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• Sheet of cells that covers a body surface or lines a body cavity
– Covering and Lining: outer layer of skin, inner and outer lining of most organs and body cavities
– Glandular: creates the glands; one or more cells that make and secrete an aqueous fluid
• Barrier that most substances received by or given off from the body must pass through
• Rest upon and supported by connective tissue
• Avascular (no blood supply), but innervated (has supplied by nerve fibers)
– Nourished by blood vessels in the underlying connective tissue
• Protection
• Absorption
• Filtration
• Excretion
• Secretion
• Sensory Reception
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• Continuous multicellular sheets composed of at least 2 primary tissue types
– Epithelium bound to underlying connective tissue proper
• Cutaneous Membrane: skin
– Organ system consisting of keratinized epithelium
(epidermis) attached to a layer of dense irregular connective tissue (dermis)
• Mucous Membranes: line body cavities that open to the exterior
– Moist membranes bathed in secertions (or urine)
• Serous Membranes: found in closed body cavities
– Thin, clear serous fluid that lubricates the membranes and surrounding organs
• Skin is the largest organ in the body
• Accounts for about 7% of total body weight in adult
• Aka, Integument = “covering”
• Composed of two distinct regions: epidermis and dermis ( 3 rd hypodermis)
– Each region of epidermis consists of several layers; each layer is called a stratum
• Protection
• Body Temperature Regulation
• Cutaneous Sensation
• Metabolic Functions
• Blood Reservoir
• Excretion
• Absorption
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• Chemical Barriers
– Skin secretions—low pH (aka, acid mantle); natural antibiotics
– Melanin—barrier preventing UV damage
• Physical/Mechanical Barriers
– Continuity of skin
– Keratinized cells—hard, “shell-like” barrier
• Biological Barriers
– Immune cells in both epidermis and dermis; i.e., dendritic cells in epidermis, macrophages in dermis
• Body Temperature Rises
– Nervous system stimulates dermal blood vessels to dilate
– Sweat glands are stimulated into vigorous secretory activity
– Visible output of sweat is called sensible perspiration
• Body Temperature Falls
– Dermal blood vessels constrict
– Warm blood bypasses skin temporarily
– Skin temperature drops to external environment temperature
– Passive heat loss from body is slowed, conserving body heat
• Cutaneous Sensation
– Receptors part of the nervous system located in skin that respond to stimuli arising outside the body (touch, pain)
• Metabolic Functions
– Vitamin D production when exposed to UV from the sun
– Keratinocyte enzymes can “disarm” many cancer-causing chemicals that penetrate epidermis
• Blood Reservoir
– Dermal vascular supply can hold about 5% of the body’s entire blood volume
• Excretion
– Nitrogen-containing wastes (i.e., ammonia, urea, and uric acid) are eliminated from the body in sweat
• Absorption
– Oxygen, Nitrogen, Carbon Dioxide, Nicotine, anything lipid soluble
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• Outer part of skin
• Composed of four to five strata depending on location (i.e., thickness of skin)
• Deep to Superficial
– Stratum Basale
– Stratum Spinosum
– Stratum Granulosum
– Stratum Lucidum
– Stratum Corneum
• aka, Stratum Germinativum- deepest layer of epidermis
– In this layer the cells of the epidermis “germinate” or are formed
• Cells undergo rapid division
• Consists of a single row of the youngest
Keratinocytes
• Located at the junction between the epidermis and dermis
• Between the Stratum Basale and Stratum
Granulosum
• Cells start to round up like little balls
– cells change shape, dehydrate, and protrude into the other layers from here—hence the name from the spiny architecture of this layer
• Melanin granules and Langerhans’ cells (nonfixed) are abundant in this layer
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• Between Stratum Spinosum and Stratum
Corneum
• Here is where keratin production occurs
– stratum gets its name from granules of keratin found there
– Keratin - a tough, durable protein that is secreted and fills this stratum for protection
• holds up to lots of wear-and-tear and waterproofs the body
• actual protective mechanism in the epidermis
• 3-5 cell layers in which drastic changes in keratinocyte appearance occurs
• Between Stratum Granulosum and Stratum
Corneum in THICK skin only
• Protects soles of feet and palms of hands from abrasion
– Occurs in reposne to pressure
• Thin transparent band that contains no pigment
– A few rows of flat, dead keratinocytes
• Outermost layer of keratinized cells
– dead cells that have flattened out
• Varying ages of cells are found in this layer with oldest near the surface and youngest closest to the layer below.
– cells will remain there for two to four weeks until they are lost or washed off.
• These cells are held together by very strong cellular connections called tight junctions and desmosomes
– Peel off in sheets when you get sunburned.
