The Integumentary System

Overview and Functional Anatomy

The integumentary system is the body's most accessible organ system, commonly referred to as the skin or integument. It comprises about 16% of total body weight and serves as the first line of defense against environmental threats. The system consists of two major components: the cutaneous membrane and accessory structures.

• Cutaneous membrane: Includes the epidermis (stratified squamous epithelium) and dermis (papillary layer of areolar connective tissue and reticular layer of dense irregular connective tissue).

• Accessory structures: Hairs, nails, exocrine glands (sebaceous and sweat glands), sensory receptors, nerve fibers, arrector pili muscles, and cutaneous plexus (blood vessels).

• Subcutaneous layer (hypodermis): Not part of the integument, but separates it from deeper fascia and is dominated by adipose tissue.

Functions of the Integumentary System

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

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

• Temperature regulation: Maintains body temperature through insulation and evaporative cooling.

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

• Keratin production: Provides abrasion resistance and water repellency.

• Vitamin D synthesis: Produces a steroid precursor converted to calcitriol, essential for calcium metabolism.

• Lipid storage: Stores lipids in dermal adipocytes and subcutaneous adipose tissue.

• Sensory detection: Detects touch, pressure, pain, and temperature, relaying information to the nervous system.

The Epidermis

Structure and Layers

The epidermis is composed of multiple cell layers (strata), primarily of keratinocytes. It forms epidermal ridges adjacent to dermal papillae, increasing surface area for attachment.

• Thin skin: Covers most of the body; contains four strata.

• Thick skin: Found on palms and soles; contains five strata.

• Fingerprints: Unique patterns of epidermal ridges used for identification.

Epidermal Layers (Deep to Superficial)

• Stratum basale: Attached to basement membrane; contains basal cells (stem cells) and Merkel cells (touch receptors).

• Stratum spinosum: 8–10 layers of keratinocytes bound by desmosomes; contains dendritic (Langerhans) cells for immune defense.

• Stratum granulosum: 3–5 layers; cells produce keratin and keratohyalin, become thinner and less permeable.

• Stratum lucidum: Only in thick skin; densely packed dead cells filled with keratin.

• Stratum corneum: Outermost layer; 15–30 layers of keratinized, dead cells tightly connected by desmosomes; water resistant.

Cells migrate from the deepest to the most superficial layer in about 7–10 days, remaining in the stratum corneum for about 2 weeks before being shed.

Skin Color

Factors Influencing Skin Color

Skin color is determined by epidermal pigmentation and dermal circulation, as well as thickness and degree of keratinization.

• Carotene: Orange-yellow pigment.

• Melanin: Brown, yellow-brown, or black pigment produced by melanocytes in the stratum basale. Differences in skin color are due to the amount of melanin produced, not the number of melanocytes.

• Hemoglobin: Red pigment in blood; increased blood flow causes redness, decreased flow causes paleness or cyanosis (bluish color).

• UV exposure: Increases melanin production.

Skin Cancer Types

• Basal cell carcinoma: Most common; originates in stratum basale, usually due to UV exposure; rarely metastasizes.

• Malignant melanoma: Dangerous; cancerous melanocytes metastasize rapidly; early detection yields high survival rates.

The Dermis and Subcutaneous Layer

Dermis Structure

The dermis lies between the epidermis and hypodermis, containing collagen and elastic fibers.

• Papillary layer: Areolar tissue; contains capillaries, lymphatic vessels, and sensory neurons.

• Reticular layer: Dense, irregular connective tissue; contains blood vessels, lymphatics, nerve fibers, and accessory organs.

Subcutaneous Layer (Hypodermis)

• Separates skin from deeper structures; dominated by adipose tissue for energy storage.

• Adipose accumulation patterns differ by sex and region.

Sensory Receptors

• Epidermis: Free nerve endings (touch, pressure), tactile discs (texture, steady pressure).

• Dermis: Tactile (Meissner’s) corpuscles (light touch), lamellar (Pacinian) corpuscles (deep pressure), Ruffini corpuscles (pressure/stretch).

Tension (Cleavage) Lines

• Formed by collagen and elastic fiber arrangement; important for surgical incisions.

• Cuts parallel to lines heal better; perpendicular cuts result in more scarring.

Burns and Clinical Aspects

Classification of Burns

• First-degree: Surface of epidermis; redness (erythema), mild inflammation.

• Second-degree: Entire epidermis and part of dermis; blistering, pain, swelling; accessory structures not affected.

• Third-degree: Destroys epidermis, dermis, and extends into subcutaneous layer; less painful due to nerve damage; requires skin grafting.

