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

The Integumentary System

Introduction

The integumentary system is the largest organ system in the human body, primarily composed of the skin and its associated structures. It serves as the body's first line of defense and plays a vital role in protection, sensation, and homeostasis.

Functions of the Skin

Main Functions

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

• Temperature Regulation: Maintains body temperature through sweat production and blood flow regulation.

• Excretion: Removes waste products such as urea and salts through sweat.

• Vitamin D3 Production: Synthesizes vitamin D3 when exposed to ultraviolet (UV) light, essential for calcium absorption.

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

Structure of the Skin

Major Layers

• Epidermis: The outermost layer, composed of stratified squamous epithelium.

• Dermis: The middle layer, made of connective tissue, providing strength and elasticity.

• Hypodermis (Subcutaneous Layer): Lies beneath the dermis, consisting mainly of adipose and areolar tissue. It is not technically part of the skin but connects the skin to underlying structures and provides insulation and energy storage.

Cell Types in the Epidermis

Key Cell Types and Their Functions

• Keratinocytes: Found throughout the epidermis; produce keratin, a protein that provides strength and waterproofing.

• Melanocytes: Located in the stratum basale; synthesize melanin, the pigment responsible for skin color and UV protection.

• Merkel (Tactile Epithelial) Cells: Found in the stratum basale; function as touch receptors.

• Dendritic (Langerhans) Cells: Located in the stratum spinosum; act as immune cells, detecting and presenting antigens.

Layers of the Epidermis

From Deepest to Most Superficial

1. Stratum Basale (Germinativum): Deepest layer; site of cell division; contains stem cells, melanocytes, and Merkel cells.

2. Stratum Spinosum: "Spiny" layer; keratinocytes connected by desmosomes; contains dendritic cells and bundles of intermediate filaments.

3. Stratum Granulosum: Contains keratinocytes with keratohyalin and lamellated granules; initiates keratinization and water barrier formation.

4. Stratum Lucidum: Present only in thick skin (palms, soles); consists of a few layers of dead keratinocytes.

5. Stratum Corneum: Outermost layer; thick layer of dead, keratinized cells; provides protection against abrasion and penetration.

Dermis

Structure and Modifications

• Papillary Layer: Superficial layer with dermal papillae that increase surface area for exchange of gases, nutrients, and waste; forms fingerprints.

• Reticular Layer: Deeper, thicker layer (about 80% of dermis); contains dense irregular connective tissue, providing strength and elasticity.

Dermal Modifications

• Dermal Papillae: Projections that interlock with the epidermis, increasing surface area and forming ridges (fingerprints).

• Cleavage (Tension) Lines: Patterns of collagen fibers; important for surgical incisions.

• Flexure Lines: Dermal folds at or near joints.

• Friction Ridges: Enhance gripping ability (e.g., fingerprints).

Hypodermis

Composition and Function

• Composed of areolar and adipose connective tissue.

• Functions: Fat storage, insulation, and anchoring the skin to underlying structures.

• Not technically part of the skin but closely associated with its function.

Skin Color

Determinants of Skin Color

• Melanin: Produced by melanocytes; amount and type determine skin color.

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

• Hemoglobin: Oxygenated pigment in red blood cells; gives a pinkish hue, especially in lighter skin.

Additional info: Bruising (hematoma) results from blood escaping into tissues, causing discoloration.

Nails

Structure and Growth

• Composed of hard keratin.

• Main parts: Free edge, nail body (plate), nail bed, cuticle (eponychium), lateral edges, nail root, nail matrix, lunule.

• Nail matrix is responsible for nail growth.

Hair

Structure and Types

• Composed of flexible, dead keratinized cells.

• Shaft: Visible part; shape determines hair type (round = straight, oval = wavy, flat/ribbonlike = kinky).

• Root: Embedded in the skin.

• Three concentric layers: Medulla (core), cortex (middle), cuticle (outermost).

Hair Follicle and Associated Structures

• Hair follicle: Extends from epidermis to dermis; contains the hair bulb at its base.

• Dermal papilla: Provides nutrients and signals for hair growth.

• Arrector pili muscle: Smooth muscle attached to follicle; contracts to produce "goosebumps."

Types of Hair

• Vellus hair: Fine, unpigmented hair found on children and adult females.

• Terminal hair: Coarser, pigmented hair found on scalp, face, axillary, and pubic regions.

Hair Color and Growth

• Hair color: Determined by type and amount of melanin produced by melanocytes in the hair bulb.

• Growth: Hair grows about 2 mm per week; scalp hair has a growth phase of about 4 years, while eyelash hair grows for a few months.

• Hair loss: Terminal hair can be replaced by vellus hair (e.g., male pattern baldness).

Glands of the Skin

Sebaceous (Oil) Glands

• Produce sebum (oil) via holocrine secretion.

• Associated with hair follicles; lubricate hair and skin, collect dirt, and have antibacterial properties.

• Activated by hormones (androgens); overactivity can lead to acne.

Sweat (Sudoriferous) Glands

• Eccrine (Merocrine) Glands: Most numerous; produce "true" sweat (water, salts, metabolic wastes); important for thermoregulation.

• Apocrine Glands: Found in axillary and genital areas; produce a thicker secretion that may function in scent communication.

• Specialized Glands: Ceruminous (earwax) and mammary (milk) glands are modified apocrine glands.

Burns and Clinical Assessment

Types of Burns

• 1st Degree: Involves only the epidermis; symptoms include redness, swelling, and pain.

• 2nd Degree: Involves epidermis and upper dermis; blisters form; heals with minimal scarring.

• 3rd Degree: Destroys entire epidermis and dermis; area appears white, red, or black; requires skin grafting.

Rule of Nines

Used to estimate the percentage of body surface area affected by burns:

Body Region	Percentage
Head	9%
Each Arm	9%
Each Leg	18%
Anterior Trunk	18%
Posterior Trunk	18%
Perineum	1%

Skin Cancer

Types and Detection

• Basal Cell Carcinoma: Least malignant; arises from stratum basale.

• Squamous Cell Carcinoma: Arises from keratinocytes of the stratum spinosum.

• Melanoma: Most dangerous; cancer of melanocytes.

ABCDE Rule for Melanoma Detection:

• A - Asymmetry

• B - Border irregularity

• C - Color variation

• D - Diameter > 6mm

• E - Evolving (change in size, shape, color, or symptoms)

Skin Through the Lifecycle

Developmental Changes

• Fetal Skin: Well-formed after the fourth month; covered with vernix caseosa and lanugo (downy hairs).

• Aging: Skin thins, becomes less elastic, and is more prone to damage and inflammation.

Review Questions

• Identify all five layers of the epidermis in thick skin. What types of cells will you find in each layer?

• What determines skin color?

• What part of the epithelial root sheath contains stem cells? What do these stem cells become?

• What are the two types of sweat gland? How do they differ?

• What can you teach someone if they are wondering how they can be proactive on screening for melanoma?