The Integumentary System

Overview

The integumentary system is composed of the skin and its derivatives, including hair, nails, and various glands. It serves as the body's primary barrier against the external environment and plays essential roles in protection, sensation, thermoregulation, and metabolic functions.

Structure and Function of Hair

Hair: Composition and Types

• 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 and heat loss, shielding from sunlight, and filtering air particles (e.g., eyelashes, nose hair).

Hair Color and Pigmentation

• Color is determined by melanin produced by melanocytes in hair follicles.

• Different types of melanin (yellow, rust, brown, black) and additional pheomelanin pigment create hair color diversity.

• Gray or white hair results from decreased melanin production and the presence of air bubbles in the hair shaft.

Hair Structure

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

• Root: The part below the scalp, where keratinization is ongoing.

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

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

• Cuticle: Outermost layer consisting of overlapping single cells.

Hair Follicle Anatomy

• Extends from the epidermal surface into the dermis.

• Hair bulb: 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: Actively dividing area of the bulb that produces hair cells; as new cells are produced, older ones are pushed upward.

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

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

Hair Thinning and Baldness

Types and Causes

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

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

• Male pattern baldness: Genetically determined and sex-influenced, caused by follicular response to DHT (dihydrotestosterone).

• Telogen effluvium (TE): Abrupt hair thinning due to a large number of follicles entering the resting phase simultaneously, often triggered by stress, 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.

• The pink appearance is due to underlying capillaries.

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

• Nail bed: Epidermis beneath the keratinized nail plate.

• Nail matrix: Thickened portion of the bed responsible for nail growth; the lunule is the white, crescent-shaped area.

• Nail folds: Skin folds that overlap the nail border; 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 most of the body except nipples and parts of external genitalia; about 3 million per person.

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

• Myoepithelial cells (with actin fibers) contract upon nervous system stimulation to force sweat out.

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), and 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.

• Secrete milky or yellowish sweat containing fatty substances and proteins, which bacteria break down to produce body odor.

Other Glands

• Ceruminous glands: Modified apocrine glands lining the external ear canal; secrete cerumen (earwax).

• Mammary glands: Specialized glands that secrete milk.

• Sebaceous (oil) glands: Widely distributed except in thick skin; most develop from and secrete into hair follicles. Secretion (sebum) is stimulated by hormones, especially androgens, and has bactericidal properties.

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 sensory information through various receptors.

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

• Excretion of nitrogenous wastes via sweat.

Types of Barriers

• Chemical barrier: Skin secretions (acid mantle) and melanin provide antimicrobial and UV protection.

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

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

Skin Disorders: Cancer and Burns

Skin Cancer

• Most skin tumors are benign and do not metastasize.

• Main risk factor: Overexposure to UV radiation.

• 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; appears as a scaly, reddened papule.

  • Melanoma: Most dangerous; highly metastatic and resistant to chemotherapy; arises from melanocytes; early detection is critical (ABCD rule: Asymmetry, Border irregularity, Color variation, Diameter >6mm).

Burns

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

• Main threats: Dehydration, electrolyte imbalance, and infection.

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

• Burns are classified by severity:

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

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

  • Third-degree: Entire thickness of skin involved; area appears gray-white, cherry red, or blackened; not painful due to nerve destruction; skin grafting usually required.

• Burns are 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

Development and Aging of the Integumentary System

Developmental Stages

• By the end of the 4th month of fetal development, the skin is formed.

• Lanugo: Delicate fetal 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 rises during adolescence, often causing acne.

• In adulthood, skin is at its optimal appearance in the 20s and 30s; aging leads to thinning, decreased elasticity, and increased risk of cancer due to reduced melanocyte and dendritic cell numbers.

Comparison Table: Eccrine vs. Apocrine Sweat Glands

Key Diagnostic Criteria: Melanoma (ABCD Rule)

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

• Border irregularity: Edges are notched or indented.

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

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

• Evolving: Changes in size, shape, or color over time (sometimes included as 'E').