The Integumentary System

Overview

The integumentary system is the body's largest organ system, primarily composed of the skin and its derivatives, including hair, nails, and various glands. It serves as a protective barrier, regulates body temperature, and provides sensory information.

Structure and Function of Hair

Hair Anatomy and Growth

• Hair consists of flexible strands of dead, keratinized cells produced by hair follicles.

• Keratin in hair is hard keratin, which is tougher and more durable than the soft keratin found in skin.

• Hair is absent from palms, soles, lips, nipples, and portions of external genitalia.

• Functions include sensing insects, guarding against physical trauma, heat loss, and sunlight, and filtering particles (e.g., eyelashes, nose hair).

Hair Structure

• Shaft: The part of the hair that extends above the scalp, where keratinization is complete.

• Root: The part of the hair within the scalp, where keratinization is ongoing.

• Medulla: The central core of large cells and air spaces.

• Cortex: Several layers of flattened cells surrounding the medulla.

• Cuticle: The outermost layer, consisting of overlapping layers of single cells.

Hair Follicle Structure

• Follicle: Extends from the epidermal surface to the dermis.

• Hair bulb: The expanded area at the deep end of the follicle.

• Hair follicle receptor (root hair plexus): Sensory nerve endings that wrap around the bulb, acting as touch receptors.

• Hair matrix: The actively dividing area of the bulb that produces hair.

• Arrector pili muscle: A small band of smooth muscle attached to the follicle, responsible for "goose bumps."

• Hair papilla: Dermal tissue containing a knot of capillaries that supplies nutrients to growing hair.

Hair Color

• Produced by melanocytes in hair follicles.

• Different proportions of melanin (yellow, rust, brown, black) create hair color; additional pheomelanin pigment may be present.

• Gray or white hair results when melanin production decreases and air bubbles replace pigment in the hair shaft.

Hair Thinning and Baldness

• Hirsutism: Excessive hairiness, often due to abnormally high androgen levels.

• Alopecia: Hair thinning in both sexes after age 40 as hair is shed faster than it is replaced.

• True (frank) baldness: Genetically determined and sex-influenced; most common type is male pattern baldness caused by follicular response to DHT (dihydrotestosterone).

• Telogen effluvium (TE): Abrupt hair thinning due to stressors such as surgery, trauma, or crash dieting.

• Other causes: Drugs (antidepressants, chemotherapeutics), hypothyroidism, mineral-deficient diets.

Nails

Structure and Function

• Nails are modifications of the epidermis containing hard keratin.

• They provide protective cover for the distal, dorsal surface of fingers and toes.

• Appear pink due to underlying capillaries.

• Main parts: visible free edge, nail plate, and embedded root.

• Nail bed: Epidermis underneath the keratinized nail plate.

• Nail matrix: Thickened portion of the bed responsible for nail growth; its proximal part, the lunule, appears white.

• Nail folds: Skin folds that overlap the border of the nail; the eponychium (cuticle) projects onto the nail body.

• Hyponychium: Area under the free edge that accumulates dirt.

Glands of the Skin

Types of Cutaneous Glands

• Sweat (sudoriferous) glands: Distributed over the entire skin surface except nipples and parts of external genitalia; about 3 million per person.

• Two main types: eccrine (merocrine) and apocrine sweat glands.

• Other glands: ceruminous glands (earwax), mammary glands (milk).

• Sebaceous (oil) glands: Widely distributed except in thick skin; usually secrete into hair follicles; become active at puberty and are stimulated by hormones.

Eccrine (Merocrine) Sweat Glands

• Most numerous; abundant on palms, soles, and forehead.

• Ducts open directly onto the skin surface (pores).

• Function in thermoregulation, regulated by the sympathetic nervous system.

• Sweat composition: 99% water, salts, vitamin C, antibodies, dermcidin (antimicrobial peptide), metabolic wastes.

Apocrine Sweat Glands

• Confined to axillary and anogenital areas.

• Ducts empty into hair follicles.

• Begin functioning at puberty; may act as sexual scent glands.

• Sweat is milky or yellowish, containing fatty substances and proteins; bacterial breakdown leads to body odor.

