The Integumentary System

Overview

The Integumentary System consists of the skin and its accessory structures, including hair, nails, and glands. The skin is the largest organ in the body, accounting for 10-15% of total body weight. It is composed of two main layers: the superficial epidermis and the deeper dermis. Below the dermis lies the hypodermis, which is not technically part of the skin but provides structural and metabolic support.

Skin Structure

• Epidermis: Keratinized stratified squamous epithelium; avascular; rests on a basement membrane.

• Dermis: Loose and dense irregular connective tissue; vascularized; contains accessory structures.

• Hypodermis: Loose connective and adipose tissue; highly vascularized; not part of the skin.

• Accessory Structures: Hair, nails, sweat glands, sebaceous glands, sensory neurons, and arrector pili muscles.

Functions of the Integumentary System

Protection

The skin protects underlying tissues from mechanical trauma, pathogens, and environmental threats. The acid mantle (slightly acidic pH) deters pathogen growth, and immune cells destroy invaders. Lipid-based chemicals repel water and salts, preventing water loss or entry.

Sensation

Sensory receptors in the skin detect potentially harmful stimuli, which is crucial for maintaining homeostasis.

Thermoregulation

Thermoregulation is achieved through negative feedback loops that maintain stable internal body temperature.

• When body temperature rises: Thermoreceptors in the brain detect the change, the hypothalamus activates heat-loss mechanisms (sweating, vasodilation), and temperature returns to normal.

• When body temperature falls: Thermoreceptors in the skin detect the change, the hypothalamus activates heat-conserving mechanisms (vasoconstriction), and temperature returns to normal.

Excretion

Small amounts of metabolic waste (lactic acid, urea) are eliminated in sweat.

Synthesis of Vitamin D

UV radiation converts a precursor in the epidermis to cholecalciferol, which is modified by the liver and kidneys to form calcitriol, essential for calcium absorption.

The Epidermis

Cell Types and Layers

The epidermis is composed of several cell types and up to five layers (strata). The main cell type is the keratinocyte, which produces keratin for strength and resistance to mechanical stress.

• Stratum Basale: Deepest layer; mitotically active stem cells; produces keratin and vitamin D precursors.

• Stratum Spinosum: Thickest layer; spiky-shaped cells; metabolically and mitotically active.

• Stratum Granulosum: Cells with cytoplasmic granules; forms water barrier.

• Stratum Lucidum: Clear layer of dead keratinocytes; found only in thick skin.

• Stratum Corneum: Several layers of dead, flattened keratinocytes; cells are shed daily.

Keratinocyte Life Cycle

Keratinocytes originate in the stratum basale and are pushed upward through the strata, eventually being sloughed off in the stratum corneum. This process takes 40-50 days.

Other Epidermal Cells

• Dendritic (Langerhans) Cells: Immune phagocytes in the stratum spinosum.

• Tactile (Merkel) Cells: Sensory receptors for light touch in the stratum basale.

• Melanocytes: Produce melanin pigment in the stratum basale.

Thick and Thin Skin

• Thick Skin: All five strata; found on palms, soles, fingers, toes; lacks hair follicles; numerous sweat glands.

• Thin Skin: Four strata (no stratum lucidum); covers most of the body; includes hair follicles, sweat, and sebaceous glands.

• Callus: Additional layers of stratum corneum formed in response to repeated pressure.

The Dermis

Structure and Layers

The dermis anchors the epidermis, houses blood supply, and contains sensory receptors. It is divided into two layers:

• Papillary Layer: Superficial; loose connective tissue; contains dermal papillae and tactile corpuscles.

• Reticular Layer: Deep; dense irregular connective tissue; contains blood vessels, sweat glands, sebaceous glands, and lamellated corpuscles.

Skin Markings

• Dermal Ridges: Form epidermal ridges and fingerprints; unique to each individual.

• Tension (Cleavage) Lines: Gaps between collagen bundles; important for surgical incisions.

• Flexure Lines: Deep creases on palms and other areas.

Melanin and Skin Color

Melanin

Melanin is produced by melanocytes in the stratum basale and stored in melanosomes. It protects DNA from UV radiation and must be continually replaced.

• Melanin synthesis increases with UV exposure.

• Melanin also regulates vitamin D synthesis.

• Variations include freckles, moles, and albinism.

Other Pigments

• Carotene: Yellow-orange pigment from diet; accumulates in stratum corneum.

• Hemoglobin: Iron-containing protein in blood; gives skin a pinkish hue in fair-skinned individuals.

Skin Color as a Diagnostic Tool

• Erythema: Increased blood flow; red appearance.

• Pallor: Decreased blood flow; pale appearance.

• Cyanosis: Low oxygen; bluish appearance.

• Jaundice: Bilirubin accumulation; yellow-orange appearance.

Accessory Structures: Hair, Nails, and Glands

Hair

Hair (pili) is composed of keratinized epithelial cells and functions in protection and sensation. It consists of the shaft (dead cells), root (living cells), and follicle. The arrector pili muscle causes piloerection (goosebumps).

• Hair Growth: Alternates between growth and resting stages; alopecia is baldness.

• Hair Types: Lanugo (fetal), terminal (thick, pigmented), vellus (thin, nonpigmented).

• Hair Color: Determined by melanin; decreases with age.

Nails

Nails are hard structures at the ends of digits, composed of stratified squamous epithelium and hard keratin. They protect fingertips and aid in manipulation.

• Nail Plate: Includes nail body and root; matrix is the site of cell division.

• Folds: Proximal, medial, lateral, and hyponychium.

• Lunula: Opaque half-moon region; nails are translucent.

Glands

• Eccrine Sweat Glands: Most prevalent; thermoregulation; watery sweat.

• Apocrine Sweat Glands: Found in axillae, anal area, areolae; thick, protein-rich sweat; odor develops after bacterial metabolism.

• Ceruminous Glands: Modified apocrine glands; produce cerumen (ear wax).

• Mammary Glands: Specialized sweat glands; produce milk.

• Sebaceous Glands: Produce sebum; holocrine secretion; increases at puberty.

Acne

Acne is caused by accumulation of sebum and dead cells in sebaceous glands, leading to comedones (blackheads) and pustules (pimples). Male sex hormones contribute to its development.

Wounds, Burns, and Skin Cancer

Wounds

Wounds are disruptions in skin integrity, including lacerations, burns, and skin cancers.

Burns

• First-Degree: Only epidermis; erythema and minor pain; no blisters.

• Second-Degree: Epidermis and part/all of dermis; pain, blistering, scarring.

• Third-Degree: Epidermis, dermis, hypodermis, and deeper tissues; not initially painful; major tissue damage and scarring.

Rule of Nines

The Rule of Nines estimates the percentage of body surface affected by burns, dividing the body into areas representing 9% each (genital area is 1%).

Skin Cancer

• Basal Cell Carcinoma: Most common; arises from keratinocytes in stratum basale; rarely metastasizes.

• Squamous Cell Carcinoma: Second most common; arises from keratinocytes in stratum spinosum; more likely to metastasize.

• Malignant Melanoma: Least common but most dangerous; arises from melanocytes; highly metastatic.

• ABCDE Rule: Asymmetry, Border irregularity, Color variation, Diameter >6 mm, Evolving shape/size.

Summary Table: Layers of the Epidermis

Summary Table: Types of Skin Cancer