Skeletal System Overview

Skeletal System Overview

• The skeletal system is composed of bones and connective tissue
• It forms a framework and determines the overall shape of the human body
• The skeleton is a collection of bones that support the rest of the body
• Skeleton derives from the Greek word meaning "dried up"

Learning Objectives

• Learn the basic functions of bones and their overall structure
• Describe bone growth and development
• Differentiate bone types, articulations, and related diseases

Basic Functions of Bones

• Support the body
• Attachment of muscles
• Protection of internal organs
• Levers for movement
• Production of blood cells
• Gives stability and shape to the body

Types of Bone Cells

• Osteocytes: Mature bone cells that produce soluble factors regulating bone formation and resorption
• Osteoblasts: Bone-forming cells that secrete collagen and organic bone matrix (osteoid) which calcifies
• Osteoclasts: Bone-destroying cells that break down bone matrix for remodeling and calcium release
• Osteogenic cells (Osteoprogenitor cells): Stem cells of bone, the source of new osteoblasts

Bone Remodeling Process

• A continual balance between osteoblasts generating new bone and osteoclasts breaking down bone

Changes in the Human Skeleton

• In embryos, the skeleton is primarily hyaline cartilage
• During development, much of the cartilage is replaced by bone
• Cartilage remains in isolated areas (e.g., bridge of the nose, parts of ribs)
• Joints

Bone Growth

• Epiphyseal plates allow for long bone growth during childhood
• Continuous formation of new cartilage
• Ossification of older cartilage
• Cartilage breakdown, and replacement with bone
• Bones remodel and lengthen until growth stops
• Bones change shape, and grow in width

Types of Skeleton

• Classified by formation (membrane/dermal, endochondral/cartilage), bone tissue (spongy, compact) and location (exoskeleton, endoskeleton, hydroskeleton)
• Exoskeleton is a characteristic of arthropods (insects, crustaceans, etc.). It's built up outside the body and limits the size of the animal
• Endoskeleton characteristic of vertebrates. It's built up inside the body and muscles attach to the outer surface (axial, appendicular)
  • Axial skeleton : skull, hyoid bone, vertebral column, thoracic cage
  • Appendicular skeleton: pectoral girdles, upper limbs, pelvic girdles, lower limbs
• Hydroskeleton found in soft-bodied invertebrates (starfish, jellyfish) and consists of a fluid-filled cavity supported by muscles

Bone Composition

• Solid network of moist, living cells (osteocytes)
• Living tissue and fibers (collagen) supported by calcium salts matrix
• 2/3 inorganic mineral matter (calcium phosphate), resisting x-rays; this makes bones strong
• 1/3 organic animal matter (collagen); gives resilience and flexibility

Bone Marrow

• Two types of bone marrow: yellow and red
• Yellow marrow found in most bones, filled with fat cells, and serves as an energy reserve
• Red marrow found in spongy bone/ends of long bones, ribs, vertebrae, and pelvis, produces blood cells (red and white).
  • Red blood cells carry oxygen
  • White blood cells fight harmful bacteria

Divisions of the Skeleton

• Axial skeleton (skull, vertebral column, ribs, sternum) forms the main axis of the body.
• Appendicular skeleton (pectoral girdle, upper limbs, pelvic girdle, lower limbs) forms the extremities of the body

Bone Markings

• Surface features of bones
• Sites of attachments (muscles, tendons, ligaments)
• Passages for nerves and blood vessels
• Categories of bone markings (projections/processes, depressions/cavities)

Types of Bones

• Long bones (e.g., humerus, tibia, femur, metacarpals) are longer than wide
• Flat bones (e.g., ribs, cranial bones, shoulder bones) have broad surfaces for protection
• Irregular bones (e.g., vertebrae) are any bones not fitting the preceding categories and varied shapes, sizes, and surface features in the skull and vertebrae
• Short bones (e.g. wrist/ankle) are cube-shaped

Bone Diseases and Conditions (fractures, bursitis, rickets, kyphosis, arthritis, osteoporosis, spina bifida, bone cancer, sprains)

• Fracture: A break in a bone, caused by forceful impact/stress; different types include simple, comminuted, compound, compression, grenstick, spiral, impacted etc.
• Bursitis: Inflammation of the bursa (fluid filled sacs around joints); due to overuse of a body part
• Rickets: Disease caused by insufficient vitamin D exposure/intake in childhood, causing soft and deformed bones
• Kyphosis: Forward bending or hunching of the vertebral column
• Arthritis: Inflammation of bone joints (degenerative, rheumatoid, etc.)
• Osteoporosis: Loss of bone matrix leading to weak and brittle bones (often in older people)
• Spina Bifida: Birth defect; vertebrae fail to close completely around the spinal cord; causes vulnerability to spinal injury/infection
• Bone Cancer: Originates in bones or spreads there; symptoms include localized swelling, dull ache
• Sprains: Injury to a ligament or tissue that covers a joint; symptoms including swelling and extreme pain; treatment often involves rest and application of ice/menthol

Joints

• The meeting point of two bones, allowing movement
• Three major types of Joints
  • Synarthroses: Immovable, connecting by fibrous tissue/cartilage
  • Amphiarthroses: Slightly moveable and connect by cartilage
  • Diarthroses: Freely moveable, filled with synovial fluid, allowing smooth movements
  • Types of Diarthroses (Ball and socket, Pivot, Condyloid, Saddle, Gliding/Planar)