Comparison of Eccrine and Apocrine Sweat Glands

Other Glands

• Ceruminous glands: Lining of external ear canal; secrete cerumen (earwax).

• Mammary glands: Secrete milk.

Sebaceous (Oil) Glands

• Secrete sebum (oily secretion) into hair follicles.

• Function: Lubricate skin and hair, prevent water loss, and have bactericidal properties.

• Become active at puberty, stimulated by hormones (especially androgens).

Functions of the Skin

Protective Functions

• Acts as a barrier to microorganisms, abrasions, temperature extremes, and chemicals.

• Prevents water loss and entry of harmful substances.

• Regulates body temperature via perspiration and blood flow.

• Provides cutaneous sensations through various sensory receptors.

• Metabolic functions: Synthesis of vitamin D, activation of hormones.

• Excretion of wastes via sweat (e.g., nitrogenous wastes).

Types of Barriers

• Chemical barrier: Skin secretions (acid mantle, defensins) and melanin provide protection against bacteria and UV radiation.

• Physical barrier: Keratinized cells and lipids block most water and water-soluble substances.

• Biological barrier: Dendritic cells and macrophages engulf foreign substances and activate the immune response.

Skin Disorders and Clinical Considerations

Skin Cancer

• Most skin tumors are benign and do not metastasize.

• Risk factors: Overexposure to UV radiation, frequent skin irritation.

• Three major types:

  • Basal cell carcinoma: Most common (~80%); slow-growing, rarely metastasizes; treated by surgical excision.

  • Squamous cell carcinoma: Second most common; can metastasize; arises from keratinocytes of the stratum spinosum; treated by excision or radiation.

  • Melanoma: Most dangerous; highly metastatic and resistant to chemotherapy; arises from melanocytes; treated by wide surgical excision and immunotherapy.

• ABCD Rule for Melanoma Detection:

  • Asymmetry: Two sides of the pigmented area do not match.

  • Border irregularity: Indentations in the border.

  • Color: Contains several colors (black, brown, tan, red, blue).

  • Diameter: Larger than 6 mm (pencil eraser size).

  • Evolving: Changes over time (Additional info: The "E" is sometimes added for evolution of the lesion).

Burns

• Caused by heat, electricity, radiation, or chemicals; result in protein denaturation and cell death.

• Main threat: Dehydration and electrolyte imbalance, leading to renal shutdown and circulatory shock.

• Rule of Nines: Body divided into 11 regions, each accounting for 9% of total body surface area (plus 1% for genitalia), used to estimate fluid loss.

Classification of Burns

• First-degree: Only epidermis damaged; redness, swelling, pain.

• Second-degree: Epidermis and upper dermis damaged; blisters appear; partial-thickness burn.

• Third-degree: Entire thickness of skin involved (full-thickness burn); skin gray-white, cherry red, or blackened; not painful (nerve endings destroyed); skin grafting usually required.

Critical Burns

• Considered critical if:

  • Over 25% of body has second-degree burns

  • Over 10% of body has third-degree burns

  • Third-degree burns on face, hands, or feet

• Treatment includes debridement, antibiotics, temporary covering, and skin grafts.

Developmental Aspects of the Integumentary System

• Fetal skin is developed by the end of the 4th month; lanugo (delicate hairs) appear in the 5th and 6th months.

• Vernix caseosa: Sebaceous gland secretion that protects fetal skin in the womb.

• In childhood, skin thickens and subcutaneous fat increases; sebaceous gland activity increases during adolescence, leading to acne.

• In adulthood, skin is optimal in the 20s and 30s; after that, cumulative environmental damage appears, cell replacement slows, and skin becomes thinner, drier, and less elastic.

• Increased risk of cancer due to decreased melanocytes and dendritic cells.

Sample Questions for Review

• What are the differences between eccrine and apocrine sweat glands?

• How can you determine whether a skin discoloration is due to melanoma?

• Which is more severe: a burn covering 90% of the body with redness and edema, or a burn covering 10% of the body with blackened, painless skin? Would either be considered critical and why?