Integumentary System: Skin Structure and Function

• The integumentary system is the first body system studied.
• Organs and tissues work together to perform functions in a body system.
• The integumentary system is comprised of the skin and epidermal derivatives.
• Accessory organs include hair follicles, sebaceous glands, sweat glands, and nails.
• Stratified squamous epithelium, glandular epithelium, dense irregular CT, smooth muscle tissue, adipose tissue, and nervous tissue form the organs.
• The skin's functions include protection, excretion, body temperature regulation, sensory reception, immunity, vitamin D synthesis, and blood reservoir.

Skin Structure

• The skin has two distinct layers: the epidermis (outermost) and the dermis (inner).
• The epidermis contains keratinized stratified squamous epithelium.
• The dermis contains keratinized epithelium, glandular epithelium, dense irregular CT (collagen), smooth muscle tissue (arrector pili muscles), nervous tissue (Meissner's & Pacinian corpuscles), and blood vessels.
• The subcutaneous layer, also known as the hypodermis, is a distinct layer beneath the skin comprised of adipose tissue.

Skin Functions

• The skin protects from water loss, injury, chemicals, and microorganisms.
• The skin's chemical barrier includes a pH of 5-6, preventing microorganism growth.
• Langerhan's cells (epidermis), macrophages, and mast cells (dermis) form a biological barrier.
• The skin excretes minimal amounts of urea and uric acid.
• The skin regulates body temperature via negative-feedback mechanisms.
• Meissner's Corpuscles, which are egg-shaped, detect light touch and are located in the dermal papillae.
• Meissner's Corpuscles are located in fingertips, palms, soles, eyelids, tip of tongue, nipples, clitoris, and tip of penis.
• Pacinian Corpuscles, which are onion-shaped, detect pressure and are located in the deep dermis and subcutaneous regions.
• UV rays in sunlight activate vitamin D synthesis which is required for bone homeostasis.
• The dermis houses approximately 10% of the body's blood vessels and the skin requires 1-2% of the body’s blood.
• Macrophages (Langerhan's cells) and T-helper cells mediate immune responses.

Epidermis

• The epidermis is composed of keratinized stratified squamous epithelium.
• Keratinization determines the four distinct layers of the epidermis.
• The stratum corneum is the outermost layer with dead epithelial cells filled with keratin.
• The stratum lucidum is translucent and separates the stratum corneum from the stratum granulosum, and is only found in the thick skin of soles and palms.
• The stratum granulosum has 3-5 layers of flattened granular cells filled with keratin granules.
• The stratum spinosum has many layers of spiky cells with large nuclei.
• The stratum basale (germinativum) is the innermost layer.
• The stratum basale sits directly above the basement membrane, contains a single row of mitosing cuboidal epithelial cells, and contains melanocytes.
• Melanocytes produce the pigment melanin.
• The main function of the epidermis is protection via keratin.
• Keratin prevents moisture loss and waterproofs and Melanin determines skin color and prevents injury by penetration, microorganisms, and chemicals entry

Dermis

• The dermis is located in the inner layer of skin and binds the epidermis to underlying tissues.
• The dermis has two layers: the papillary layer (20%) and reticular layer (80%).
• The papillary layer is made up of loose areolar CT and is below the epidermis.
• The surface forms dermal papillae (finger-like projections into the epidermis) that form fingerprints in thick skin.
• Meissner's corpuscles, sensory receptors for light touch, are housed in dermis.
• The reticular layer is made up of dense irregular CT.
• Bundles of collagen, elastic, and reticular fibers, giving the skin strength and resiliency, are found in dermis.
• Pacinian corpuscles are sensory receptors for deep pressure in dermis.
• The dermis houses epidermal derivatives or accessory organs.
• The main function of the dermis is in nourishment of the epidermis.

Subcutaneous Layer

• The subcutaneous layer (hypodermis) lies beneath the skin and is comprised of adipose tissue and blood vessels.
• Insulation is its primary function.

Hair Follicles

• Hair follicles are accessory organs of the skin.
• The hair follicle root (or base) lies in the deep dermis, the follicle runs throughout dermis, and the hair shaft lies in the epidermis.
• Epithelial cells in the root undergo active mitosis and cells in follicle mature and accumulate keratin
• The full of keratin forms the exposed hair or hair shaft.
• Melanin provides pigment.
• The arrector pili muscle, a bundle of smooth muscle, is associated to every hair follicle, causing hair to stand on end ("goose bumps") when frightened or cold.

Nails

• Nails are composed of epithelium undergoing keratinization with active mitosis in the lunula.
• Nails function in manipulation and protection of digit ends.

Sebaceous Glands

• Sebaceous glands, which are associated with every hair follicle, are holocrine glands (simple cuboidal epithelium).
• Secretion from the sebaceous glands (holocrine) is sebum (i.e., oil), which is comprised of fat and cellular material.
• Sebum is secreted into the hair follicle and keeps skin and hair soft, pliable, and waterproof.
• Acne is the result of hypersecretion of sebum that clogs and inflames ducts.
• Seborrhea is caused by hyperproduction of sebum, resulting in oily scales.

