Skin Organization

Parts of the Integumentary System

The integumentary system consists of the skin and its derivatives, including hair, nails, and cutaneous glands. It serves as the body's primary barrier against the external environment.

• Skin: Composed of the epidermis, dermis, and hypodermis.

• Hair and Nails: Specialized structures derived from the epidermis.

• Cutaneous Glands: Include sweat and sebaceous glands.

Epidermis vs. Dermis

The skin is organized into two main layers: the epidermis and dermis, each with distinct tissue types, blood supply, and functions.

• Epidermis: Stratified squamous epithelium; avascular; provides protection.

• Dermis: Dense irregular connective tissue; vascular; supports and nourishes the epidermis.

Hypodermis

The hypodermis, also known as the subcutaneous layer, lies beneath the dermis and consists mainly of adipose tissue. It is not technically part of the skin but anchors the skin to underlying structures.

• Location: Deep to the dermis.

• Tissue Type: Adipose and areolar connective tissue.

• Functions: Insulation, energy storage, and shock absorption.

Additional info: All four tissue types (epithelial, connective, muscle, and nervous) are represented in the skin and its appendages.

Epidermal Cells and Layers

Cell Types and Functions

The epidermis contains several specialized cell types, each with unique functions:

• Keratinocytes: Produce keratin, a protein that provides strength and waterproofing.

• Melanocytes: Synthesize melanin, the pigment responsible for skin color and UV protection.

• Dendritic (Langerhans) Cells: Immune cells that defend against pathogens.

• Tactile (Merkel) Cells: Sensory receptors for touch.

Key Structures

• Keratin: Fibrous protein for protection.

• Desmosomes: Cell junctions providing mechanical strength.

• Melanin: Pigment stored in melanosomes.

Epidermal Layers (Strata)

The epidermis is organized into distinct layers, each with specific functions. From deep to superficial:

1. Stratum Basale: Site of cell division; contains stem cells and melanocytes.

2. Stratum Spinosum: Provides strength; contains desmosomes and dendritic cells.

3. Stratum Granulosum: Cells begin to lose nuclei and organelles; keratinization starts.

4. Stratum Lucidum: Present only in thick skin; translucent layer.

5. Stratum Corneum: Dead, keratinized cells; provides barrier function.

• Cell Division: Occurs in the stratum basale.

• Loss of Nuclei/Organelles: Begins in the stratum granulosum.

• Keratinization: Process by which cells become filled with keratin and die.

• Cell Replacement and Shedding: Surface cells are continuously shed and replaced.

Thick vs. Thin Skin

• Thick Skin: Found on palms and soles; contains all five strata; lacks hair and sebaceous glands.

• Thin Skin: Covers most of the body; lacks stratum lucidum; contains hair and sebaceous glands.

Example: If basal cells stop dividing, the epidermis would thin and eventually fail to regenerate, leading to increased vulnerability.

Dermis and Skin Markings

Papillary vs. Reticular Layers

The dermis is divided into two layers:

• Papillary Layer: Areolar connective tissue; contains dermal papillae, capillaries, and sensory receptors.

• Reticular Layer: Dense irregular connective tissue; contains collagen and elastic fibers, blood vessels, nerves, and skin appendages.

Dermal Structures

• Dermal Papillae: Projections that increase surface area and form friction ridges (fingerprints).

• Collagen Fibers: Provide strength.

• Elastic Fibers: Allow stretch and recoil.

• Cleavage Lines: Regions of less dense collagen; important for surgical incisions.

• Flexure Lines: Skin folds at joints.

• Striae (Stretch Marks): Tearing of dermal collagen.

• Blisters: Separation of epidermis from dermis by fluid.

Example: Damage to the epidermis heals quickly; damage to the dermis may result in scarring and loss of function.

Skin Color and Medical Diagnosis

Normal Pigments

• Melanin: Brown-black pigment; protects against UV.

• Carotene: Yellow-orange pigment from diet.

• Hemoglobin: Red pigment in blood; gives pinkish hue.

Clinical Findings

• Cyanosis: Blue skin; low oxygenation.

• Pallor: Pale skin; reduced blood flow or anemia.

• Erythema: Redness; increased blood flow.

• Jaundice: Yellow skin; bilirubin accumulation.

• Bruising: Purple/blue marks; blood leakage.

Example: Anemia causes pallor due to reduced hemoglobin, while reduced blood flow causes pallor by vasoconstriction.

Additional info: Skin color changes require context (history, symptoms) for accurate diagnosis.

Hair and Nails

Structure and Function

• Hair: Composed of hard keratin; functions include protection, sensation, and thermoregulation.

• Nails: Hard keratin; protect fingertips and aid in manipulation.

Hair Anatomy

• Shaft: Visible part above skin.

• Root: Embedded in skin.

• Bulb: Expanded base; site of growth.

• Cuticle: Outer layer.

• Cortex: Middle layer.

• Medulla: Central core.

• Follicle Sheaths: Surround hair root; include inner and outer layers.

• Hair Matrix: Site of cell division.

• Hair Papilla: Provides nutrients.

• Root Hair Plexus: Sensory nerve endings.

• Arrector Pili: Smooth muscle causing hair to stand.

