Integumentary System

• An organ is a body structure with two or more tissue types performing specific functions.
• Skin is the body's largest organ by weight.
• Accessory structures of the skin include hair, nails, glands, and sensory receptors.
• Skin and its accessory organs constitute the integumentary system.
• Skin acts as a barrier between the internal and external environments.

Skin and its tissues

• Skin consists of two major layers: the epidermis and the dermis.
• The epidermis is a thin, outer layer composed of stratified squamous epithelium.
• The dermis is a thicker, inner layer of connective tissue, blood vessels, smooth muscle, and nervous tissue.
• A basement membrane separates and anchors the epidermis from the dermis.
• The subcutaneous layer (hypodermis) is located underneath the dermis and consists of areolar and adipose tissues.
• The subcutaneous layer binds skin to underlying tissues but is not part of the skin.
• Adipose tissue in the subcutaneous layer insulates to conserve body heat.
• Major blood vessels that supply the skin are located in the subcutaneous layer.

The epidermis

• Consists of stratified squamous epithelium and lacks blood vessels.
• There are usually 4 layers in the epidermis, but there are 5 layers in thick skin.
• The deepest layer, the stratum basale (or stratum germinativum), consists of dividing cells nourished by dermal blood vessels.
• As basal cells divide, older cells, called keratinocytes, migrate toward the skin surface due to the accumulation of fibrous protein, keratin.
• As keratinocytes migrate outward, they harden, dehydrate, and die through keratinization.
• The epidermis protects against water loss, mechanical injury, chemicals, and microorganisms.
• The stratum basale is the innermost, dividing layer.
• The stratum corneum is the outermost layer of dead, flattened, keratinized cells which are continually shed.
• The stratum lucidum is found only in the thick skin of the palms and soles, between the stratum granulosum and the stratum corneum.

Melanocytes

• Specialized cells in the epidermis that produce the pigment, melanin.
• Melanocytes are found in the deepest layer of the epidermis and in the dermis.
• Melanin protects DNA of skin cells against damaging effects of UV radiation from the sun.
• Melanin granules shade the nucleus of a cell from UV radiation to prevent mutation in the DNA.
• Melanin pigment provides skin and hair color - the more melanin, the darker the skin or hair.
• Melanin is transferred from melanocytes to nearby cells through cytocrine secretion.
• Eumelanin is brownish-black, while pheomelanin (found in areas such as the lips) is reddish-yellow.

Skin color

• All people have about the same quantity of melanocytes.
• Skin color is genetically determined depending on: the amount of melanin melanocytes produce, and the size and distribution of melanin granules.
• Skin color results from a combination of genetic, environmental, and physiological factors.
• Exposure to sunlight, UV light from sun lamps, and X-rays darkens skin because of an increase in melanin production.
• Oxygen-rich blood has a bright red color due to hemoglobin and causes light-skinned people to look pinkish.
• Oxygen-poor blood has a deep red tone causing light-skinned people to appear bluish, which is called cyanosis.
• Yellowish skin color can come from eating too many orange or yellow foods with carotene.
• Bright yellow skin color can be caused by jaundice due to liver disease.

The dermis

• Binds the epidermis to underlying tissues.
• The border between the epidermis and dermis is uneven due to epidermal ridges (which extend down toward dermis) and dermal papillae (which. extend upward toward epidermis).
• Genetically determined friction ridges formed by dermal papillae determine unique fingerprints.
• The dermis consists of areolar and dense connective tissue, with collagen and elastic fibers within a gel-like ground substance.
• The fibers provide toughness and elasticity to skin.
• Dermal blood vessels carry nutrients to upper layers of skin, and also help regulate body temperature.
• The dermis also contains nerve fibers, sensory receptors, hair follicles, sebaceous glands, and sweat glands.

Nails

• Protective coverings over the ends of fingers and toes
• Consist of a nail plate overlying a layer of skin surface, called the nail bed
• Lunula is a half-moon-shaped structure at base of nail plate; visible portion of nail matrix; the most actively growing region of the nail
• As new cells are produced, older ones are pushed outward and become keratinized, just as in other parts of the skin
• Keratin of nails that is harder than that of the stratum corneum of the rest of the skin
• Nail appearance is affected by genetics, injury, nutritional deficiencies, disease and aging, and can be used in medical diagnosis.

Hair and hair follicles

• Hair is found in nearly all regions of the skin except palms, soles, lips, nipples, and portions of external genitalia.
• Each hair develops from epithelial stem cells at the base of a tube-like depression called the hair follicle.
• Hair follicles dip down into the dermis or sometimes the subcutaneous layer.
• The deepest part of the hair root is called the hair bulb and is located at the base of the hair follicle.
• The hair matrix lies within the hair bulb and is composed of epithelial cells nourished by dermal blood vessels in the hair papilla at the base of the hair bulb.
• As new cells form in the bulb, old cells are pushed outward and become keratinized, and die, forming the hair shaft.
• Hair is composed of dead, keratinized epithelial cells.
• Hair color is determined by genetics, which determine the type and amount of melanin produced by epidermal melanocytes.
• Melanin produced by melanocytes is responsible for most hair colors.
• Dark hair contains eumelanin (brownish-black), while blonde and red hair contain more pheomelanin (reddish-yellow).
• Genetic lack of melanin, or older age, causes albinism (white).
• A mixture of pigmented and unpigmented hair is gray.
• A bundle of smooth muscle cells, called an arrector pili muscle, attaches to each hair follicle; when it contracts, it causes goose bumps due to response to cold temperature or emotional upset.

