The Integumentary System

Overview of the Integumentary System

The integumentary system is composed of the skin and its accessory structures, including hair, nails, and glands. It serves as the body's first line of defense and plays a vital role in maintaining homeostasis.

• Skin (Cutaneous Membrane): The largest organ in the human body, accounting for 10-15% of total body weight.

• Accessory Structures: Includes hair, nails, sweat glands, and sebaceous (oil) glands.

• Functions: Protection, sensation, thermoregulation, excretion, and synthesis of vitamin D.

Skin Structure

Layers of the Skin

The skin is composed of two main layers, each with distinct structures and functions.

• Superficial Epidermis: Made of keratinized stratified squamous epithelium resting on a basement membrane. It is avascular and consists of several layers (strata) of cells.

• Deep Dermis: Composed of loose connective tissue and dense irregular connective tissue. It is vascularized and provides structural support.

• Hypodermis (Superficial Fascia/Subcutaneous Tissue): Not technically part of the skin, but lies below the dermis. Contains loose connective and adipose tissue and is highly vascularized.

Functions of the Integumentary System

Major Functions

• Protection: Acts as a barrier against mechanical injury, pathogens, and harmful substances.

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

• Thermoregulation: Regulates body temperature through sweat production and blood flow.

• Excretion: Removes waste products through sweat.

• Synthesis of Vitamin D: Initiates the production of vitamin D when exposed to UV light.

Epidermis

Structure and Cell Types

The epidermis is the outermost layer of the skin, composed of several cell types and up to five distinct layers (strata).

• Keratinocytes: Make up about 95% of epidermal cells. They produce keratin, a protein that provides strength and waterproofing. Held together by desmosomes.

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

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

• Tactile (Merkel) Cells: Sensory receptors for light touch, especially abundant in fingertips and lips.

Layers (Strata) of the Epidermis

The epidermis is organized into layers from deep to superficial:

1. Stratum Basale: Deepest layer; single row of cuboidal/columnar cells. Mitotically active and produces keratin and vitamin D precursors.

2. Stratum Spinosum: Thickest layer; cells have a spiky appearance. Metabolically and mitotically active.

3. Stratum Granulosum: 3-5 rows of cells with granules. Keratinocytes release contents that form a water barrier.

4. Stratum Lucidum: Clear, thin layer of dead keratinocytes; found only in thick skin (palms, soles).

5. Stratum Corneum: Multiple layers of dead, flattened keratinocytes. Cells are shed (about 1 million per day).

Mnemonic: "Let's Get Sun Burnt" (Lucidum, Granulosum, Spinosum, Basale, Corneum)

Keratinocyte Life Cycle

• Cells in the stratum basale and spinosum divide and push older cells toward the surface.

• Cells become more keratinized as they move up and are eventually sloughed off in the stratum corneum (takes 40-50 days).

Thick vs. Thin Skin

Dermis

Structure and Layers

The dermis is the deep layer of the skin, providing strength, flexibility, and housing blood vessels and sensory receptors.

• Papillary Layer: Superficial; made of loose connective tissue. Contains dermal papillae, which supply blood to the epidermis and house tactile (Meissner) corpuscles for light touch.

• Reticular Layer: Deep; made of dense irregular connective tissue. Contains blood vessels, sweat glands, sebaceous glands, hair follicles, and lamellated (Pacinian) corpuscles for deep pressure and vibration.

Skin Markings

• Dermal Ridges: Form fingerprints; unique to each individual.

• Tension (Cleavage) Lines: Patterns of collagen bundles; important for surgical incisions.

• Flexure Lines: Deep creases in areas of frequent movement (e.g., palms).

Skin Color and Pigmentation

Melanin

• Produced by melanocytes in the stratum basale and stored in melanosomes.

• Protects DNA from UV radiation by forming a shield over the nucleus.

• Melanin synthesis increases with UV exposure; immediate effects include oxidation of existing melanin, while long-term effects involve increased production.

• Variations: Freckles (localized increased melanin), moles (nevus, increased melanocytes), albinism (lack of tyrosinase, no melanin).

Other Pigments

• Carotene: Yellow-orange pigment from diet; accumulates in stratum corneum.

• Hemoglobin: Red pigment in blood; gives skin a pinkish hue, especially in fair-skinned individuals.

Skin Color as a Diagnostic Tool

Accessory Structures

Hair (Pili)

• Composed of keratinized epithelial cells; found everywhere except thick skin and some genital areas.

• Functions: Protection (e.g., eyelashes, scalp), sensation (sensory neurons detect movement).

• Types: Lanugo (fetal, nonpigmented), terminal (thick, pigmented), vellus (thin, nonpigmented).

• Growth Cycle: Growth stage (mitosis in matrix), resting stage (follicle dormant), varies by location and individual.

• Color: Determined by melanin type and amount; gray/white hair results from decreased melanin with age.

Nails

• Composed of hard keratin; protect distal digits and aid in manipulation.

Glands

• Sudoriferous (Sweat) Glands:

  • Eccrine: Most common; secrete watery sweat for thermoregulation.

  • Apocrine: Found in axillae, anal, and areolar regions; secrete protein-rich sweat, which becomes odorous when metabolized by bacteria.

  • Ceruminous: Modified apocrine glands in the ear; produce cerumen (ear wax).

  • Mammary: Specialized glands that produce milk.

• Sebaceous Glands: Secrete sebum (oily, waxy substance) via holocrine secretion; lubricates skin and hair, increases at puberty.

Common Skin Pathologies

Wounds

• Any disruption of skin integrity; includes lacerations, burns, and skin cancers.

Burns

• Rule of Nines: Used to estimate the percentage of body surface area affected by burns for fluid replacement.

Skin Cancer

• Basal Cell Carcinoma: Most common; arises from stratum basale; rarely metastasizes.

• Squamous Cell Carcinoma: Second most common; arises from stratum spinosum; may metastasize.

• Malignant Melanoma: Least common but most dangerous; arises from melanocytes; high risk of metastasis.

ABCDE Rule for Melanoma:

• Asymmetry

• Border irregularity

• Color variation

• Diameter > 6 mm

• Evolving shape/size

Additional info:

• Cellulite is a normal condition resulting from the arrangement of collagen bands and adipose tissue in the hypodermis.

• Topical medications can be absorbed through the skin if they are nonpolar/hydrophobic.

• Wrinkles result from decreased collagen, elastic fibers, and adipose tissue, and are accelerated by UV exposure and smoking.