Hair Types

• Vellus Hair: Fine, pale hair; covers children and adult females.

• Terminal Hair: Coarse, pigmented hair; found on scalp, eyebrows, and at puberty.

Example: Cutting hair removes the shaft, but damaging the matrix prevents regrowth.

Nail Matrix

• Location: Proximal nail bed.

• Function: Responsible for nail growth.

Hair Loss and Telogen Effluvium

Types and Causes

• Alopecia: General hair loss.

• Pattern Baldness: Genetic; influenced by dihydrotestosterone (DHT).

• Telogen Effluvium: Sudden diffuse shedding; triggered by stress, hormonal changes, surgery, or crash dieting.

Hair Cycle Changes

• Timing: Shedding occurs weeks to months after trigger.

• Recovery: Hair regrows if stress resolves.

• Other Factors: Medications, hypothyroidism, protein deficiency.

Example: History and timing of hair loss help distinguish telogen effluvium from other causes.

Cutaneous Glands

Types and Functions

• Eccrine (Merocrine) Glands: Widely distributed; open to skin surface; secrete watery sweat for cooling.

• Apocrine Glands: Found in axilla and groin; open to hair follicles; secrete viscous sweat; involved in odor.

• Sebaceous Glands: Associated with hair follicles; secrete oily sebum; lubricates skin and hair.

• Ceruminous Glands: Ear canal; produce earwax.

• Mammary Glands: Produce milk.

Secretion Mechanisms

• Merocrine Secretion: Exocytosis; no cell damage.

• Holocrine Secretion: Cell ruptures to release contents.

Other Features

• Myoepithelial Cells: Contract to expel sweat.

• Nervous Control: Regulates sweating.

• Dermcidin: Antimicrobial peptide in sweat.

• Seborrhea/Cradle Cap: Overactive sebaceous glands.

Example: Eccrine glands cool the body; apocrine glands contribute to odor; sebaceous glands cause oily skin.

Gland Comparison Table

Functions of Skin

Barrier Functions

• Chemical Barriers: Acid mantle, antimicrobial peptides.

• Physical Barriers: Keratinized cells, tight junctions.

• Biological Barriers: Dendritic cells, macrophages.

Temperature Regulation

• Dermal Blood Vessels: Vasodilation increases heat loss; vasoconstriction conserves heat.

• Sweating: Evaporation cools the body.

• Sensible vs. Insensible Perspiration: Sensible is visible sweat; insensible is continuous, unnoticed water loss.

Sensory Receptors

• Merkel Cells: Touch; in epidermis.

• Meissner Corpuscles: Light touch; in dermal papillae.

• Root Hair Plexus: Hair movement.

• Free Nerve Endings: Pain and temperature.

• Lamellar (Pacinian) Corpuscles: Deep pressure; in dermis.

Vitamin D Synthesis

• Cholecalciferol: Produced in skin by UV exposure.

• Calcidiol: Formed in liver.

• Calcitriol: Active form; produced in kidneys.

• Function: Promotes calcium absorption in intestines.

Example: Impaired kidney function disrupts vitamin D activation, affecting calcium metabolism.

Other Functions

• Metabolic Functions: Synthesis of chemicals.

• Blood Reservoir: Stores blood.

• Excretion: Removes waste via sweat.

Example: Reduced sweating impairs temperature regulation, increasing risk of overheating.

Skin Cancer

Types and Characteristics

• Benign: Non-spreading.

• Malignant: Can metastasize (spread).

• Basal Cell Carcinoma: Most common; arises from basal cells; rarely metastasizes.

• Squamous Cell Carcinoma: Arises from keratinocytes; moderate risk of spread.

• Melanoma: Arises from melanocytes; high risk of metastasis.

ABCD Rule for Melanoma Detection

• A: Asymmetry

• B: Border irregularity

• C: Color variation

• D: Diameter > 6 mm

Example: Small lesions can be dangerous if malignant cells invade and spread.

Burns and Loss of Skin Function

Burn Types and Effects

• First-Degree: Superficial; affects epidermis; redness and pain.

• Second-Degree: Partial-thickness; affects epidermis and part of dermis; blisters, pain.

• Third-Degree: Full-thickness; destroys epidermis and dermis; no pain (nerve endings destroyed), requires grafting.

Injury Classification

• Superficial: Epidermis only.

• Partial-Thickness: Epidermis and part of dermis.

• Full-Thickness: Entire skin destroyed.

Complications

• Immediate: Fluid loss, electrolyte imbalance, circulatory shock.

• Later: Infection, impaired kidney function.

Rule of Nines

Used to estimate burn area in adults:

Treatment

• Fluid Replacement: Prevents shock.

• Removal of Dead Tissue: Reduces infection risk.

• Temporary Coverings/Skin Grafts: Promote healing.

• Antibiotics: Prevent/treat infection.

Example: Deeper burns may hurt less due to nerve destruction; large burns affect multiple organs.

Studying for Application Questions

Approach

• Identify the structure involved.

• Recall its normal function.

• Explain the likely consequence of any change.

• Practice labeling diagrams and explaining functions.

Additional info: Application questions require integration of structural, functional, and clinical knowledge.