INTEGUMENTARY SYSTEM

Introduction

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

• Main components: Skin, hair, nails, sweat glands, and sebaceous glands.

• Primary functions: Protection, sensation, temperature regulation, excretion, and synthesis of vitamin D.

STRUCTURE OF SKIN

Overview of Skin Layers

The skin varies in thickness depending on the region of the body and consists of two main layers: the epidermis and the dermis. Beneath these lies the hypodermis (subcutaneous tissue), which is not technically part of the skin but is closely associated with its function.

• Epidermis: The superficial, avascular layer made of keratinized stratified squamous epithelium. Contains four main cell types and is organized into 4-5 layers (strata).

• Dermis: The deeper, vascular connective tissue layer. Contains typical connective tissue cells, nerve fibers, blood vessels, and lymphatics. Organized into two layers: papillary (superficial) and reticular (deep).

• Hypodermis: Also called superficial fascia or subcutaneous tissue. Composed of areolar connective tissue, blood vessels, and adipose tissue. Anchors skin to underlying structures and provides insulation and cushioning.

Cell Types of the Epidermis

• Keratinocytes: Most abundant epidermal cells; produce keratin, a protein that provides toughness and waterproofing. Formed in the stratum basale and pushed upward, dying as they reach the surface. The epidermis renews every 25-45 days.

• Melanocytes: Located in the deepest layer (stratum basale); produce melanin pigment, which protects against UV radiation.

• Langerhans cells (Epidermal dendritic cells): Immune cells that act as macrophages and activate immune responses.

• Merkel cells (Tactile epithelial cells): Found at the epidermis/dermis boundary; function as touch receptors.

Layers (Strata) of the Epidermis

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

• Stratum corneum: Outermost layer; 20-30 layers of dead, flat keratinocytes. Provides a tough, waterproof barrier.

• Stratum lucidum: Thin, translucent layer of dead keratinocytes; present only in thick skin (palms, soles).

• Stratum granulosum: Cells begin to flatten; keratohyaline and lamellated granules accumulate, contributing to keratin formation and waterproofing.

• Stratum spinosum: Several layers of keratinocytes connected by desmosomes; contains melanin and dendritic cells.

• Stratum basale: Deepest layer; single row of mitotically active keratinocytes, melanocytes, and some tactile cells.

Thick skin (palms, fingertips, soles) contains all five layers; thin skin (elsewhere) lacks the stratum lucidum and has thinner other strata.

Dermis Structure

• Papillary layer: Superficial; areolar connective tissue with collagen and elastic fibers. Contains dermal papillae, which house capillaries and sensory receptors for touch and temperature.

• Reticular layer: Deep; dense irregular connective tissue with thick bundles of collagen and elastic fibers. Provides strength and resilience; contains tension lines important for surgical incisions.

Hypodermis

• Composition: Areolar and adipose tissue, blood vessels.

• Functions: Anchors skin to underlying tissues, insulates, and cushions.

PIGMENT & SKIN COLOUR

Determinants of Skin Colour

Skin colour is determined by the presence and relative amounts of three main pigments:

• Melanin: Produced by melanocytes in the stratum basale. Provides brown to black hues; protects against UV damage. Genetic factors determine type and amount of melanin.

• Carotene: Yellow/orange pigment from plant foods (e.g., carrots, sweet potatoes). Accumulates in the stratum corneum and hypodermis; precursor to vitamin A.

• Hemoglobin: Oxygen-carrying protein in red blood cells. Oxygenated blood imparts a reddish colour; deoxygenated blood can cause a bluish tint (cyanosis).

Variation in skin colour among humans is primarily genetic, with environmental factors (e.g., sun exposure) also playing a role.

APPENDAGES OF SKIN

Overview

Skin appendages are derivatives of the epidermis and include hair, hair follicles, nails, sweat glands, and sebaceous glands. Each appendage contributes to homeostasis and protection.

Hair and Hair Follicles

• Hair: Flexible strands of dead keratinized cells. Functions include protection (scalp, eyes, nose), sensation, and regulation of heat loss.

• Hair follicle: Invagination of the epidermis; contains the hair root, matrix (growth area), papilla (blood supply), and root hair plexus (sensory nerve endings).

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

• Hair structure: Medulla (central core), cortex (middle layer with melanin), cuticle (outer layer of overlapping cells).

• Types of hair: Vellus (fine, pale) and terminal (coarse, dark).

• Growth cycles: Active phase (weeks-years) and resting phase (1-3 months); rate ~2.5 mm/week.

Nails

• Structure: Hard keratin plates covering dorsal surfaces of distal digits. Parts include free edge, body, root, nail bed, nail matrix (growth area), lunula (white area over matrix), and nail folds.

• Function: Protection and enhancement of fine touch.

Sudoriferous (Sweat) Glands

• Eccrine (merocrine) glands: Most numerous; found on palms, soles, forehead. Secrete watery sweat (99% water, salts, vitamin C, antibodies, dermcidin, metabolic wastes) via exocytosis. Function: temperature regulation.

• Apocrine glands: Located in axillary and anogenital areas; secrete sweat into hair follicles. Sweat contains proteins and fatty substances; becomes odorous when decomposed by bacteria. Functioning begins at puberty; may be linked to sexual scent.

Sebaceous (Oil) Glands

• Structure: Branched alveolar glands; secrete sebum into hair follicles.

• Function: Softens and lubricates hair and skin, slows water loss, bactericidal.

• Secretion method: Holocrine (cells rupture to release contents).

• Clinical relevance: Overactivity can lead to acne; blocked ducts form 'blackheads.'

FUNCTIONS OF INTEGUMENTARY SYSTEM

Protection

• Chemical barriers: Acidic secretions (sweat, sebum), melanin (UV protection).

• Physical barriers: Keratinized cells, glycolipids (waterproofing), prevent trauma and bacterial invasion.

• Biological barriers: Langerhans cells (epidermis), macrophages (dermis).

Regulation of Permeability

• Permits passage of gases, fat-soluble vitamins, some drugs, and plant oleoresins.

• Alcohol and certain chemicals can increase skin permeability.

Excretion

• Removes wastes such as NaCl and water via sweat.

Thermoregulation

• Regulates heat loss and conservation through sweat production and blood flow adjustments.

Sensation

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

Blood Reservoir

• Holds a significant portion of the body's blood volume; can shunt blood to other organs as needed.

Vitamin D Synthesis

• UV radiation stimulates production of vitamin D precursor in the skin.

Additional info:

• Clinical conditions such as burns, infections, and autoimmune diseases can disrupt integumentary system function.

• Proper care and protection of the skin are essential for overall health.