The Skin
Structure—two primary layers called epidermis and dermis
Epidermis: the outermost and thinnest primary layer of skin
Stratum germinativum: the innermost layer of cells that continually reproduce; new cells move
toward the surface; contains pigment cells called melanocytes, that produce the brown pigment
melanin
Stratum corneum: the outermost layer of keratin-filled cells
As cells approach the surface, they are filled with a tough, waterproof protein called keratin and
eventually flake off
Vitiligo: acquired loss of epidermal melanocytes
Blisters are caused by the breakdown of the union between cells or primary layers of skin
Dermis: The deeper and thicker of the two primary skin layers
Ridges and grooves in the dermis form a pattern unique to each individual
Deeper areas of the dermis are filled with a network of tough, elastic fibers
The dermis also contains nerve endings, muscle fibers, hair follicles, sweat and sebaceous
glands, and many blood vessels
Appendages of the skin
Hair
The soft hair of the fetus and newborn is called lanugo
Hair growth requires an epidermal tube-like structure called the hair follicle
Hair growth begins from hair papilla
The hair root lies hidden in the follicle; the visible part of hair is called the shaft
Arrector pili: specialized smooth muscle that produces “goose bumps” and causes hair to stand
up straight
Receptors
Specialized nerve endings that make it possible for skin to act as a sense organ
Meissner’s corpuscle is capable of detecting light touch
The Pacinian corpuscle is capable of detecting pressure
Nails
Produced by epidermal cells over terminal ends of fingers and toes
The visible part is called the nail body
The root lies in a groove and is hidden by the cuticle
The crescent-shaped area nearest the root is called the lunula
The nail bed may change color with changes in blood flow (oxygenation)
Skin glands
Eccrine sweat glands
The most numerous, important, and widespread of the sweat glands
Produce perspiration which flows out through pores on skin surface
Function throughout life and assist in body heat regulation
Apocrine sweat glands
Found primarily under the arms and around genitalia
Secrete a thicker, milky secretion quite different from eccrine perspiration
Breakdown of secretions by skin bacteria produces odor
Sebaceous glands
Secrete oil for hair and skin
The level of secretion increases during adolescence
The amount of secretion is regulated by sex hormones
Sebum in sebaceous gland ducts may darken to form blackheads
Acne vulgaris involves inflammation of the sebaceous gland ducts
Functions of the Skin
Protection: a first line of defense
Against infection by microbes
Against ultraviolet rays from sun
Against harmful chemicals
Against cuts and tears
Temperature regulation
Skin can release almost 3000 calories of body heat per day
Mechanisms of temperature regulation
Regulation of sweat secretion
Regulation of the flow of blood close to the surface of the skin
Sense organ activity
Our skin functions as an enormous sense organ
Skin receptors serve as receivers for the body, keeping it informed of changes in its
environment
Skin Lesions
Elevated lesions cast a shadow outside their edges
Papule: a small, firm raised lesion (e.g. warts)
Plaque: a large raised lesion (caused by friction)
Vesicle: a small, fluid-filled blister (e.g. non-genital herpes)
Pustule: a pus-filled lesion (e.g. acne)
Crust: a scab (dried blood and/or exudate)
Wheal (hive): a raised, irregularly-shaped, firm lesion with a light center
Flat lesions do not cast a shadow
Macule: a flat, discolored region (e.g. freckle)
Patch: a macule larger than 1cm (e.g. vitiligo)
Depressed lesions cast a shadow within their edges
Excoriation: a lesion with missing epidermis, as in a scratch wound
Atrophy: areas showing loss of tissue (e.g. striae)
Ulcer: a craterlike lesion caused by disintegration of the skin (e.g. bedsore)
Fissure: a deep crack or break from the epidermis to the dermis (e.g. athlete’s foot)
Skin Infections
Impetigo is a highly contagious staphylococcal infection
Tinea is a fungal infection (mycosis) of the skin; several forms occur
Boils (furuncles) are staphylococcal infections of hair follicles
Scabies is a parasitic infection (mite) passed through skin-to-skin contact
Vascular and Inflammatory Skin Disorders
Decubitus ulcers (bedsores) develop when pressure slows down blood flow to local areas of the
skin
Urticaria (hives) are red lesions caused by fluid loss from blood vessels
Scleroderma is a disorder of vessels and connective tissue characterized by hardening of the
skin
Psoriasis is a chronic inflammatory condition accompanied by scaly plaques
Eczema is a common inflammatory condition characterized by papules, vesicles, and crusts; it is
not a disease itself but a symptom of an underlying condition
Skin Cancer
Squamous cell carcinoma is the most common type, characterized by hard, raised tumors; slowgrowing, painless, but likely to metastasize
Basal cell carcinoma is typically found on the upper face; it is characterized by papules with a
central crater; rarely spreads
Melanoma is a malignancy in a nevus (mole); it is the most serious type of skin cancer
Skin Cancer
The most important causative factor in common skin cancers is exposure to sunlight
Kaposi sarcoma, characterized by purple lesions, is associated with AIDS and other immune
deficiencies; it may be caused by a virus