BIOL 2120K: Human A&P I - Integumentary System Notes (copy)

Skin (Integument)

• Consists of two distinct regions:
  • Epidermis: Superficial region made of epithelial tissue.
  • Dermis: Mostly fibrous connective tissue (CT) underlies the epidermis.
• Hypodermis:
  • Mostly adipose tissue.
  • Not part of the skin but shares some of the skin’s functions.
  • Functions include shock absorption and heat insulation.
  • Also called superficial fascia.
  • Anchors skin to dense fibrous CT of skeletal muscles.

Epidermis

• Keratinized stratified squamous epithelium.
• Four (or five) distinct layers (Come Let's Get Sun Burned):
  • Stratum corneum: Most superficial layer; keratinized dead cells.
  • Stratum lucidum: Translucent layer; only in thick skin, e.g., palmar or plantar skin.
  • Stratum granulosum: Organelles deteriorating; cytoplasm full of granules.
  • Stratum spinosum: Intermediate filaments attached to desmosomes; contains melanosomes and dendritic cells.
  • Stratum basale: Deepest layer; one row of actively mitotic stem cells.

Cells of Epidermis

• Keratinocytes:
  • Predominant cell type in epidermis.
  • Produce fibrous protein keratin and are tightly connected by desmosomes.
  • Cells above the granular layer are dead (stratum corneum and lucidum).
• Melanocytes:
  • 10-25% of cells in the deepest epidermis.
  • Produce pigment melanin that is packaged into melanosomes.
  • Protect the apical surface of the keratinocyte nucleus from UV damage.
• Dendritic (Langerhans’) cells and macrophages:
  • Key activators of the immune system.
• Tactile (Merkel) cells:
  • Sensory touch receptors.

Cell Differentiation in Epidermis

• Change of cells from stratum basale to stratum corneum:
  • Organelles break down.
  • Plasma membrane thickens.
• Cells slough off as dandruff and dander.
  • Shed approximately 50,000 cells every minute in a lifetime.
• The main structural features of the skin epidermis include different cell types and layers, each with specific functions.

Dermis

• Strong, flexible connective tissue.
• Cell types include fibroblasts and macrophages; occasionally mast cells and other WBCs.
• Fibers:
  • Collagen, elastic, and reticular fibers.
• Embedded in ECM bind body together.
  • “Hide” is used to make leather.
• Contains:
  • Blood vessels.
  • Lymph vessels.
  • Nerve fibers.
  • Epidermal hair follicles.
  • Oil and sweat glands.

Two Layers of Dermis: Papillary Layer

• Loose areolar CT:
  • Collagen and elastic fibers.
  • Blood vessels.
  • Phagocytes patrol for microorganisms.
• Superficial peglike projections called dermal papillae:
  • Appear in thick skin.
  • Cause epidermal ridges called friction ridges.
  • Fingerprint pattern.
  • Enhance gripping ability.
  • Contribute to touch sense.

Two Layers of Dermis: Reticular Layer

• Approximately 80% of dermal thickness.
• Dense irregular CT:
  • Elastic and collagen fibers.
• Two dermal modifications:
  • Cleavage lines: collagen fibers run parallel to skin surface
    • Externally invisible; important surgical land markings; heal more readily.
  • Flexure lines: dermal folds at or near joints
    • Dermis tightly secured to deeper structures; skin does not slide easily.

Other Skin Markings

• Striae:
  • Silvery-white scars.
  • Dermal tearing caused by extreme stretching.
  • E.g., “stretch marks” associated with pregnancy, rapid weight or muscle gain, puberty.
• Blister:
  • Fluid-filled pocket that separates epidermal and dermal layers.
  • Caused by acute, short-term trauma.

Skin Color

• Three pigments contribute to skin color:
  • Melanin
  • Carotene
  • Hemoglobin

Melanin

• Combination of polymers made of tyrosine.
• Produced in melanocytes.
  • Same relative number in all people.
• Sun exposure stimulates melanocytes.
  • Melanin migrates to keratinocytes to form “pigment shields” for nuclei.
• Freckles and pigmented moles are local accumulations of melanin.
• Tinea versicolor (“sunspots”) is a fungal infection that turns off melanocytes.

Carotene

• Yellow to orange pigment.
  • Accumulates in stratum corneum and hypodermis.
  • Most obvious in palms and soles.
  • Can be converted to vitamin A for vision and epidermal health.
• Carotenemia:
  • Increased β-carotene levels in blood.
  • Mostly due to prolonged and excessive consumption of carotene-rich foods, such as carrots, squash, and sweet potatoes.

Hemoglobin

• Protein molecule in red blood cells that transports O2 and CO2 between lungs and body’s tissues.
• Heme pigment gives:
  • Blood red color.
  • Pinkish hue of fair skin.
• Anemia: Lack of healthy RBCs.

