The Integumentary System

Overview and Components

The integumentary system is the body's largest organ system, serving as the primary barrier between internal tissues and the external environment. It plays crucial roles in protection, sensation, and regulation.

• Skin: The main organ, providing a protective covering.

• Sudoriferous (Sweat) Glands: Involved in thermoregulation and excretion.

• Sebaceous (Oil) Glands: Secrete sebum to lubricate and protect skin and hair.

• Hairs: Sensory and protective functions.

• Nails: Protect distal phalanges and aid in manipulation.

• Subcutaneous Tissue: Anchors skin, provides insulation and shock absorption.

Clinical Importance

Understanding the integumentary system is essential for evaluating and treating skin injuries, such as burns, and for recognizing signs of systemic diseases.

Structure of the Skin

Layers of the Skin

The skin consists of two primary layers, each with distinct structure and function:

• Epidermis: Outermost layer, composed of epithelial tissue; avascular.

• Dermis: Underlies the epidermis; made of dense connective tissue; vascular.

• Subcutaneous Tissue (Hypodermis): Not technically part of the skin, but functionally related. Composed mainly of adipose tissue and some areolar connective tissue. Functions include anchoring skin to underlying structures (mainly muscles), shock absorption, and insulation.

The Epidermis

Cell Types of the Epidermis

The epidermis is primarily made of keratinized stratified squamous epithelium and contains four main cell types:

• Keratinocytes: Produce keratin, a protein that provides protective properties. These are the most abundant cells in the epidermis. Millions are shed daily, renewing the epidermis every 25–45 days.

• Melanocytes: Spider-shaped cells in the deepest epidermis. They produce melanin, a pigment packaged into vesicles called melanosomes. Melanosomes are transferred to keratinocytes, where melanin shields the nucleus from UV damage.

• Dendritic Cells (Langerhans Cells): Star-shaped macrophages that patrol the deep epidermis, ingesting foreign substances and activating the immune system.

• Tactile Epithelial Cells (Merkel Cells): Sensory receptors for touch, located at the epidermal-dermal junction.

Layers (Strata) of the Epidermis

The epidermis is organized into four or five distinct layers, depending on skin thickness:

• Stratum Basale: Deepest layer; single row of stem cells actively dividing. Also called stratum germinativum due to mitosis.

• Stratum Spinosum: Several cell layers thick; cells contain intermediate filaments attached to desmosomes, allowing resistance to tension. Contains abundant melanosomes and dendritic cells.

• Stratum Granulosum: Four to six cell layers; cells flatten, nuclei and organelles disintegrate, and keratinization begins. Contains keratohyaline granules (form keratin fibers) and lamellar granules (water-resistant glycolipid).

• Stratum Lucidum: Present only in thick skin (palms, fingertips, soles); thin, translucent band of dead keratinocytes.

• Stratum Corneum: Outermost layer; ~30 rows of flat, anucleated, keratinized dead cells. Functions as a barrier against environmental damage, water loss, abrasion, and penetration.

Thick vs. Thin Skin

• Thick Skin: Contains all five layers, found on palms, fingertips, and soles.

• Thin Skin: Contains only four layers (no stratum lucidum).

Functions of the Integumentary System

Protection

The skin acts as a barrier against physical, chemical, and biological threats.

• Chemical Barriers: Sweat (antimicrobial proteins), sebum, defensins, acid mantle (low pH), and melanin (UV protection).

• Physical Barriers: Keratinized cells and glycolipids block water and most substances; some chemicals can penetrate (e.g., lipid-soluble substances, heavy metals).

• Biological Barriers: Dendritic cells and macrophages activate immune responses.

Body Temperature Regulation

• Insensible Perspiration: ~500 ml/day of unnoticeable sweat at rest.

• Sensible Perspiration: Up to 12 L/day during heat or exercise; sweat cools the body.

• Vasoconstriction: In cold, dermal blood vessels constrict to reduce heat loss.

Cutaneous Sensations

• Touch: Meissner's corpuscles and tactile epithelial cells.

• Deep Pressure: Pacinian corpuscles.

• Hair Movement: Hair follicle receptors.

• Pain: Free nerve endings.

Metabolic Functions

• Synthesis of vitamin D for calcium absorption.

• Keratinocytes disarm carcinogens and activate hormones (e.g., cortisone to hydrocortisone).

• Production of collagenase for collagen turnover.

Blood Reservoir

• Skin can hold up to 5% of total blood volume; blood can be shunted to other organs as needed.

Excretion

• Excretes nitrogenous wastes (ammonia, urea, uric acid), salt, and water.

Clinical Aspects

Skin Color and Pigments

• Melanin: Produced by melanocytes; protects against UV damage. Skin color differences are due to amount and form of melanin.

• Carotene: Yellow to orange pigment, most visible in palms and soles; can be converted to vitamin A.

• Hemoglobin: Pinkish hue in fair skin due to transparency and underlying blood.

Skin Color Changes and Disease

• Cyanosis: Blue skin due to low oxygenation.

• Pallor: Pale skin from stress, anemia, or low blood pressure.

• Erythema: Redness from fever, inflammation, or allergy.

• Jaundice: Yellow skin from liver disorders.

• Bruises: Blood leakage under skin.

• Hyperpigmentation: May indicate endocrine disorders.

