and prevents dehydration. It contains nerve endings that constitute the four sensory senses, touch pressure, cold, warmth, and pain. It communicates with the external openings of the digestive, respiratory, and urogenital systems through the mucous membranes. The skin is composed of five layers (strata) called stratified squamous epithelium. The top most layer is the epidermis. It consists of three sub layers known as Stratum corneum, Stratum lucidum and Stratum germinativum. The second layer is the dermis and the third layer is the subcutaneous. The three combining forms for skin are derm/o, dermat/o, and cutane/o. The epithelium consists of several layers of keratin-containing cells in which the surface cells are flattened and scale-like and the deeper cells are polyhedral in form. Keratin filaments become progressively more abundant toward the surface, which on the dry surfaces of the body may consist of a layer of dead corneocytes. THE EPIDERMIS The stratum corneum, the outermost layer of the epidermis is composed of dead cells filled with a hard protein material called keratin. This keratin prevents the evaporation of body water and also prevents the entry of water. The stratum corneum also acts as a barrier to pathogens and chemicals because most of the microorganisms and chemicals cannot penetrate the unbroken skin. The stratum lucidum, a layer of lightly staining corneocytes in the deepest level of the stratum corneum; found primarily in the thick epidermis of the palmar and plantar skin. The stratum germinativum is the inner epidermal layer where cells are being continuously produced. Every three to four weeks the dead cells are pushed to the surface (stratum corneum) and are replaced with new cells. Keratinocytes and melanocytes are two important types of cells produced in this layer. Keratinocytes are the cells that produce the hard protein substance called keratin. Melanocytes produce the pigment called melanin, which gives the color to skin. Keratin is an insoluble, fibrous protein in the horny layer of the epidermis and helps to protect the body, by producing extra thickness. The thickness varies in different parts of the body and is the most in the palms and feet, resulting in callus formation with use. Melanin is a pigment produced by the melanocytes. The more melanin present, the darker the color of skin and hair and lesser the melanin, the lighter the color of skin and hair. Melanin absorbs the ultraviolet rays of the sun protecting our skin from damage. At the same time, the ultraviolet rays react with the chemicals within the skin to convert them into vitamin D, a necessary nutrient, and is absorbed through the skin. THE DERMIS OR CORIUM It is the layer immediately under the epidermis and is composed of living tissues, that consist of numerous capillaries, collagen, elastic fibers, lymphatics, and nerve endings. The sweat (sudoriferous) glands, oil (sebaceous) glands, and hair roots are also located in the dermis. The sudoriferous glands are small structures that open as pores on the surface of the skin, and are of two types. They are known as Eccrine and Apocrine. Eccrine: is found all over the body and has ducts. A tubular structure giving exit to the secretion of a gland, or conducting any fluid that open directly onto the surface of the skin and secrete a watery fluid in response to warm temperature and helps to cool the body through evaporation. Apocrine: becomes active at puberty and secretes a “milky” sweat that is broken down by bacteria that causes odor. Specialized apocrine glands produce cerumen (wax), found in the ear. The sebaceous (oil) glands: produce an oily secretion that lubricates the skin and hair. The ducts of the oil glands open into the hair follicles or directly to the surface of the skin. They secrete a liquid substance called sebum, otherwise known as oil. Their function is to prevent drying of the skin and the cracking of the hair, which could be a potential entryway for bacterial invasion. THE SUBCUTANEOUS LAYER OR HYPODERMIS It is the connective tissue layer underlying the dermis and is composed of adipose (fat) tissue and connects the skin to underlying organs like the muscles and bones. It regulates the temperature of the body. OTHER APPENDAGES OF THE SKIN Hair appears all over the body except on the palms and soles and is composed of keratin. In the dermis, there are cavities known as hair follicles and hair grows from a root within these follicles. This root is known as hair root. The part of the hair that is seen above the skin is known as hair shaft. Hair grows in two phases: a growing phase known as anagen and a resting phase known as telogen. Nails protect the fingertips and are composed of hardened keratin. The thin fold of skin at the base of the nail is known as the cuticle and the nail grows from a root within this cuticle. TERMS adipose: fat albinism: deficiency or absence of pigment in the skin, hair, and eyes, or eyes only, due to an abnormality in production of melanin. causalgia: persistent severe burning sensation subcutaneous: beneath the skin. diaphoresis: excessive sweating erythema: redness of the skin due to capillary dilatation anhidrosis: inability