SKIN STRUCTURE

Skin is made up of two layers.

• A) Epidermis - outer layer; its made up of 4 types of cells in 4-5 layers
• 1) There are four types of epidermal cells.
• a) keratinocytes - make up the outer, protective layer of skin cells
• b) melanocytes - produce melanin to color the skin and protect it from UV rays
• c) Merkel cells - associated with sensory receptors
• d) Langerhans cells - macrophages that help fight off infections
• 2) There are usually four layers of skin. However, the soles of the feet and palms of the hand have five layers. The extra fifth layer is the stratum lucidum (clear layer)
• a) Stratum basale (basal layer) - the deepest layer; mostly a single layer of keratinocytes that produce new skin
• b) Stratum spinosum (spiny layer)
• c) Stratum granulosum (granular layer)
• d) Stratum lucidum (clear layer) - the extra layer on palms & soles
• e) Stratum corneum (horny layer) - the outer layer of keratinized dead cells; it's 20-30 cell layers thick; waterproof
• B) Dermis - inner layer; your "hide"
• 1) made of strong, flexible connective tissue
• 2) full of nerves, blood vessels, hair follicles, oil & sweat glands
• 3) It's composed of two layers.
• a) papillary - thin; superficial; full of blood vessels; the papillary layer pushes up to form dermal ridges; these ridges (which produce epidermal ridges above) create the fingerprint pattern; sweat glands at the top edge of the ridges produce an oil pattern which we call fingerprints
• b) reticular - deeper layer which comprises 80% of the dermis; dense irregular connective tissue; contains thick bundles of interlacing collagen fibers

There are three pigments that make up skin color.

• 1) melanin
• a) made in the skin
• b) protects the body from UV radiation; the more melanin ... the more protection
• c) more melanin = darker skin; less melanin = lighter skin
• d) freckles and moles are small areas of concentrated melanin
• e) melanin production increases with exposure to the sun; more melanin is made (and the skin darkens) to better protect the skin
• 2) carotene
• a) yellow/orange pigment
• b) found in carrots and certain other plants
• c) accumulates in the stratum corneum; so the color becomes most evident in areas of thick stratum corneum (palms & soles)
• 3) hemoglobin - blood under the skin gives it a pinkish hue, especially in light-colored people

• A) Hair (pili) & hair follicles
• 1) Structure of a hair
• a) composed of the shaft (long part) and the root (in the follicle)
• b) made of 3 layers of keratin
• c) hair color is created by different levels of melanocytes (yellow, rust, brown, black) in the follicle
• 2) Structure of a hair follicle
• a) contains the root of the hair at the hair bulb (enlarge area at the base)
• b) a knot of sensory nerve endings (root hair plexus) is wrapped around the hair bulb
• c) attached to an arrector pili muscle, which erects the hair (goosebump)
• 3) Distribution, types and growth of hair
• a) Distribution of hair
• i) found all over, except for lips, nipple, palms, soles, external genitalia
• ii) there are about 100,000 in the scalp
• b) Two types of hair
• i) vellus - fine body hair; baby hair
• ii) terminal hair - thicker, darker, courser hair on the scalp, eyebrows, beard, axillary & pubic regions
• c) Rate of hair growth - 2 mm per week (that's about an inch every 3 months)
• 4) Hair thinning & baldness
• a) Alopecia - loss of hair that is not replaced; baldness; occurs in men & women
• b) Male pattern baldness - occurs in men only; caused by genetics
• B) Nails - hardened keratin at the tips of the fingers & toes
• 1) Cuticle - edge where nail meets skin
• 2) Lunula - thickened half-moon shaped area above the nail matrix; distal to the cuticle
• C) Sweat (sudoriferous) glands
• 1) Occur everywhere (except for the nipples and external genitalia); 2.5 million per person
• 2) There are 2 types.
• a) eccrine
• i) most common; especially on the palms, soles, and forehead
• ii) they secrete sweat, which is composed of: 99% water, salts, antibodies, metabolic wastes, lactic acid, and vitamin C
• b) apocrine
• i) mostly in the axillary (underarm) and apogenital areas
• ii) larger than eccrine glands
• iii) empty directly into hair follicles
• iv) though odorless when produced, bacteria quickly act as it reaches the surface and create a musky odor
• v) the name is incorrect; they are actually merocrine glands
• vi) ceruminous glands are a type of apocrine gland; located in the ear canal, they produce cerumen (earwax)
• vii) mammary glands are another specialized type that produce milk (in women)
• D) Oil (sebaceous) glands
• 1) Occur everywhere (except for the palms and soles)
• 2) Produce sebum to soften & lubricate the hair and skin; it also helps to waterproof the skin

• A) Protection
• 1) Physical (mechanical) barrier - between internal tissues and the environment
• 2) Biological barrier - macrophages & Langerhans cells help destroy foreign matter
• 3) Chemical barrier - chemicals secreted by the skin kill bacteria; melanin protects against UV radiation
• B) Body temperature regulation
• 1) As temperature rises ...
• a) sweat is produced (evaporation of sweat cools you off)
• b) dermal blood vessels dilate (enlarge); heat is transferred to the external environment
• 2) As temperature falls, dermal blood vessels constrict (shrink), forcing the blood to stay in the core areas and conserving heat
• C) Cutaneous sensations: nerves in the skin allow us to feel pain, touch, pressure, heat, and cold
• D) Metabolic functions - sunlight is converted into vitamin D in the skin; and several other chemicals are produced
• E) Blood reservoir - 5% of the body's blood is in the integument; when other areas of the body need blood (i.e. during exercise), they can "borrow" blood from the skin
• F) Excretion - nitrogeneous wastes are secreted in sweat

• A) Burns
• 1) Over 2,000,000 people are treated for burns each year; about 12,000 of them die
• 2) There are 3 levels of burns.
• a) First-degree burns - only the epidermis is burned; least severe
• b) Second-degree burns - the epidermis and upper dermis is burned; blisters occur
• c) Third-degree burns - epidermis and dermis are burned; most severe; nerve damage (ends burned off); skin is blanched (gray-white), red, or black; skin must be replaced (grafting)
• B) Skin Cancer - most often caused by UV radiation, it can be influenced by chemicals, infections, physical trauma
• 1) Basal cell carcinoma
• a) the least malignant (cancerous; spreads throughout the body) type
• b) the most common type
• c) removal of the cancer area by surgery cures 99% of all cases
• 2) Squamous cell carcinoma
• a) starts in the keratinocytes of the stratum spinosum
• b) surgical removal and radiation therapy cures most cases
• 3) Malignant melanoma
• a) cancer of the melanocytes
• b) the most dangerous type
• c) accounts for 5% of skin cancers
• d) 1/3 of cases develop from pigmented moles

• A) The lanugo coat of delicate, downy hairs cover the fetus during the 5th and 6th months of pregnancy; it's shed by the 7th month.
• B) At birth, the baby is protected by the vernix caseosa; produced by the sebaceous glands, it protects the fetus in the amniotic sac.
• C) Dermititis - inflammation of the skin; various causes
• D) Aging - as skin gets older, its rate of epidermal cell replacement slows down, the skin thins, and susceptibility to injury increases; fewer lubricating substances are produced; elastic fibers lose elasticity