Integument = skin

Only one organ in this system --but it's the largest organ of the body

Therefore, the skin is an organ AND a system

Functions of skin

	Protection - mechanical, UV radiation, immune "first line" and "second line," water conservation
	Excretion - sweat glands excrete "waste"
	Chemical synthesis - vitamin D
	Thermoregulation - can regulate heat loss or conservation
	Sensation - various sense of touch, temperature, vibration, pain

General structure     GA   pp

	Epidermis - stratified squamous epithelial outer layer
	Dermis - dense fibrous connective inner layer (usually thicker than the epidermis)
	Hypodermis - loose fibrous tissue under skin (hypodermis is NOT part of skin, but where else can we discuss it?)
	Thickness varies Thick skin - Thickest skin found on palms, fingertips, sole Thin skin - Thinnest skin found on scalp, near lips

Composed of different strata, the outermost of which are comprised of dead cells

	Cells divide only in base layer, then die as they are pushed outward
	Form a "keratinized" layer on surface that protects

Strata (listed from deep to superficial)     GA    pp

	Stratum germinativum ("growth layer")  - made up of two sublayers: Stratum basale ("base layer") Keratinocytes Keratin (tough, water-resistant protein) Melanocytes Melanin (brown protein pigment) is produced by melanocyte cells in base layer Melanin, being dark, absorbs UV radiation Melanin is released by melanocytes, which have long extensions, and absorbed into surrounding keratinocytes Two categories of melanin: Eumelanins - dark brown Pheomelanins - light brown/red/orange Cells of stratum basale are closest to the blood supply in the dermis and are thus the healthiest    pp Only these cells can reproduce Stratum spinosum ("spiny layer") Cells are pushed from below and become "squished" and look "spiny" on cross sectional view Because the cells are farther away from the dermal blood supply, they are less healthy and thus don't reproduce Cells get sicker and sicker as they are pushed away from the dermis
	Stratum granulosum ("grainy layer") As cells from stratum germinativum die, they enter stratum granulosum All cells here are dead (and look grainy when stained/ no nuclei) --but biochemical changes are happening inside them Keratohyalin (a precursor to keratin) forms in granules here
	Stratum lucidum ("light layer" or "clear layer") Keratohyalin is transformed into eleidin, which is almost transparent, making this layer look almost "clear"
	Stratum corneum ("horny layer" meaning "like an animal's horn" --not what YOU were thinking!) Keratin is fully formed Flattened cells filled with keratin form "keratinized layer" and make this "keratinized stratified squamous epithelium"

Epidermis joins dermis at the glue-like dermal-epidermal junction

Dermis is irregular dense fibrous connective tissue

Two layers of the dermis:

	Papillary region     GA Outer (superficial) region of dermis has bumps called dermal papillae FYI - bumps in the human body are OFTEN called papillae (sing. papilla = "nipple") Increase surface area for glue to "hold" more tightly Arranged in rows to form "prints" of hands/fingers and feet/toes to improve grip (and to identify the perpetrators of crimes against professors)
	Reticular region  Deeper area of the dermis has irregular swirls of collagen fibers plus nerves and nerve endings, blood vessels, sweat glands, and more Reticular = "network"

Scars

	Form when a denser-than-usual mass of fibers are produced to replace those damaged in an injury
	Keloid scars are abnormally large scars
	Stretch marks occur when the skin is overstretched and the dermis tears beneath the epidermis   pp
	Read about dermal cleavage lines in the textbook

Also called "subcutaneous tissue" (sub = "under"  cutaneous = "skin")

Also called "superficial fascia"

	Fascia ( = "gang") is made up of loose bundles ("gangs") of fibers
	Superficial fascia is under the skin; deep fascia is among the muscles

Loose fibrous tissue

	areolar (loose, ordinary)
	adipose (formed when fat cells of areolar tissue grow large and dominate the tissue)

NOT part of skin proper

Functions

	Protection
	Thermoregulation
	Shape
	Storage of fat (fuel storage)

Hair     GA

	Grows from hair follicles (follicle = "little pocket") Hair papilla is bump at base of pocket Covered with stratum germinativum Place from which the hair grows Arrector pili Small muscles attached to follicle and dermal-epidermal junction Contract when a person is chilled or stressed Additional arrector muscles in the skin around the nipple cause the nipple to become erect    pp A&P trivia: do you know what the word horripilation means? (click it to find out!)
	Hair shaft (visible part) and root (not seen; in follicle)     GA Dense, cylindrical modification of keratinized layer Inner part: medulla Outer part: cortex  Shape can affect curling (flatter = curlier)
	Usually pigmented with melanin  except in white hair (such as in the platinum blonde highlights sported by your professor) cortex can be pigmented differently than medulla

Nails     GA

	Platelike, denser version of hair (never pigmented in humans)
	Nail body is visible part; nail root is still in follicle (nail groove) Usually no pigmentation (easy to see changes in blood color under nail) May have pigmented streaks Lunula (= "little moon") Whitish crescent at base of nail Base of nail root is fused with connective tissue covering bone of finger- stabilizing nail
	Nail bed is skin under the nail Stratum germinativum only

Sweat glands (exocrine)

	Sweat contains mostly water plus some excretions and secretions (such as pheromones [sex attractants])
	Different types    pp Eccrine - primarily thermoregulatory Apocrine - involved in stress response

Sebaceous glands (exocrine)

Secrete  sebum  or "skin oil" conditions hair and skin to prevent damage

Surface film

	Combination of sweat, sebum, dead cells, bacteria, etc.
	A mini ecosystem

Blood in the dermis is like the fluid in a radiator, bringing heat from other parts of the body

Blood flow in the dermis increases or decreases as the body tries to gain or lose heat

Conduction

Heat is conducted from blood through skin to air (or water or the chair you are sitting on right now)

Convection

Warm air rises (because it is less dense than cool air), creating a convection current that draws cooler air in behind it

Radiation

Heat travels as radiant energy (like light or radio waves), specifically as infrared (light waves beyond visible red)

Evaporation

When water turns to steam (evaporation) heat moves (from skin) to the water molecules as they leave as water vapor