Integumentary System

Overview

The Integumentary System is the body's largest organ system, consisting of the skin and its associated structures. It serves multiple essential functions, including protection, sensation, thermoregulation, excretion, and vitamin D synthesis.

Major Components of the Integument

Cutaneous Membrane

• Integument: The collective term for the skin and its appendages.

• Cutaneous Membrane: Another term for the skin, composed of the epidermis and dermis.

• Epidermis: The superficial layer of the skin, made of stratified squamous epithelium.

• Dermis: The deeper connective tissue layer beneath the epidermis.

• Hypodermis: Also called the subcutaneous layer; lies below the dermis and consists mainly of adipose tissue, providing insulation and cushioning.

Functions of the Integumentary System

• 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 vessel dilation/constriction.

• Excretion: Removes waste products via sweat.

• Vitamin D Synthesis: Initiates synthesis of vitamin D when exposed to ultraviolet (UV) radiation.

Epidermis

Cell Types and Structure

• Keratinocyte: The primary cell type in the epidermis, producing the protein keratin for strength and waterproofing.

• Keratin: A tough, fibrous protein that protects skin and underlying tissues.

• Desmosome: Specialized cell junctions that tightly bind keratinocytes together.

Strata of the Epidermis

• Strata: Layers of the epidermis, each with distinct cell types and functions.

• Stratum Corneum: Outermost layer; dead, flattened keratinocytes provide a durable barrier.

• Stratum Lucidum: Thin, clear layer found only in thick skin (palms, soles).

• Stratum Granulosum: Cells contain granules of keratin precursors; site of keratinization.

• Stratum Spinosum: Several layers of keratinocytes; contains dendritic cells.

• Stratum Basale: Deepest layer; site of cell division and contains melanocytes and Merkel cells.

• Sloughing: The process of shedding dead cells from the stratum corneum.

Specialized Cells

• Dendritic (Langerhans) Cell: Immune cells that detect and respond to pathogens.

• Merkel Cell: Sensory cells associated with touch.

• Melanocyte: Cells that produce melanin, the pigment responsible for skin color.

Skin Types

• Thick Skin: Found on palms and soles; contains all five epidermal strata and lacks hair.

• Thin Skin: Covers most of the body; fewer layers and contains hair follicles.

Dermis

Structure and Layers

• Dermal Papilla: Projections of the dermis that interlock with the epidermis, increasing surface area.

• Papillary Layer: Superficial layer of the dermis; contains loose connective tissue and sensory receptors.

• Reticular Layer: Deep layer; dense irregular connective tissue, provides strength and elasticity.

Sensory Receptors

• Tactile (Meissner) Corpuscle: Detects light touch.

• Lamellated (Pacinian) Corpuscle: Detects deep pressure and vibration.

Fingerprints

• Epidermal Ridges: Ridges formed by the epidermis, contributing to fingerprints.

• Dermal Ridges: Ridges in the dermis that support epidermal ridges.

• Fingerprint: Unique pattern formed by epidermal ridges; used for identification.

Skin Pigmentation

• Melanin: Pigment produced by melanocytes; protects against UV radiation.

• Melanosome: Organelle in melanocytes where melanin is synthesized and stored.

• Ultraviolet (UV) Radiation: Stimulates melanin production; excessive exposure can damage DNA.

• Freckle: Localized area of increased melanin production.

• Mole (Nevus): Benign growth of melanocytes.

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

• Hemoglobin: Red pigment in blood; gives skin a pinkish hue when blood vessels are close to the surface.

Hair

Structure

• Hair Shaft: Visible part of the hair above the skin surface.

• Hair Root: Portion of hair below the skin surface.

• Hair Follicle: Tubular structure surrounding the hair root; consists of epithelial and dermal components.

• Epithelial Root Sheath: Inner layer of the follicle derived from epidermis.

• Dermal Root Sheath: Outer layer of the follicle derived from dermis.

• Arrector Pili Muscle: Smooth muscle attached to hair follicle; causes piloerection (goosebumps).

• Piloerection: The process of hair standing upright due to contraction of arrector pili muscle.

• Cuticle: Outermost layer of hair; provides protection.

• Cortex: Middle layer; contains pigment and provides strength.

• Medulla: Central core; may be absent in fine hair.

• Hair Bulb: Enlarged base of the hair follicle; site of hair growth.

• Matrix: Actively dividing cells in the hair bulb responsible for hair growth.

• Hair Papilla: Dermal tissue at the base of the follicle; supplies nutrients.

Types of Hair

• Lanugo: Fine, unpigmented hair found on fetuses.

• Vellus Hair: Fine, lightly pigmented hair covering most of the body.

• Terminal Hair: Coarse, pigmented hair found on scalp, eyebrows, and after puberty in certain regions.

Nails

Structure

• Nail Plate: Hard, keratinized portion of the nail.

• Nail Body: Visible part of the nail plate.

• Nail Root: Proximal part of the nail plate under the skin.

• Lateral Nail Fold: Skin fold on the sides of the nail.

• Medial Nail Fold: Skin fold on the medial side (if present).

• Proximal Nail Fold: Skin fold at the base of the nail.

• Lunula: Crescent-shaped, whitish area at the base of the nail.

• Eponychium: Cuticle; protects the nail matrix.

• Hyponychium: Skin under the free edge of the nail.

• Free Edge: Distal part of the nail plate extending beyond the finger.

• Nail Matrix: Area of actively dividing cells responsible for nail growth.

• Nail Bed: Skin beneath the nail plate.

Glands

Sweat (Sudoriferous) Glands

• Sweat (Sudoriferous) Gland: Produces sweat for thermoregulation and excretion.

• Merocrine Secretion: Secretion by exocytosis; most sweat glands use this method.

• Eccrine Sweat Gland: Most numerous; produces watery sweat for cooling.

• Apocrine Sweat Gland: Found in axillary and genital areas; produces thicker, odoriferous sweat.

• Ceruminous Sweat Gland: Located in the ear canal; produces cerumen (earwax).

• Cerumen: Earwax; protects and lubricates the ear canal.

• Mammary Sweat Gland: Modified sweat gland; produces milk.

• Milk: Nutrient-rich secretion for infant nourishment.

Sebaceous Glands

• Sebaceous Gland: Produces sebum, an oily substance that lubricates skin and hair.

• Sebum: Oily secretion that prevents drying and protects against bacteria.

• Acini: Cluster of cells in sebaceous glands that produce sebum.

• Holocrine Secretion: Secretion method where entire cells disintegrate to release their contents (used by sebaceous glands).

Summary Table: Layers of the Skin

Summary Table: Types of Glands

Additional info:

• Vitamin D synthesis begins in the skin with exposure to UV light, converting 7-dehydrocholesterol to cholecalciferol (vitamin D3).

• Keratinization is the process by which keratinocytes mature, fill with keratin, and eventually die, forming the protective stratum corneum.

• Fingerprints are unique to each individual and are used in forensic identification.

• Holocrine secretion is unique to sebaceous glands, while most other glands use merocrine or apocrine methods.