The Integumentary System: Structure and Function

The Integumentary System

Overview

The integumentary system is composed of the skin and its accessory structures, including hair, nails, and glands. As the largest organ system, it plays a vital role in protection, sensation, thermoregulation, excretion, and vitamin D synthesis.

• Skin (Cutaneous Membrane): Largest organ, accounting for 10-15% of body weight. It consists of two main layers:

  • Superficial Epidermis: Keratinized stratified squamous epithelium resting on a basement membrane.

  • Deep Dermis: Loose and dense irregular connective tissue.

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

• Hypodermis (Superficial Fascia/Subcutaneous Tissue): Not part of the skin; consists of loose connective and adipose tissue, highly vascularized.

5.1 Skin Structure

Basic Anatomy

The skin is organized into distinct layers and contains various structures essential for its function.

• Epidermis: Outermost, avascular layer; consists of dead cells at the surface.

• Dermis: Vascularized, deeper layer; supports and nourishes the epidermis.

• Accessory Structures: Include nails, hair, sweat glands, and sebaceous glands.

Cellulite

Cellulite refers to the dimpled appearance of skin due to collagen bands forming around adipose tissue in the hypodermis. It is influenced by genetics, sex, fat distribution, and age, and is considered normal. Diet and exercise may minimize but not eliminate cellulite.

5.1 Functions of the Integumentary System

Key Functions

• Protection: Shields underlying tissues from mechanical trauma, pathogens, and environmental threats. The skin's acid mantle (slightly acidic pH) deters pathogen growth.

• Sensation: Sensory receptors detect stimuli, crucial for homeostasis.

• Thermoregulation: Maintains internal temperature via negative feedback loops.

• Excretion: Eliminates small amounts of metabolic waste (e.g., lactic acid, urea) in sweat.

• Vitamin D Synthesis: UV exposure converts a precursor in the epidermis to cholecalciferol, which is modified by the liver and kidneys to form calcitriol, essential for calcium absorption.

Thermoregulation: Negative Feedback Loops

• When body temperature rises:

  • Stimulus: Increased body temperature

  • Receptor: Thermoreceptors in the brain

  • Control Center: Hypothalamus (heat-loss center)

  • Effector/Response: Sweating and vasodilation

  • Return to Normal: Response stops when temperature normalizes

• When body temperature falls:

  • Stimulus: Decreased body temperature

  • Receptor: Thermoreceptors in the skin

  • Control Center: Hypothalamus (heat-promoting center)

  • Effector/Response: Vasoconstriction

  • Return to Normal: Response stops when temperature normalizes

5.2 The Epidermis

Structure and Cell Types

The epidermis is the superficial layer of the skin, composed of several cell types and up to five layers (strata).

• Keratinocytes: 95% of epidermal cells; produce keratin for strength and resistance to stress; connected by desmosomes.

Layers (Strata) of the Epidermis (Deep to Superficial)

• Stratum Basale (Germinativum): Single layer of stem cells; mitotically active; produces keratin and vitamin D precursors.

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

• Stratum Granulosum: 3-5 rows; contains granules; forms water barrier.

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

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

Mnemonic: Brilliant Studying Gives Loads of Confidence (Basale, Spinosum, Granulosum, Lucidum, Corneum)

Keratinocyte Life Cycle

• Cells in the stratum corneum are continually shed and replaced.

• Keratinocytes in the stratum basale and spinosum divide and push older cells upward.

• The process from formation to shedding 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 vs. Thin Skin

• Thick Skin: Palms, soles, fingers, toes; all five strata; no hair follicles; many sweat glands.

• Thin Skin: Covers most of the body; lacks stratum lucidum; has hair follicles, sweat, and sebaceous glands.

• Callus: Thickened stratum corneum due to repeated pressure.

5.3 The Dermis

Structure and Layers

The dermis is the deep layer of the skin, providing structural support and housing blood vessels, nerves, and accessory structures.

• Papillary Layer: Superficial; loose connective tissue; forms dermal papillae that supply blood and house tactile corpuscles.

• Reticular Layer: Deep; dense irregular connective tissue; contains blood vessels, glands, hair follicles, and lamellated corpuscles for deep pressure and vibration.

Skin Markings

• Dermal Ridges: Prominent in thick skin; form unique epidermal ridges (fingerprints).

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

• Flexure Lines: Deep creases in areas of frequent movement (e.g., palms).

Skin Wrinkles

• Caused by decreased collagen, elastic fibers, proteoglycans, and adipose tissue with age.

• Accelerated by UV exposure and smoking.

• Temporary treatments include Botox, fillers, peels, and topical creams.