Skin Glands

• Skin glands (sweat glands or sudoriferous glands) are epidermal derivatives.
• There are two types, distinguished by secretion: merocrine (eccrine) and apocrine.

Merocrine Glands

• In the deep dermis the merocrine glands coil, the duct runs in the dermis and the pore lies at surface
• Merocrine glands respond to elevated temperature/exercise
• Secretion is water plus salts and wastes (urea and uric acid) but is odorless.
• The glands function throughout life and are not associated with hair follicles.
• These glands are located on the forehead, neck, and back.

Apocrine Glands

• The ducts terminate into hair follicles
• Apocrine glands respond to stress/emotions, resulting in odor in secretion
• Apocrine glands, which are associated with hair follicles, begin functioning at puberty and continue through life
• Apocrine glands are located in the armpits and groin.
• Apocrine secretion is sweat (water, salts, urea, uric acid) plus oil and cellular debris.

Modified Apocrine Glands

• Modified apocrine glands include ceruminous glands in the external ear (secretion = earwax) and mammary glands in the breasts (secretion= milk).
• Structures are epithelial and specialized parts of the epidermis

Regulation of Body Temperature

• Normal body temperature is near 98.6 F (37 C)
• Heat production is mostly a by-product of cellular metabolism.
• Heat loss is controlled by regulating dermal blood flow via vasodilation (increases dermal blood flow) and vasoconstriction (decreases dermal blood flow).
• Heat loss occurs through radiation, conduction, convection, and evaporation.
• Radiation is the most common mode with infrared heat rays moving from high heat (blood) to low heat (the environment).
• Conduction involves less heat loss as heat moves by physical contact transferring heat to an object.
• Convection involves heat loss to surrounding air.
• Evaporation depends on conditions; more heat is lost through the evaporation of sweat from the skin's surface when heat increases.
• When body temperatures are low, the hypothalamus signals for decreased sweating (decreased heat loss by evaporation) and dermal vasoconstriction (decreasing heat loss by radiation).
• If body temperatures remain low, heat must be produced via shivering and the resulting muscle contractions.

Temperature Regulation Problems

• Hyperthermia is an elevated body temperature brought on either by humid air reduces evaporation, or when air temperatures exceed normal body temperature, the body gains heat.
• Hypothermia is a low body temperature which may be induced during surgical procedures
• If core body temperature drops below 94 F it is very dangerous
• Limbs can withstand about 65 F because they contain no vital organs

Skin Color

• Amount of melanin produced varies (DNA)
• People of different races contain virtually the same number of melanocytes.
• Environmental Factors affect melanin production by affecting gene expression.
• UV rays affect melanin production, as do chemicals and drugs (antihistamines & antibiotics)
• Physiologic Factors affect skin color but not melanin production.
• Carotene build-up in the stratum corneum causes an orange tint.
• Hemoglobin (Hb) in dermal blood vessels causes a pink coloring.
• A lack of Hb in dermal blood vessels causes a blue tint (cyanosis).
• Inability to breakdown Hb (liver problems) causes a yellow tint (jaundice).

Healing of Wounds and Burns

• Inflammation is a process that involves blood flow changes and chemical signaling leading to healing

Cuts

• Epidermal cuts heal via increased cell division in the stratum basale.
• Deep cuts involving blood vessel damage result in inflammation, blood clotting, scab formation, fibroblast infiltration and repair, scab falling off, and a potential scar.

Burns

• Superficial partial-thickness burns (1st degree) only affect the epidermis, resulting in reddening due to increased blood flow and mild pain, common in sunburn and will heal in a few days to 2 weeks
• Deep partial-thickness burns (2nd degree) affect the epidermis and some of the dermis, resulting in reddening and blistering caused by blood vessel damage and moderate pain and are common to physical contact with hot objects.
• If not infected, Deep partial-thickness burns will heal in 2-6 weeks without scars.
• Full-thickness burns (3rd degree) affect the epidermis, entire dermis, and potentially subcutaneous damage, causing dry, leathery tissue with a red or black color and severe pain and are usually caused by prolonged heat or chemical contact.
• Healing rarely occurs in Full thickness burns; skin replacements (grafts) are usually needed, resulting in extensive scarring.
• An autograft is a transplant from an undamaged area of the patient's own body.
• A homograft is a temporary transplant from a cadaver.
• Skin substitutes are also used .
• Determining the body surface affected by a burn is vital for determining treatment and prognosis and is estimated by the "rule of nines”.

Life Span Changes

• Aging skin results in wrinkling, sagging, and age spots or liver spots.
• Efficient regulation of body temperature declines with age.
• The number of sweat glands changes and Capillary beds in the skin shrink.
• Synthesis of vitamin D declines as skin ages, which affects skeletal health.

Homeostatic Imbalances of the Skin

• Homeostatic imbalances of the skin include epidermolysis bullosa, psoriasis, contact dermatitis, rashes, skin cancer (carcinoma), folliculitis, hair loss, acne, hypothermia, albinism, elevated body temperature, and jaundice.