Integumentary System

Structure and Functions of the Integumentary System

The integumentary system consists of the skin and its accessory structures, providing protection, sensation, and other vital functions.

• Epidermis: Superficial layer composed of keratinized stratified squamous epithelium.

• Dermis: Deeper connective tissue layer containing collagen and elastic fibers, blood vessels, and sensory structures.

• Subcutaneous (hypodermis): Loose connective tissue and adipose; anchors skin to underlying structures.

Functions:

• Protection: Against mechanical injury, UV radiation, pathogens, and dehydration.

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

• Thermoregulation: Via sweating and blood vessel dilation/constriction.

• Excretion: Sweat removes salts, water, and waste.

• Vitamin D synthesis: UV exposure stimulates vitamin D production.

• Storage: Fat in the hypodermis stores energy and cushions organs.

Layers and Cell Types of the Epidermis

The epidermis is organized into five layers (strata) in thick skin and four in thin skin. Each layer has distinct features and functions.

• Stratum corneum: Dead, keratin-filled cells; barrier to infection and water loss.

• Stratum lucidum (only in thick skin): Clear, densely packed keratinocytes; additional protection (palms, soles).

• Stratum granulosum: Keratohyalin granules; waterproofing; forms keratin.

• Stratum spinosum: Desmosomes, Langerhans cells.

• Stratum basale: Basal cells, melanocytes, Merkel cells; cell division, pigment, sensory input.

Major Cell Types:

• Keratinoctyes: Produce keratin for strength and waterproofing.

• Melanocytes: Produce melanin (pigment).

• Langerhans cells: Immune defense.

• Merkel cells: Sensory receptors for touch.

Skin Color

Individual differences in skin color are due to genetic and environmental factors.

• Melanin: Amount and type (eumelanin = brown-black, pheomelanin = red-yellow).

• Carotene: Yellow-orange pigment from diet (accumulates in stratum corneum/fat).

• Hemoglobin: Oxygen-rich blood gives skin pinkish hue.

Genetic + Environmental factors:

• Genetics determine baseline melanin production.

• UV exposure increases melanin synthesis (tanning).

Dermis and Subcutaneous Layer

The dermis provides strength, elasticity, and houses sensory and vascular structures. The subcutaneous layer anchors skin and stores energy.

• Papillary layer: Loose areolar tissue; contains dermal papillae, capillaries, sensory neurons.

• Reticular layer: Dense irregular connective tissue with collagen and elastin; houses hair follicles, sweat glands, sebaceous glands.

Subcutaneous layer (Hypodermis):

• Areolar + adipose tissue.

• Anchors skin to muscles, cushions, insulates, stores energy.

Burns and Skin Grafts

• Autograft: From patient’s own body.

• Allograft: From donor or cadaver.

• Xenograft: From animal source (temporary).

• Synthetic graft: Engineered skin substitute.

Accessory Structures: Hair, Nails, and Glands

Accessory structures perform specialized functions in protection, sensation, and secretion.

• Hair: Protection (UV, debris), sensory input, insulation.

• Nails: Protection of fingertips, aid in grasping.

• Glands:

  • Sebaceous (oil): Lubricate and waterproof skin/hair.

  • Sweat glands: Thermoregulation, excretion.

  • Ceruminous glands: Produce earwax (protection).

  • Mammary glands: Produce milk.

Hair Structure and Production

• Hair shaft: Visible portion.

• Hair root: Beneath skin surface.

• Hair follicle: Surrounds root; contains dermal papilla (blood supply).

• Hair bulb: Base with actively dividing cells.

Production:

• Hair grows from matrix cells in the bulb.

• Cells keratinize and die as they move outward.

Texture: Determined by follicle shape (round = straight, oval = wavy, flat = curly).

Color: Due to melanin type and amount; decreases with age (gray/white).

Nail Structure and Function

• Nail body: Visible portion.

• Nail root: Embedded in skin.

• Nail bed: Skin beneath nail.

• Lunula: Crescent-shaped whitish area.

• Cuticle (eponychium): Fold of skin at nail base.

• Hyponychium: Skin under free edge.

Function: Protection, manipulation, sensory enhancement.

Glands of the Skin

Skin Response to Injury and Repair

• Inflammation: Bleeding; mast cells trigger inflammation.

• Migration: Scab forms; macrophages clean area; epithelial cells migrate under scab.

• Proliferation: Fibroblasts produce collagen; tissue fills in.

• Scarring (Maturation): Collagen remodels; scar tissue may remain (fibrosis).

Skeletal System

Divisions of the Skeleton

The skeleton is divided into two main parts:

• Axial skeleton: Skull, vertebral column, ribs, sternum (80 bones).

• Appendicular skeleton: Limbs and girdles (shoulder and pelvic) (126 bones).

Bone Shapes, Markings, and Components

• Shapes: Long (femur), short (carpals), flat (sternum), irregular (vertebrae), sesamoid (patella).

• Markings: Projections (tubercle, spine), depressions (fossa), openings (foramen).

• Components: Compact bone, spongy bone, marrow cavity, periosteum, endosteum.

Bone Cell Types and Functions

Major Bones and Markings

• Axial:

  • Skull: Frontal, parietal, temporal, occipital, sphenoid, ethmoid

  • Vertebral column: Cervical (7), thoracic (12), lumbar (5), sacrum, coccyx

  • Thoracic cage: Sternum, ribs (12 pairs)

• Appendicular:

  • Pectoral girdle: Clavicle, scapula

  • Upper limb: Humerus, radius, ulna, carpals, metacarpals, phalanges

  • Pelvic girdle: Coxal bones (ilium, ischium, pubis)

  • Lower limb: Femur, tibia, fibula, tarsals, metatarsals, phalanges

Differences in Skulls: Infants, Children, Adults

• Infants: Fontanelles (soft spots); unfused sutures allow growth and flexibility.

• Adults: Sutures fused; larger facial bones relative to cranium.

Spinal Column: Structure and Function

• Curvatures: Cervical, thoracic, lumbar, sacral (normal balance).

• Functions: Protect spinal cord; support head/trunk; allow flexibility.

• Components: Vertebral body, arch, processes, intervertebral discs.

Appendicular Skeleton

Bone Shapes, Markings, and Components

Major Bones and Markings of the Appendicular Skeleton

Arthrology (Joints)

Categories of Joints

Types of Fibrous Joints:

• Suture: Found between skull bones; immovable (synarthrosis).

• Syndesmosis: Bones connected by a ligament; slightly movable (amphiarthrosis).

• Gomphosis: Peg-in-socket joint (tooth in alveolar socket); immovable (synarthrosis).

Types of Cartilaginous Joints:

• Synchondrosis: Connected by hyaline cartilage; immovable (synarthrosis).

• Symphysis: Connected by fibrocartilage; slightly movable (amphiarthrosis).

Synovial Joint Structure and Accessory Structures

A synovial joint consists of a joint capsule, synovial cavity, articular cartilage, and synovial fluid. Accessory structures such as menisci, bursae, ligaments, and fat pads enhance stability, reduce friction, and absorb stress.

Special Movements of Skeletal Articulations

Additional info:

• Bone matrix is primarily composed of collagen fibers and hydroxyapatite (calcium phosphate crystals).

• Osteogenesis (bone formation) occurs via intramembranous or endochondral ossification.

• Vitamin D is essential for calcium absorption and bone health.

• Common bone fractures include simple (closed), compound (open), and comminuted (multiple fragments).

• Osteoporosis is a condition characterized by decreased bone density and increased fracture risk.