Accessory Structures

Hair

Hair consists of dead, keratinized cells and serves protective and sensory functions.

• Absent on palms, soles, lips, nipples, and parts of external genitalia.

• Functions: Sensing insects, protection from trauma, heat loss, sunlight, filtering particles (nose, eyelashes).

Structure of Hair

• Root: Portion within scalp; keratinization ongoing.

• Shaft: Portion above scalp; keratinization complete.

• Medulla: Central core of large cells and air spaces.

• Cortex: Layers of flattened cells surrounding medulla.

• Cuticle: Outermost layer of overlapping single cells.

Hair Pigments

• Produced by melanocytes in hair follicle; combinations of melanins create hair colors.

• Red hair contains pheomelanin; gray/white hair results from decreased melanin and air bubbles.

Hair Follicle Structure

• Extends from epidermal surface into dermis or subcutaneous tissue.

• Bulb: Deep end; contains hair papilla (capillaries for nutrients).

• Root hair plexus: Sensory nerve endings.

• Arrector pili: Smooth muscle causing 'goose bumps.'

Hair Types and Growth

• Vellus Hair: Fine, pale body hair.

• Terminal Hair: Coarse, long hair (scalp, eyebrows, axillary, pubic regions).

• Growth cycles: Active and resting phases; scalp hair grows for ~4 years, eyebrows for a few months.

Hair Thinning and Baldness

• Age-related thinning; male pattern baldness is genetically determined and androgen-influenced.

• Telogen effluvium: Abrupt hair thinning due to stress, hormonal changes, or illness.

Nails

Nails are scale-like modifications of the epidermis, containing hard keratin and serving as protective covers for fingers and toes.

• Root: Embedded in skin.

• Nail Plate/Body: Visible attached portion.

• Free Edge: Extends beyond finger/toe.

• Nail Bed: Epidermis under nail plate.

• Nail Matrix: Responsible for nail growth.

• Nail Folds: Skin folds overlapping nail border.

• Cuticle (Eponychium): Projects onto nail body.

• Hyponychium: Area under free edge; accumulates dirt.

Clinical Nail Changes

• Yellow nails: Respiratory or thyroid disorders.

• Thickened yellow nails: Fungal infection.

• Spoon nails (koilonychia): Iron deficiency.

• Beau's lines: Horizontal lines indicating severe illness.

Glands of the Skin

Sweat (Sudoriferous) Glands

• Eccrine (Merocrine) Glands: Most numerous; abundant on palms, soles, forehead. Secrete watery sweat for thermoregulation.

• Apocrine Glands: Located in axillary and anogenital areas; secrete viscous sweat containing fatty substances and proteins. Function at puberty; may act as scent glands.

• Modified Apocrine Glands:

  • Ceruminous Glands: Secrete earwax.

  • Mammary Glands: Secrete milk.

Oil (Sebaceous) Glands

• Widely distributed except in thick skin; secrete sebum into hair follicles.

• Activated at puberty; sebum softens hair/skin and has bactericidal properties.

Summary Table: Cutaneous Glands

Skin Disorders

Skin Cancer

• Basal Cell Carcinoma: Most common, least malignant; arises from stratum basale; cured by excision.

• Squamous Cell Carcinoma: Second most common; arises from stratum spinosum; can metastasize.

• Melanoma: Cancer of melanocytes; most dangerous, highly metastatic, resistant to chemotherapy.

ABCDE Rule for Skin Cancer Detection

• A: Asymmetry

• B: Border irregularity

• C: Color variation

• D: Diameter >6 mm

• E: Evolution (changes over time)

Burns

• Caused by heat, electricity, radiation, or chemicals; damage by protein denaturation.

• Immediate threat: dehydration and electrolyte imbalance, leading to renal shutdown and shock.

• Rule of Nines: Body divided into 11 sections, each 9% of surface area (genitals 1%) to estimate fluid loss.

Burn Classification

• First-degree: Epidermal damage only; redness, swelling, pain.

• Second-degree: Epidermal and upper dermal damage; blisters.

• Third-degree: Entire skin thickness; gray-white, cherry red, or blackened; no pain (nerve endings destroyed); requires skin grafting.

Critical Burns

• >25% body with second-degree burns

• >10% body with third-degree burns

• Face, hands, or feet with third-degree burns

• Treatment: Debridement, antibiotics, temporary covering, skin grafts

Developmental Aspects

Embryonic and Fetal Development

• Epidermis develops from ectoderm; dermis and subcutaneous tissue from mesoderm.

• By 4th month, skin is well developed; lanugo coat (fine hair) appears in 5th–6th months, replaced by vellus hair in 7th month.

Postnatal and Aging Skin

• Vernix caseosa: White, protective substance produced by sebaceous glands in fetus.

• Milia: Small white spots on newborn skin.

• Skin thickens, subcutaneous fat increases, gland activity rises (acne).

• Optimal skin appearance in 20s–30s; after 30, environmental damage accumulates (scaling, dermatitis).

• Aging: Epidermal replacement slows, skin thins, subcutaneous fat and elasticity decrease (wrinkles, cold intolerance), increased cancer risk, hair thinning.

• Prevention: UV protection (UVA = aging rays, UVB = burning rays).

Additional info: For more detailed mechanisms of skin repair, wound healing, and advanced clinical correlations, refer to specialized dermatology or physiology texts.