Integumentary System: Structure, Function, and Clinical Relevance

Membranes of the Body

Types of Membranes

The body contains several types of membranes, each with distinct structures and functions. These include mucous membranes, serous membranes, cutaneous membranes, and synovial membranes.

• Mucous Membrane: Lines body cavities open to the exterior (e.g., digestive, respiratory, urinary tracts). Composed of simple squamous epithelium over areolar connective tissue. Secretes mucus for protection and moisture.

• Serous Membrane: Lines closed ventral body cavities (e.g., pleura, pericardium, peritoneum). Made of simple squamous epithelium (mesothelium) over areolar connective tissue. Secretes serous fluid to reduce friction.

• Cutaneous Membrane: Commonly known as skin. Composed of keratinized stratified squamous epithelium (epidermis) attached to a thick layer of connective tissue (dermis). Functions in protection, sensation, and regulation.

• Synovial Membrane: Made only of connective tissue (no epithelium). Lines joint cavities, secretes synovial fluid for lubrication.

Functions of Membranes

• Protection: Prevents dehydration, blocks pathogens, and houses sensory receptors.

• Secretion: Mucous and serous membranes secrete fluids for lubrication and protection.

• Regulation: Maintains homeostasis by controlling fluid movement and temperature.

Layers of the Integumentary System

Main Layers

The integumentary system consists of three primary layers:

1. Epidermis: Outermost protective shield, composed of epithelial tissue, avascular.

2. Dermis: Underlies the epidermis, made up of mostly dense connective tissue, vascular.

3. Hypodermis (Subcutaneous Layer): Not part of the skin but shares functions. Mostly adipose tissue with some connective tissue. Anchors skin to underlying structures, provides shock absorption and insulation.

Functions of the Layers

• Epidermis: Protection against environmental hazards, water loss, and pathogens.

• Dermis: Provides strength, flexibility, and houses blood vessels, nerves, and glands.

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

Strata of the Epidermis

Layers (from deepest to superficial)

1. Stratum Basale: Deepest layer, contains mitotic stem cells, melanocytes, and Merkel cells.

2. Stratum Spinosum: Contains keratinocytes and Langerhans cells (immune defense).

3. Stratum Granulosum: Keratinization begins, cells flatten and die.

4. Stratum Lucidum: Only in thick skin (palms, soles); adds protection.

5. Stratum Corneum: Most superficial, dead keratinized cells, barrier to environment.

Mnemonic

Come, Let's Get Sun Burned (Corneum, Lucidum, Granulosum, Spinosum, Basale)

Functions of Epidermal Cells

• Keratinocytes: Produce keratin, provide strength and water resistance.

• Melanocytes: Produce melanin, protect DNA from UV radiation.

• Langerhans Cells: Dendritic cells, part of immune defense.

• Merkel Cells: Function as touch receptors.

Accessory Structures of the Integumentary System

Hair

• Structure: Made of hard keratin, more rigid than soft keratin in skin.

• Parts: Shaft (visible), root (embedded), bulb (growth zone), papilla (brings blood and nerves), medulla/cortex/cuticle (layers).

• Function: Protection (scalp, UV, debris), thermoregulation (heat loss), sensory (touch).

Nails

• Structure: Hard keratin, nail bed, matrix, lunula, cuticle, hyponychium.

• Function: Protects distal phalanx, aids in picking up objects.

Glands

• Sudoriferous (Sweat) Glands: Eccrine (cooling, excrete waste), apocrine (odor, emotional sweating), modified (ceruminous, mammary).

• Sebaceous (Oil) Glands: Secrete sebum for lubrication, antibacterial properties.

Skin Color and Clinical Relevance

Pigments

• Melanin: Produced by melanocytes, protects against UV damage.

• Carotene: Yellow-orange pigment from diet.

• Hemoglobin: Red pigment in blood, visible in fair skin.

Development of Skin Cancer

Skin cancer is caused by DNA mutations, often due to UV exposure. Types include:

• Basal Cell Carcinoma: Most common, least malignant, arises from stratum basale.

• Squamous Cell Carcinoma: From keratinocytes in stratum spinosum, can spread if untreated.

• Melanoma: Cancer of melanocytes, most dangerous, highly metastatic.

Warning Signs: "ABCDE" Rule

• A: Asymmetry

• B: Border irregularity

• C: Color variation

• D: Diameter >6mm

• E: Evolving shape, size, or color

Skin Conditions and Aging

Common Conditions

• Blisters: Fluid-filled sacs between epidermis and dermis, caused by friction or burns.

• Stretch Marks: Dermal tearing due to rapid skin stretching (pregnancy, weight gain).

• Calluses: Thickened skin from repeated friction or pressure, increased keratinization.

Photoaging

• UV light damages collagen and elastin fibers in the dermis.

• Leads to wrinkles, sagging, uneven pigmentation, and increased risk of skin cancer.

Aging Effects

• Epidermal and dermal replacement slows, skin becomes thinner and more fragile.

• Fewer sebaceous glands, dry and itchy skin.

• Collagen and elastin loss, wrinkles, less elasticity.

• Subcutaneous fat decreases, less cushioning, more cold sensitivity.

• Fewer melanocytes and Langerhans cells, higher cancer and infection risk.

• Hair follicle activity slows, hair thins, pigment loss (gray/white), terminal hair may convert to vellus hair (wispy, thinning).

Keratin and Its Significance

Keratin Structure and Function

• Keratin: Fibrous structural protein found in hair, nails, and skin.

• Keratin Composition: Rich in the amino acid cysteine.

• Keratin Strength: Cysteine forms disulfide bonds ($S-S$) between keratin molecules, providing toughness and durability.

Types of Keratin

• Hard Keratin: Found in hair and nails, more rigid.

• Soft Keratin: Found in the skin, more flexible.

Summary Table: Membrane Types

Membrane Type	Location	Structure	Main Function
Mucous	Lines cavities open to exterior (digestive, respiratory, urinary)	Epithelium + areolar connective tissue	Secretion, protection
Serous	Lines closed body cavities (pleura, pericardium, peritoneum)	Mesothelium + areolar connective tissue	Secretion, reduces friction
Cutaneous	External body covering (skin)	Keratinized stratified squamous epithelium + dense connective tissue	Protection, sensation, regulation
Synovial	Lines joint cavities	Connective tissue only	Lubrication, reduces friction

Additional info:

• Keratinization is a process where keratinocytes fill with keratin, die, and form the stratum corneum, creating a strong, protective barrier.

• Photoaging and skin cancer are major clinical concerns related to UV exposure and aging.