The Integumentary System: Structure and Function

The Integumentary System

Overview

The integumentary system is composed primarily of the skin and its derivatives, including hair, nails, and glands. It serves as the body's first line of defense, regulates temperature, and provides sensory information.

• Skin consists of two main regions: epidermis (superficial) and dermis (deep).

• Hypodermis (subcutaneous layer) lies beneath the dermis, primarily composed of adipose tissue.

• Functions include protection, sensation, thermoregulation, and synthesis of vitamin D.

Skin Structure

Main Layers

• Epidermis: Keratinized stratified squamous epithelium; avascular; four or five layers depending on location.

• Dermis: Strong, flexible connective tissue; contains blood vessels, nerves, and accessory structures.

• Hypodermis: Not part of skin proper; anchors skin to underlying structures.

Epidermis

Layers of the Epidermis

• Stratum Basale (Basal Layer): Deepest layer; single row of stem cells; site of mitosis; contains melanocytes (10–25%).

• Stratum Spinosum (Prickly Layer): Several cell layers; cells contain intermediate filaments; abundant melanosomes and dendritic cells.

• Stratum Granulosum (Granular Layer): Thin; cells flatten; keratinization begins; cells accumulate keratohyaline and lamellar granules.

• Stratum Lucidum (Clear Layer): Only in thick skin (palms, soles); few rows of dead keratinocytes.

• Stratum Corneum (Horny Layer): 20–30 layers of dead, flat, anucleate keratinized cells; provides protection.

Cells of the Epidermis

• Keratinocytes: Produce keratin; most abundant; arise from stratum basale.

• Melanocytes: Synthesize melanin; found in stratum basale.

• Dendritic (Langerhans) cells: Macrophages; activate immune response.

• Tactile (Merkel) cells: Sensory touch receptors.

Cell Differentiation in Epidermis

• Cells migrate from stratum basale to stratum corneum.

• Apoptosis occurs as cells move upward.

• Dead cells slough off; about 50,000 cells/minute.

Dermis

Layers of the Dermis

• Papillary Layer: Areolar connective tissue; contains dermal papillae (projections); capillary loops; touch receptors (Meissner's corpuscles); phagocytes.

• Reticular Layer: ~80% of dermal thickness; dense irregular connective tissue; elastic fibers provide stretch-recoil; collagen fibers provide strength and bind water.

Dermal Papillae and Skin Markings

• Dermal papillae form friction ridges (fingerprints).

• Flexure lines: Dermal folds near joints.

• Striae: Stretch marks due to dermal tears.

• Blisters: Fluid-filled pockets separating epidermis and dermis.

Skin Color

Pigments Contributing to Skin Color

Pigment	Description
Melanin	Brown-black pigment; produced by melanocytes; protects against UV radiation.
Carotene	Yellow-orange pigment; accumulates in stratum corneum and hypodermis.
Hemoglobin	Red pigment in blood; visible in fair skin.

Melanin

• Two forms: eumelanin (brown-black), pheomelanin (red-yellow).

• Color differences due to amount and form.

• UV exposure increases melanin production.

Carotene and Hemoglobin

• Carotene: Most apparent in palms and soles; precursor to vitamin A.

• Hemoglobin: Oxygenated blood gives pinkish hue.

Skin Color in Diagnosis

• Cyanosis: Blue skin due to low oxygenation.

• Erythema: Redness due to increased blood flow.

• Pallor: Pale skin due to decreased blood flow.

• Jaundice: Yellow skin due to bilirubin accumulation.

• Bronzing: Metallic appearance; Addison's disease.

• Black-and-blue marks: Bruising (hematomas).

Appendages of the Skin

Hair

• Dead keratinized cells; functions in protection, sensation, and thermoregulation.

• Regions: shaft (above skin), root (within skin), follicle (surrounds root).

• Hair color determined by melanin; red hair contains pheomelanin.

Hair Follicles

• Extend from epidermal surface to dermis.

• Two-layered wall: outer connective tissue root sheath, inner epithelial root sheath.

• Associated with arrector pili muscle (goosebumps).

Types and Growth of Hair

Type	Description
Vellus Hair	Pale, fine body hair of children and adult females.
Terminal Hair	Coarse, long hair of scalp, eyebrows, and after puberty, axillary and pubic regions.

• Average growth: 2.25 mm/week.

• Lose 90 scalp hairs daily.

Hair Thinning and Baldness

• Alopecia: Hair thinning in both sexes after age 40.

• True (frank) baldness: Genetically determined; caused by follicular response to DHT (dihydrotestosterone).

Nails

• Scale-like modifications of epidermis; protective cover for distal, dorsal surface of fingers and toes.

• Contain hard keratin; nail matrix responsible for growth.

Sweat Glands

• Also called sudoriferous glands; distributed over entire skin except nipples and parts of external genitalia.

• Two main types: eccrine and apocrine.

Eccrine Sweat Glands

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

• Ducts connect to pores; function in thermoregulation.

• Secretion: sweat (99% water, salts, vitamin C, antibodies, metabolic wastes).

Apocrine Sweat Glands

• Confined to axillary and anogenital areas.

• Secrete viscous milky or yellowish sweat; odorless until bacterial decomposition.

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

• Modified apocrine glands: ceruminous glands (earwax), mammary glands (milk).

Summary Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum Basale	Deepest	Single row of stem cells; mitosis; melanocytes
Stratum Spinosum	Above basale	Several layers; intermediate filaments; dendritic cells
Stratum Granulosum	Middle	Keratinization begins; granules accumulate
Stratum Lucidum	Only in thick skin	Few rows of dead keratinocytes
Stratum Corneum	Superficial	20–30 layers of dead, keratinized cells

Key Equations

• There are no specific mathematical equations for the structure of the integumentary system, but physiological processes such as sweat production and heat loss can be described by general principles of diffusion and thermodynamics.

Clinical Applications

• Skin color changes can indicate underlying health conditions (e.g., cyanosis, jaundice).

• Hair and nail growth patterns may reflect nutritional or hormonal status.

• Disorders: alopecia, vitiligo, psoriasis, dermatitis.

Additional info: The notes have been expanded to include definitions, examples, and clinical relevance for a comprehensive review suitable for college-level Anatomy & Physiology students.