Skin Glands

• Sebaceous glands secrete an oily substance called sebum, that waterproofs and softens the hair and skin
• Sebaceous glands are holocrine glands; entire cells filled with secretion are released
• Sebaceous glands associated with hair follicles
• Sweat glands (sudoriferous glands) are merocrine glands, which means secretions exit cells via exocytosis
• Eccrine sweat glands respond to body temperature; they are more abundant and many are found on forehead, neck, and back; and secrete onto skin surface
• Apocrine sweat glands (incorrectly named) are merocrine glands that become active at puberty, respond to fear, emotional upset, pain, or sexual arousal, most numerous in axilla and groin, sweat contains proteins & fats that produce body odor, secrete into hair follicles.
• Ceruminous glands secrete wax in the ear canal, to trap dust and pathogens
• Mammary glands secrete milk to nourish a baby

Skin Functions

• Important for maintaining homeostasis
• Protective coating prevents many substances and pathogenic microorganisms from entering body and protects underlying tissues
• Waterproofing slows down water loss by diffusion
• Melanin pigment protects against UV radiation from the sun
• Houses sensory receptors for touch, pressure, pain, temperature
• Excretes wastes
• Conducts some chemical reactions in the synthesis of vitamin D; vital for bone and tooth development
• Body temperature regulation, via sweat gland activation, and vasodilation or vasoconstriction of dermal blood vessels
• Synthesizes certain biochemicals

Role of Skin in Body Temperature Regulation

• Proper temperature regulation is vital to maintaining optimal metabolic reaction rates
• The skin plays a major role in temperature regulation; the hypothalamus controls the process.
• Active cells, such as those of the heart and skeletal muscle, and the liver, produce heat.
• Heat escapes the skin through radiation and evaporation of sweat.
• Vasodilation of dermal blood vessels; more blood enters skin and is released to regulate excess heat (hyperthermia).
• Sweat glands are activated, as sweat is released, it evaporates, cooling the skin in response to excess heat (hyperthermia).
• Vasoconstriction of dermal blood vessels, keeping blood in core in response to excess cooling (hypothermia).
• Inactivation of the sweat glands in response to excess cooling (hypothermia).
• Shivering, an involuntary form of skeletal muscle contraction, causes an increase in cellular respiration resulting in heat is released a byproduct, in response to excess cooling (hypothermia).

Healing of Wounds

• Inflammation, in which blood vessels dilate and become more permeable, causing tissues to become red and swollen, is the body's normal response to injury or stress

Characteristics of Inflammation

• Redness is caused by Vasodilation, where there is more blood in area
• Heat is caused by large amount of blood accumulating in area and as a by-product of increased metabolic activity in tissue
• Swelling is caused by increased permeability of blood vessels, as fluids leaving blood go into tissue spaces causing edema
• Pain is cased by injury to neurons and increased pressure from edema
• Dilated blood vessels bring in fluids, oxygen, nutrients, and immune cells to aid healing
• Superficial cuts are filled in by reproducing epithelial cells
• Deeper injuries that extend into the dermis or subcutaneous layer, require a more extensive response

The Healing of a Deep Wound

• Deep injury, extending into the dermis or subcutaneous layer involves the formation of a blood clot from the released blood
• The blood clot and dried tissue fluids form a scab to cover the wound
• Fibroblasts migrate into the area, and secrete collagen fibers to bind the edges of the wound together
• Phagocytic cells remove debris and dead cells
• Damaged tissue is replaced and the scab sloughs off Extensive collagen fiber production in the area may form an elevated area called a scar Large wounds leave scars and healing may be accompanied by the formation of rounded masses called granulations
• A new branch of a blood vessel grows into the area
• The vessel is accompanied by a cluster of fibroblasts that begin producing collagen fibers and repairing the wound
• Eventually, blood vessels are resorbed, the fibroblasts vacate, and a scar of collagen fibers is left behind

Common Skin Disorders

• Acne is disease of the sebaceous glands that produces blackheads and pimples
• Keloid is an elevated, enlarging fibrous scar usually initiated by an injury
• Alopecia is classified as hair loss, usually sudden
• Mole (mohl) is a Benign skin tumor (nevus) that is usually pigmented; colors range from brown to black
• Athlete's foot is caused by Fungus (Tinea pedis) infection usually in the skin of the toes and soles
• Pediculosis is a disease produced by an infestation of lice
• Birthmark is a congenital blemish or spot on the skin, visible at birth or soon after
• Pruritus is itching of the skin
• Boil (boyl) is caused by Bacterial infection (furuncle) of the skin, produced when bacteria enter a hair follicle
• Psoriasis is a chronic skin disease characterized by red patches covered with silvery scales
• Carbuncle is a bacterial infection, similar to a boil, that spreads into the subcutaneous tissues
• Pustule is an elevated, pus-filled area on the skin
• Cyst is a liquid-filled sac or capsule
• Scabies is a disease resulting from an infestation of mites
• Dermatitis is simply inflammation of the skin
• Seborrhea is hyperactivity of the sebaceous glands, causing greasy skin and dandruff
• Eczema is classified as a noncontagious skin rash that produces itching, blistering, and scaling
• Ulcer is an open sore
• Erythema is the reddening of the skin due to dilation of dermal blood vessels in response to injury or inflammation
• Urticaria is an allergic reaction of the skin that produces reddish, elevated patches (hives)
• Herpes is an infectious disease of the skin, caused by the herpes simplex virus and characterized by recurring formations of small clusters of vesicles
• Vitiligo is the loss of melanocytes in parts of the epidermis, producing whitened areas of skin
• Impetigo is a contagious disease of bacterial origin characterized by pustules that rupture, and become covered with loosely held crusts
• Wart is a flesh-colored raised area caused by a viral infection

Test your knowledge of the skin's structure and function. This quiz covers epidermis, dermis, cutaneous glands, sensory receptors, and thermoregulation. Explore the role of melanin, keratinocytes in the integumentary system.