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• The inner subdivision of skin
– Contains strong, flexible connective tissue
• Serves several functions
– Makes Sweat (Sudoriferous Glands)
– Secretes Oils (Sebaceous Glands)
– Gives rise to Hair & Nails
– Contains many sensory receptors and blood vessels
• Contains two types of connective tissue fibers
– Collagen
• Gives skin strength and resistance to stretching
– Elastin
• Allows your skin to stretch and then recoil back to normal length
• Two Layers
• Papillary Layer—forms projections into the epidermis
– Contains sweat and oil glands, hair follicles, blood vessels, lymph vessels, and the sensory nerves that sense changes on the surface of the epidermis
• Reticular Layer—innermost layer of dermis
– Contains the network of collagen and elastic fibers that gives skin its properties of strength and flexibility
– 80% of the thickness of skin
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• Subcutaneous tissue just deep to the skin; aka, superficial fascia
• Adipose tissue and areolar connective tissue
• Anchors skin to the underlying structures
– mostly muscles
• Strictly speaking, this is not part of the skin, but it shares some of the skin’s protective functions.
• Determined by the amount of melanin produced
– darker skin = more melanin production
• Melanocytes produce melanin
– located between the epidermis and dermis.
• Brown pigment serves two important purposes
– Primary way skin gets its color
– Protects from harmful UV rays
• 2 Additional Pigments
– Carotene: yellow to orange pigment (palms and soles)
– Hemoglobin: reddish pigment yields pinkish hues
• Exocrine Glands = glands that secrete through a tube/duct onto the surface; generally, have a local effect.
• Sweat (Sudoriferous) Glands
– Sweat is composed mostly of water (99%); 1% = potassium ions, lactic acid, ammonia, and sodium chloride
(making sweat taste salty)
• Types of sweat glands
– Merocrine: thin sweat
– Apocrine: thick sweat
– Ceruminous: cerumen (ear wax)
– Mammary: milk during lactation
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• Most numerous on the skin
• Produce watery perspiration that cools your body temperature and aids in excretion
• Most abundant on the soles, palms, and forehead
• On average, you will sweat five hundred milliliters per day without realizing it
– With heavy exercise, you could lose up to one liter of sweat per hour.
• Associated with hair follicles
– Help reduce friction between the hair follicle and the surrounding skin.
• Anogenital and axillary regions, areola, beard region of men
• Scents associated with body odor
– particularly active with stress and sexual stimulation
• Produce an oily secretion called sebum
– keeps the skin and hair supple and prevents it from becoming dry, brittle, and scaly
• Acne is a condition in which the sebaceous glands ducts become blocked
– secretions accumulate, and a bacterium colonizes the area
– genetic predisposition, or by hormone fluctuations
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• One hair is called a pilus; many hairs are called pili
• Filamentaous strands of dead keratinized cells produced by hair follicles
– hard keratin- tougher and more durable than the soft keratin of the skin
• Hair originates in the dermis of the skin at the bulb
• Composed of three layers – core called a medulla, cortex, outermost cuticle
• Pigmented by melanocytes at the base of the hair and transferred to the cells of the cortex
– Yellow, brown, black
– Red hair also has an iron-containing pigment called trichosiderin.
• Functions
– Maintain warmth
– Alert body to insects on skin
– Guarding scalp against heat loss, sunlight and physical trauma
• Hair is distributed over the entire skin surface except
– Palm, soles, lips, nipple and portions of external genitalia
• Scale-like modification of the epidermis
• Forms a clear protective covering
• Changes in nail appearance may help diagnose certain conditions
– Yellow-tinged nails = respiratory or thyroid gland disorders
– Yellow-tinged and thickened nails = fungal infection
– Outwardly concave nail (spoon nail) = iron deficiency
– Horizontal lines (Beau’s Lines) = malnutrition
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• 3 Major Types of Skin Cancer
– Basal Cell Carcinoma: cells of Stratum Basale proliferate and invade the dermis and hypodermis
• Least malignant, most common
– Squamous Cell Carcinoma: arises from keratinocytes of the Stratum Spinosum
• Scalp, ears, lower lip
– Melanoma: cancer of melanocytes
• Most dangerous – highly metastatic
• Asymmetry: two sides of pigmented area don’t match
• Border: irregular and exhibits indentations
• Color: pigmented area is black, brown, tan and sometimes red or blue
• Diameter: larger than 6mm (pencil eraser)
• Elevation: raised
• Critical if…
– 25% of the body has second degree burns
– 10% of the body has third degree burns
– Third degree burns on face, hands, feet
• First Degree: only the epidermis
• Second Degree: epidermis and upper regions of the dermis
• Third Degree: entire thickness of the skin is damaged
– Painless because the nerves have been damaged
– Dehydration, infection
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