Skin Color in Diagnosis

• Cyanosis: Low oxygenation of hemoglobin.
• Erythema: Fever, hypertension, inflammation, allergy.
• Pallor: Anemia, low blood pressure, fear, anger.
• Jaundice: Liver disorder; bilirubin deposited in body tissues.
• Bronzing: Inadequate steroid hormones in Addison’s disease.
• Bruises: Clotted blood beneath skin.

Appendages of Skin

• Derivatives of epidermis:
  • Hairs and hair follicles.
  • Nails.
  • Sweat glands.
  • Sebaceous (oil) glands.

Hairs

• Flexible strands of dead, hard-keratinized cells.
  • More durable than soft keratin of skin.
  • Individual cells do not flake off.
• Functions:
  • Body hairs: Sense insects on skin before they bite or sting us.
  • Hair on the scalp: Guards head against physical trauma, heat loss, and sunlight.
  • Nose hairs: Filter foreign particles during inhalation.
  • Eyelashes: Shield eyes.

Structure of Hair and Hair Follicle

• Hair has three concentric layers of keratinized cells: medulla, cortex, and cuticle.
• Hair follicle:
  • Sac from which hair grows and into which sebaceous (oil) glands open.
  • Extends from epidermal surface to dermis.
• Arrector pili:
  • Smooth muscle attached to follicle.
  • Contraction causes “goosebumps”.

Hair Pigments

• Two types of melanin: eumelanin and pheomelanin.
  • Black hair contains virtually only eumelanin.
  • Red hair is rich in pheomelanin.
• Gray/white hair is due to reduced melanin production or increased air bubbles in the shaft.

Types and Growth of Hair

• Vellus hair:
  • Pale, fine body hair of children and adult females.
• Terminal hair:
  • Coarse, long hair of eyebrows, scalp.
  • At puberty:
    • Appear in axillary and pubic regions of both sexes.
    • Face and neck of males.
• Hair follicles cycle between active and regressive phases.
  • Not all follicles are in the telogen stage at the same time.

Hair Thinning and Baldness

• Alopecia:
  • Hair thinning in both sexes after mid-age.
  • Much more dramatic in males:
    • Coarse terminal hairs are replaced by vellus hairs.
• True (or frank) baldness:
  • Genetically determined and sex-influenced condition.
  • Primary baldness gene on the X chromosome.
    • Affected by other factors, e.g., diet, hormone, stress.

Nail

• Scalelike modification of epidermis.
  • Forms protective cover for the distal, dorsal surface of fingers and toes.
  • Contains hard keratin (like hairs).
• Each nail has:
  • Free edge.
  • Nail plate or body (visible attached portion).
  • Root (embedded in skin).
• Nail bed:
  • Deeper layers of epidermis extend beneath the nail.
• Nail matrix:
  • Thickened proximal portion of nail bed.
  • Responsible for nail growth.

Multicellular Exocrine Glands

• Composed of duct and secretory unit; surrounded by supportive CT.
• Classified by structure:
  • Simple glands (unbranched duct) or compound glands (branched duct).
  • Cell shape: tubular, alveolar, or tubuloalveolar.

Multicellular Exocrine Glands (cont.)

• Classified by type of secretion:
  • Merocrine (exocytosis):
    • E.g., most sweat glands, salivary glands, and pancreas.
  • Holocrine (cell ruptures):
    • E.g., sebaceous glands.
  • Apocrine (only apex ruptures):
    • E.g., lactating mammary glands or some sweat glands.

Sweat Glands

• Also called sudoriferous glands (sudor = sweat).
  • Approximately all skin surfaces except nipples and parts of external genitalia.
  • Approximately 3 million per person.
• Contain myoepithelial cells (myo = muscle):
  • Contract upon nervous system stimulation to force sweat into ducts.
• Two main types:
  • Merocrine (eccrine) sweat gland.
  • Apocrine sweat gland.

Merocrine Vs. Apocrine Sweat Glands

• Merocrine (eccrine) sweat glands:
  • Most numerous.
  • Abundant on palms, soles, and forehead.
  • Ducts connect to sweat pores.
  • Secretions include:
    • 99% water, salts, vitamin C, antibodies, dermcidin, metabolic wastes.
  • Function in thermoregulation.
• Apocrine sweat glands:
  • Larger.
  • Confined to axillary and anogenital areas.
  • Ducts empty into hair follicles.
  • Secretions include sweat, fatty substances, and proteins:
    • Viscous; milky or yellowish.
    • Odorless until bacterial interaction leads to body odor.
  • Begin functioning at puberty.
  • May act as sexual scent gland.
  • Modified apocrine glands:
    • Ceruminous glands: secrete cerumen (earwax).
    • Mammary glands: secrete milk.