to tolerate heat; absence of sweat glands ichthyosis: congenital disorders of keratinization characterized by noninflammatory dryness and scaling of the skin keratosis: any lesion on the epidermis marked by the presence of circumscribed overgrowths of the horny layer leukoplakia: a white patch of oral mucous membrane lipoma: a benign neoplasm of adipose tissue, comprised of mature fat cells. melanoma: a malignant neoplasm, derived from cells that are capable of forming melanin. dermatomycosis: fungus infection of the skin caused by dermatophytes, yeasts, and other fungi. onycholysis: loosening of the nails, beginning at the free border, and usually incomplete. dermatophytosis: an infection of the hair, skin, or nails caused by any one of the dermatophytes. pilosebaceous: relating to the hair follicles and sebaceous glands. seborrhea: over activity of the sebaceous glands, resulting in an excessive amount of sebum. squamous: relating to or covered with scales. steatoma: benign tumor of fat tissue. trichopathy: any disease of the hair. subungual: below the nail. xanthoma: a yellow nodule or plaque, especially of the skin, composed of lipid-laden histiocytes. xeroderma: dry skin. DISEASES AND CONDITIONS cyst: a thick walled, closed sac, or pouch containing fluid or semisolid material fissure: a groove or crack-like sore macule: a discolored flat lesion papule: small less than 1-cm diameter solid elevation of the skin. polyp: mushroom-like growth extending on a stalk from the surface of the mucous membrane. pustule: small elevation of the skin containing pus. ulcer: open sore or erosion of the skin or mucous membrane. vesicle: small collection of clear fluid. wheal: smooth slightly elevated edematous area that is redder or paler than the surrounding skin. SYMPTOMS: alopecia: absence of hair from areas where it normally grows. ecchymosis: bluish-black mark on the skin. petechiae: small pinpoint hemorrhage. pruritus: itching. urticaria: acute allergic reaction in which red round wheals develop on the skin. purpura: merging ecchymosis and petechiae over any part of the body. vitiligo: loss of pigment in areas of the skin. ABNORMAL CONDITIONS: acne: papular and pustular eruption of the skin. burns: injury to tissues caused by heat contact. eczema: inflammatory skin disease with erythematous, papulovesicular lesions. gangrene: death of tissue associated with loss of blood supply. impetigo: bacterial inflammatory skin disease. psoriasis: chronic recurrent dermatosis marked by itchy, scaly, red patches covered by silvery-gray scales. scabies: a contagious parasitic infection of the skin with intense pruritus. scleroderma: chronic progressive disease of the skin with hardening and shrinking of connective tissue. tinea: ringworm infecting the skin. SKIN NEOPLASM callus: increased growth of cells in the horny layer of epidermis due to pressure or friction. keloid: hypertrophied, thickened scar after trauma or surgical incision. keratosis: thickened area of the epidermis. leukoplakia: white thickened patches on mucous membrane tissues of the tongue or cheek. nevus: pigmented lesion of the skin. wart: epidermal growth caused by a virus. CANCEROUS LESIONS: basal cell carcinoma: malignant tumor of the basal cell layer of the epidermis Kaposi sarcoma: malignant, vascular, neoplastic growth characterized by cutaneous nodules on the lower extremities. malignant melanoma: cancerous growth composed of melanocytes. squamous cell carcinoma: malignant tumor of the squamous epithelial cells of the epidermis. LABORATORY TESTS AND SURGICAL PROCEDURES bacterial analysis: samples of purulent material or exudates are sent to the laboratory for examination to determine what type of bacteria are present fungal tests: scrapings from skin lesions are placed on a growth medium for several weeks and then examined microscopically for evidence of fungal growth. CLINICAL PROCEDUERS cryosurgery: tissue is destroyed by the application of intensely cold liquid nitrogen. Mohs’ surgery: thin layers of a malignant growth are removed and each is examined under the microscope. allergy skin testing: tests used to determine which allergens (substance causing allergy) are responsible for causing an allergic reaction. scratch test: it is a form of skin test in which antigen is applied through a scratch in the skin. patch tests : a test of skin sensitiveness: a small piece of paper, tape, or a cup, wet with a non-irritating diluted test fluid, is applied to skin of the upper back or upper outer arm and after 48 hours the area previously covered is compared with the uncovered surface; an erythematous reaction with vesicles occurs if the substance causes contact allergy. intradermal tests: injection of an antigen into the skin for allergy testing. biopsies: Obtaining a tissue sample for microscopic evaluation to establish diagnosis. 1. excisional: complete removal of skin lesion. 2. incisional: partial removal by cutting. 3. punch: partial removal by means of a special surgical instrument. ABBREVIATIONS AE acrodermatitis enteropathica bx biopsy DLE discoid lupus erythematosus PPD purified protein derivative subq subcutaneous
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