Chapter 5: The Integumentary System – Structure, Function, and Clinical Relevance

Integumentary System Overview

Introduction

The integumentary system is the body's largest organ system, primarily composed of the skin and its accessory structures. It serves as the first line of defense against environmental hazards and plays a vital role in maintaining homeostasis.

• Cutaneous membrane (skin): Consists of the epidermis (superficial epithelium) and dermis (deep connective tissue).

• Accessory structures: Includes hair, hair follicles, nails, and exocrine glands.

• Subcutaneous layer (hypodermis): Composed of loose connective tissue, attaches skin to underlying tissues, and provides insulation and padding.

Functions of the Integumentary System

Role in Homeostasis

The integumentary system performs several essential functions that contribute to the body's overall homeostasis.

• Regulation of body temperature: Through sweat production and blood vessel dilation/constriction.

• Vitamin D synthesis: Converts inactive vitamin D to its active form upon exposure to sunlight, which is crucial for calcium absorption.

• Sensory reception: Contains receptors for touch, pain, pressure, and temperature.

• Protection: Acts as a physical barrier against pathogens, chemicals, and mechanical injury.

• Blood reservoir: Stores blood that can be redirected to other organs as needed.

Structural Features of the Skin

Layers of the Skin

The skin is composed of multiple layers, each with distinct structures and functions.

• Epidermis: Outermost layer, made of stratified squamous epithelium. Provides waterproofing and protection.

• Dermis: Middle layer, composed of connective tissue. Contains blood vessels, nerves, hair follicles, and glands.

• Hypodermis (subcutaneous layer): Deepest layer, primarily adipose tissue. Provides insulation and energy storage.

Epidermis Structure and Function

• Keratinocytes: Main cell type, produce keratin for strength and waterproofing.

• Melanocytes: Produce melanin pigment, which protects against UV radiation.

• Langerhans cells: Immune cells that help detect and fight pathogens.

• Merkel cells: Sensory cells associated with touch.

The epidermis is organized into layers (from superficial to deep):

• Stratum corneum: Multiple layers of dead, keratinized cells.

• Stratum lucidum: Present only in thick skin (palms, soles).

• Stratum granulosum: Cells begin to die and accumulate keratin.

• Stratum spinosum: Provides strength and flexibility.

• Stratum basale: Deepest layer, site of cell division.

Dermis and Hypodermis Structure and Function

• Papillary layer: Superficial dermis, areolar connective tissue, contains capillaries and sensory receptors.

• Reticular layer: Deep dermis, dense irregular connective tissue, provides strength and elasticity.

• Hypodermis: Not technically part of the skin, but supports it. Contains adipose tissue for insulation and energy storage.

Individual Differences in Skin Color

Determinants of Skin Color

Skin color is determined by the amount and type of pigments present in the skin.

• Melanin: Produced by melanocytes; provides brown to black coloration and UV protection.

• Carotene: Yellow-orange pigment stored in the stratum corneum and adipose tissue.

• Hemoglobin: Red pigment in blood, contributes to skin's pinkish hue.

Genetic factors, environmental exposure, and certain disorders (e.g., albinism, vitiligo) can affect skin color.

Accessory Structures and Glands

Hair

• Structure: Composed of the cuticle (outer layer), cortex (middle layer with pigment), and medulla (central core).

• Function: Protection from UV light, sensory input, and insulation.

Nails

• Structure: Modification of the stratum corneum at the tips of fingers and toes.

• Function: Protects the distal phalanges and aids in manipulation of objects.

Glands

• Sebaceous (oil) glands: Secrete sebum to lubricate skin and hair; have antibacterial properties.

• Sudoriferous (sweat) glands: Include eccrine (widely distributed, produce watery sweat for cooling) and apocrine (associated with hair follicles, produce thicker secretions).

• Ceruminous glands: Modified sweat glands in the ear canal, produce earwax.

• Mammary glands: Specialized for milk production.

Skin Cancers and the ABCDE Rule

Types of Skin Cancer

• Basal cell carcinoma: Most common, least malignant; arises from stratum basale cells.

• Squamous cell carcinoma: Second most common; arises from keratinocytes of the stratum spinosum.

• Malignant melanoma: Most dangerous; arises from melanocytes, highly metastatic.

ABCDE Rule for Early Detection

• Asymmetry: Uneven shape.

• Border: Irregular, notched edges.

• Color: Multiple colors or uneven distribution.

• Diameter: Larger than 6 mm.

• Evolution: Changes in size, shape, color, or symptoms.

Burns: Types, Classifications, and Rule of Nines

Classification by Depth

• First-degree burns: Affect only the epidermis; redness and mild pain.

• Second-degree burns: Affect epidermis and part of dermis; blistering, pain, and inflammation.

• Third-degree burns: Destroy epidermis, dermis, and may extend into hypodermis; skin may appear white, red, or blackened; nerve endings destroyed.

Rule of Nines

The Rule of Nines is used to estimate the percentage of body surface area affected by burns, which is important for fluid resuscitation and treatment planning.

Body Region	Percentage
Head	9%
Each upper limb	9%
Each lower limb	18%
Anterior trunk	18%
Posterior trunk	18%
Genitalia	1%

Effects of Aging on the Skin

Age-Related Changes

• Thinning of epidermis and dermis: Skin becomes more fragile.

• Decreased cell division: Slower healing and regeneration.

• Reduced sweat and oil gland activity: Skin becomes dry and less able to regulate temperature.

• Loss of elasticity: Wrinkles and sagging due to decreased collagen and elastin.

• Reduced blood supply: Slower nutrient delivery and waste removal.

Summary Table: Layers of the Skin

Layer	Main Cell Types	Key Functions
Stratum corneum	Dead keratinocytes	Protection, waterproofing
Stratum lucidum	Clear, dead cells (only in thick skin)	Extra protection
Stratum granulosum	Keratinocytes	Keratin formation
Stratum spinosum	Keratinocytes, Langerhans cells	Strength, immune defense
Stratum basale	Stem cells, melanocytes, Merkel cells	Cell division, pigment production, sensation

Key Equations

• Vitamin D Synthesis: $\text{7-dehydrocholesterol} \xrightarrow{UV~light} \text{cholecalciferol (vitamin D}_3\text{)}$ $\text{cholecalciferol} \xrightarrow{liver} \text{calcidiol} \xrightarrow{kidney} \text{calcitriol}$

Example: A patient with third-degree burns covering the anterior trunk and both lower limbs would have burns over 54% of their body surface area (18% + 18% + 18%).

Additional info: The notes include references to healthcare disparities in skin cancer detection and outcomes, emphasizing the importance of recognizing signs in diverse skin tones.