Effects of Burns

• Threaten fluid/electrolyte balance, thermoregulation, and protection from infection (sepsis is a major risk).

• Severity depends on depth and area affected; assessed clinically by pin prick and "rule of nines" for surface area estimation.

Rule of Nines Table

Additional info: Child proportions differ due to larger head size.

Burn Treatment and Skin Grafts

• Emergency treatment: Replace fluids/electrolytes, provide nutrients, prevent infection, assist tissue repair.

• Skin grafts: Split-thickness (epidermis + superficial dermis), full-thickness (epidermis + full dermis).

• Sources: Autograft (patient's own skin), allograft (cadaver), xenograft (animal).

• Advances: Laboratory-grown epidermis improves survival rates.

Accessory Structures of the Skin

Hair Follicles

Hair follicles produce hairs that protect the skull and provide sensory input. Hair is composed of dead, keratinized cells.

• Terminal hairs: Large, coarse, pigmented (scalp, armpit).

• Vellus hairs: Small, short, delicate (general body surface).

• Hair shaft: Visible part; hair root: anchors hair.

• Root hair plexus: Sensory nerves at follicle base.

• Arrector pili: Smooth muscle causing hair to stand erect.

• Sebaceous gland: Lubricates hair and skin.

Exocrine Glands

• Sebaceous glands: Holocrine glands releasing sebum (triglycerides, cholesterol, proteins, electrolytes); lubricates and is antimicrobial.

• Sweat glands: Merocrine secretion; myoepithelial cells squeeze gland. Two types:

  • Apocrine: Axillae, nipples, pubic region; sticky, odorous secretion; hormone-influenced; includes ceruminous and mammary glands.

  • Merocrine (eccrine): Directly onto skin surface; most numerous on palms/soles; watery secretion with electrolytes; important for thermoregulation and excretion.

Nails

• Thick sheets of keratinized epidermal cells; protect tips of fingers and toes.

• Nail body: Visible portion; bordered by grooves and folds; covers nail bed.

• Lunula: Pale crescent at proximal nail.

• Nail root: Site of nail production.

• Eponychium: Cuticle; extension of stratum corneum.

• Hyponychium: Thickened stratum corneum under free edge.

• Nail appearance can indicate health conditions (e.g., pitted nails in psoriasis, concave nails in blood disorders).

Age-Related Changes in the Integument

Effects of Aging

• Fewer melanocytes: Pale skin, increased sun sensitivity.

• Drier epidermis: Reduced sebaceous gland activity.

• Thinning epidermis: Weaker connections, increased injury risk.

• Reduced vitamin D production: Muscle weakness, brittle bones.

• Diminished immune response: Fewer dendritic cells, higher infection risk.

• Thinning dermis: Fewer elastic fibers, sagging/wrinkling.

• Decreased perspiration: Higher risk of overheating.

• Reduced blood supply: Skin feels cold, slower heat loss.

• Slower skin repair: Healing takes longer.

• Fewer active follicles: Thinner, finer, gray/white hair; altered fat distribution.

Endocrine and Integumentary System Interactions

Vitamin D3 Synthesis and Bone Health

• Vitamin D3 obtained from sunlight and diet (fish, oils, fortified milk).

• UV radiation converts steroid in epidermal cells to cholecalciferol.

• Liver and kidneys convert cholecalciferol to calcitriol.

• Calcitriol enables calcium and phosphate absorption in the intestine, essential for bone growth and maintenance.

• Inadequate calcitriol: Impaired bone growth (rickets in children, decreased bone density in elderly).

• Skin production of cholecalciferol decreases with age, increasing fracture risk and slowing healing.

Skin Repair and Regeneration

Phases of Skin Regeneration

• Inflammation phase: Injury causes bleeding and mast cell activation; inflammation results in swelling, redness, heat, pain.

• Migration phase: Blood clot (scab) forms; basal cells migrate; macrophages remove debris; granulation tissue forms if dermis is damaged.

• Proliferation phase: Epidermal cells migrate under scab; phagocytic activity nearly complete; fibroblasts produce collagen and ground substance.

• Scarring phase: Scab shed; epidermis complete; fibroblasts continue to produce scar tissue (inflexible, fibrous, noncellular).

Example: Blister repair takes 3–4 weeks in young adults, 6–8 weeks in elderly.

Summary Table: Epidermal Layers

Key Equations

• Vitamin D3 Synthesis:

• Calcitriol Formation:

Additional info: These equations summarize the biochemical pathway for vitamin D synthesis and activation.