Sebaceous (Oil) Glands

• Widely distributed except for thick skins.
  • Most develop from hair follicles and secrete into hair follicles.
• Secrete sebum:
  • Slows down water loss from skin.
  • Softens and lubricates hair and skin.
  • Retards bacterial multiplication.
• Relatively inactive until puberty:
  • Stimulated by hormones (especially androgens).

Functions of Integumentary System

• Protection
• Body temperature regulation
• Cutaneous sensation
• Metabolic functions
• Blood reservoir
• Excretion

Protection

• Physical barriers:
  • Glycolipids block most water and water-soluble substances.
  • Keratinized stratum corneum allows limited penetration, e.g., organic solvents, lipid-soluble substances, plant oleoresins, some salts, and some drugs.
• Biological barriers:
  • Dendritic cells in epidermis:
    • Present antigens to lymphocytes.
  • Macrophages in dermis:
    • Phagocytize pathogens.

Protection (cont.)

• Chemical barriers:
  • Low pH of skin secretions functions like acid mantle, e.g., sweat (pH 4.5-7.0), sebum (pH 4.5-5.5).
  • Natural antibiotics kill bacteria, e.g., dermcidin, defensins, cathelicidins.
  • Melanin defends against UV radiation damage to DNA.

Body Temperature Regulation

• Cold external environment:
  • Dermal blood vessels constrict.
  • Skin temperature drops.
  • Slows heat loss.
• Hot external environment:
  • Dermal blood vessels dilate.
  • Increased sweat gland activity.
  • Cools down the body.

Cutaneous Sensations

• Cutaneous sensory receptors detect temperature, touch, pressure, and pain.
• Types of receptors mentioned:
  • Free nerve endings (pain, heat, cold).
  • Merkel disks (touch).
  • Krause end bulbs (touch).
  • Meissner corpuscles (touch).
  • Pacinian corpuscles (pressure).
  • Root hair plexus (touch).
  • Ruffini endings (pressure).

Metabolic Functions, Blood Reservoir, and Excretion

• Metabolic functions:
  • Convert chemicals:
    • Cholesterol \rightarrow vitamin D precursor for Ca^{2+} absorption.
    • Cortisone \rightarrow hydrocortisone for anti-inflammation.
  • Synthesis of proteins:
    • Collagenase: enzyme for the natural turnover of collagen.
• Blood reservoir:
  • Holds up to 5% of the body’s blood volume.
• Excretion:
  • Nitrogenous wastes (urea, uric acid, and ammonia) and salt (Na+) in sweat.

Skin Cancer

• Risk factors:
  • Overexposure to UV radiation (most important).
  • Frequent irritation of the skin, e.g., infections, chemicals, or physical trauma.
• Three major types of skin cancer:
  • Basal cell carcinoma: cells of stratum basale.
  • Squamous cell carcinoma: keratinocytes of the stratum spinosum.
  • Melanoma: melanocytes.

Basal Cell Carcinoma

• Least malignant; most common.
  • Open sores, red patches, pink growths, shiny bumps, or scars.
• Stratum basale cells proliferate and slowly invade the dermis and hypodermis.
• Cured by surgical excision in 99% of cases.

Squamous Cell Carcinoma

• Can metastasize; second most common type.
  • Scaly reddened papule on scalp, ears, lower lip, and hands.
• Involves keratinocytes of stratum spinosum.
• Good prognosis if treated by radiation therapy or removed surgically.

Melanoma

• Cancer of melanocytes.
  • Highly metastatic and resistant to chemotherapy.
• Treated by wide surgical excision accompanied by immunotherapy.
• Key to survival is early detection – ABCDE rule.

Burns

• Tissue damage caused by heat, electricity, radiation, chemicals.
  • Denature proteins and kill cells.
• Immediate threat of life due to dehydration and electrolyte imbalance:
  • Renal failure and circulatory shock.
• To evaluate burns:
  • The rule of nines divides the body into 11 areas.
    • Each accounts for 9% of the total body area; 1% from the genital area.
    • Used to estimate the volume of fluid loss.

Burns Classified by Severity

• Partial-thickness burns:
  • First degree: epidermal damage only.
    • Localized redness, edema (swelling), and pain.
  • Second degree: epidermal and upper dermal damage.
    • Blisters appear.
• Full-thickness burns:
  • Third degree: entire thickness of skin involved.
    • Skin gray-white, cherry red, or blackened.
    • Not painful (nerve endings destroyed) or swollen.
    • Skin grafting usually necessary.

Severity and Treatment of Burns

• Critical if:
  • >25% of the body has second-degree burns.
  • >10% of the body has third-degree burns.
  • Face, hands, or feet bear third-degree burns.
• Treatment includes:
  • Debridement of burned skin.
  • Temporary covering.
  • Antibiotics.
  • Skin grafts.