The Integumentary System

Overview and Major Components

The integumentary system consists primarily of the skin (integument) and its accessory structures. It is the body's largest organ system, accounting for a significant portion of total body weight and surface area. The skin serves as the first line of defense against environmental hazards.

• Skin (Integument): Comprises about 16% of total body weight and covers approximately 1.5–2 m2 in adults.

• Major Components:

  • Cutaneous membrane (skin itself)

  • Accessory structures (hair, nails, exocrine glands)

Cutaneous Membrane

Layers of the Skin

The cutaneous membrane is divided into two main layers: the epidermis and the dermis. Beneath these lies the hypodermis, which is not technically part of the skin but supports its structure and function.

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

• Dermis: The deeper layer, subdivided into:

  • Papillary layer: Contains areolar connective tissue and forms dermal papillae.

  • Reticular layer: Contains dense irregular connective tissue, providing strength and elasticity.

• Hypodermis (Subcutaneous Layer): Composed of areolar and adipose connective tissue; binds skin to underlying organs and is not considered part of the skin.

Accessory Structures of the Skin

Types and Functions

Accessory structures originate from the epidermis and include hair, nails, and exocrine glands. Blood vessels and nerve fibers support the skin's functions.

• Hair: Provides protection and sensory input.

• Nails: Protect the tips of fingers and toes.

• Exocrine glands: Include sweat and sebaceous glands, which help regulate temperature and excrete wastes.

Functions of the Integumentary System

Protective and Regulatory Roles

The integumentary system performs several vital functions to maintain homeostasis and protect the body.

• Protection: Shields underlying tissues and organs from impact, abrasion, fluid loss, and chemical attack.

• Excretion: Removes salts, water, and organic wastes via integumentary glands.

• Temperature Regulation: Maintains normal body temperature through radiation, conduction, convection, and evaporation.

• Melanin Production: Protects against ultraviolet (UV) radiation.

• Keratin Production: Provides waterproofing and resistance to abrasion.

• Vitamin D3 Synthesis: Essential for calcium metabolism.

• Lipid Storage: Stores energy in adipose tissue.

• Sensory Detection: Detects touch, pressure, pain, and temperature.

Epidermis

Structure and Cell Types

The epidermis is a stratified squamous epithelium dominated by keratinocytes. It consists of several layers (strata) that are continuously produced by stem cell divisions and shed at the surface.

• Keratinocytes: Main cell type, produce keratin.

• Strata: Multiple layers, each with distinct functions.

• Epidermal ridges: Increase surface area for attachment to the dermis.

Types of Skin

• Thin Skin: Covers most of the body; contains 4 strata; thickness ~0.08 mm.

• Thick Skin: Found on palms and soles; contains 5 strata; thickness ~0.5 mm; lacks hair and sebaceous glands.

Epidermal Layers (Strata)

• Stratum basale (basal layer): Attached to basement membrane; contains stem cells; takes 7–10 days for cells to reach the surface.

• Stratum spinosum (spiny layer): 8–10 layers of keratinocytes; contains dendritic (Langerhans) cells for immune response.

• Stratum granulosum (grainy layer): 3–5 layers; cells produce keratohyalin; cells become thinner and less permeable.

• Stratum lucidum: Only in thick skin; flattened, densely packed, keratinized, dead cells.

• Stratum corneum: 15–30 layers of dead, keratinized cells; waterproof; allows for insensible perspiration; cells remain for about 2 weeks before shedding.

Blood Supply and Metabolic Demand

• The epidermis lacks blood vessels and relies on diffusion from capillaries in the dermis.

• Cells closest to the dermis have the highest metabolic demand.

Clinical Conditions

• Psoriasis: Rapid cell division in the stratum basale leads to red, scaly patches.

• Bed sores (pressure ulcers): Caused by prolonged pressure, leading to tissue breakdown.

Specialized Cells in the Epidermis

Cell Types and Functions

• Melanocytes: Produce melanin from tyrosine; found in the stratum basale; melanin absorbs UV rays and protects DNA.

• Skin color: Determined by melanin production, not the number of melanocytes; influenced by genetics and UV exposure.

• Albinism: Deficiency or absence of melanin synthesis.

• Blood supply: Hemoglobin gives skin a red color when oxygenated; cyanosis (bluish tint) occurs when oxygen is low.

Skin Cancers

Types and Causes

• Basal cell carcinoma: Most common; originates in the stratum basale; caused by mutations, often due to UV exposure.

• Squamous cell carcinoma: Originates in the squamous cells; associated with UV exposure, chronic wounds, and chemical exposure.

• Malignant melanoma: Cancer of melanocytes; aggressive and linked to excessive UV exposure.

Dermis

Papillary Layer

The papillary layer is composed of highly vascularized areolar tissue and contains capillaries, lymphatic vessels, and sensory neurons. Dermal papillae form fingerprints, which are genetically determined but can be influenced by environmental factors during development.

Reticular Layer

The reticular layer provides strength and elasticity to the skin. It contains dense irregular connective tissue filled with collagen and elastic fibers, as well as roots of hair follicles, sebaceous glands, and sweat glands.

• Lines of cleavage: Collagen and elastic fibers are arranged in parallel bundles to resist forces; cuts parallel to these lines heal better.

Hypodermis (Subcutaneous Layer)

Structure and Function

The hypodermis is not part of the skin but stabilizes its position and allows for independent movement. It is dominated by adipose tissue, which serves as an energy reserve.

Sensory Receptors in the Skin

Types and Functions

• Free nerve endings: Detect pain and temperature.

• Tactile (Meissner's) corpuscles: Detect light touch and pressure.

• Lamellated (Pacinian) corpuscles: Detect deep pressure and vibration.

• Ruffini corpuscles: Detect pressure and stretching.

Burns

Classification and Effects

Burns damage the integrity of the skin and are classified by depth and severity.

• First-degree: Only the epidermis is affected.

• Second-degree: Affects the entire epidermis and possibly part of the dermis.

• Third-degree: Destroys epidermis and dermis, extending into the hypodermis; skin cannot repair itself.

Burns covering more than 20% of the skin threaten homeostasis by disrupting fluid/electrolyte balance, thermoregulation, and protection from infection. Sepsis is a leading cause of death in burn victims.

Rule of Nines (Burn Surface Area Estimation)

Treatment of Burns

• Replace lost fluids

• Provide nutrients for healing

• Prevent infection

• Assist tissue repair (e.g., skin grafts)

Accessory Structures: Hair and Nails

Development and Function

• Hair follicles: Develop from epidermal cords; produce hair for protection and sensation.

• Nails: Form from thickened epidermis at the tips of fingers and toes; composed of dead, tightly compressed cells packed with keratin; nail production occurs at the nail root.

• Diagnostic value: Nail appearance can reflect health conditions.

Age-Related Changes in the Integumentary System

Effects of Aging

• Fewer melanocytes and reduced activity; increased sensitivity to sun.

• Thinner epidermis; decreased sebaceous gland secretion.

• Reduced basal cell activity; diminished immune response.

• Thinner dermis; sagging and wrinkling due to fewer fibers.

• Decreased perspiration; reduced blood supply.

• Slower skin repair and less active hair follicles.