Bone Tissue, Skeletal System, and Joints: Study Guide

Bone Tissue and the Skeletal System

Functions of the Skeletal System

The skeletal system performs several essential functions that support and maintain the human body.

• Support: Provides a structural framework for the body and supports soft tissues.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Serves as levers for muscles to act upon, enabling movement.

• Mineral Storage: Stores minerals, especially calcium and phosphate, which can be released into the bloodstream as needed.

• Blood Cell Formation (Hematopoiesis): Occurs in red bone marrow, producing red and white blood cells and platelets.

• Fat Storage: Yellow bone marrow stores triglycerides as an energy reserve.

Bone Anatomy

Bones have distinct anatomical regions, each with specific functions.

• Diaphysis: The shaft of a long bone; provides leverage and weight support.

• Epiphyses: The ends of a long bone; involved in joint articulation and muscle attachment.

• Metaphysis: The region between diaphysis and epiphysis; contains the epiphyseal plate (growth plate) in growing bones.

• Medullary Cavity: Central cavity within the diaphysis; contains yellow marrow in adults.

• Articular Cartilage: Hyaline cartilage covering epiphyses; reduces friction and absorbs shock at joints.

• Periosteum: Dense connective tissue membrane covering the bone's outer surface; contains nerves, blood vessels, and osteogenic cells.

• Endosteum: Thin membrane lining the medullary cavity; contains bone-forming and bone-resorbing cells.

Bone Classification

Bones are classified by shape and structure, each serving different functions.

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Nearly equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin, flattened, and often curved (e.g., sternum, skull bones).

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvic bones).

• Sesamoid Bones: Small, round bones embedded in tendons (e.g., patella).

• Sutural (Wormian) Bones: Small bones within cranial sutures.

Microscopic Structure of Bone

Bone tissue contains specialized cells and structures for growth, repair, and maintenance.

• Osteogenic Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; synthesize bone matrix.

• Osteocytes: Mature bone cells; maintain bone tissue.

• Osteoclasts: Large cells that break down bone matrix (bone resorption).

These cells coordinate during bone growth, repair, and remodeling.

Compact Bone Structures

• Osteon (Haversian System): Structural unit of compact bone; consists of concentric lamellae around a central canal.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Volkmann's (Perforating) Canals: Connect central canals transversely, allowing vascular and nerve supply.

• Lamellae: Layers of bone matrix.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae; allow nutrient and waste exchange between osteocytes and blood vessels.

Spongy Bone

• Trabeculae: Lattice-like network of bone tissue; provides strength with minimal weight.

• Bone Marrow: Fills spaces between trabeculae; site of hematopoiesis.

• Comparison: Compact bone is dense and forms the outer layer; spongy bone is lighter, found at bone ends and inside flat bones.

Bone Formation (Ossification)

Bone develops by two main processes:

• Intramembranous Ossification: Bone forms directly from mesenchymal tissue. Forms flat bones of the skull and clavicle. Steps include mesenchymal cell differentiation, osteoid secretion, and bone matrix formation.

• Endochondral Ossification: Bone forms by replacing a cartilage model. Most bones (e.g., long bones) develop this way. Involves formation of a cartilage template, development of primary ossification center, and growth at the epiphyseal plate.

Bone Growth

• Length Growth: Occurs at the epiphyseal plate through interstitial growth of cartilage, followed by ossification. Zones of cartilage include resting, proliferating, hypertrophic, and calcification zones. Epiphyseal closure marks the end of lengthwise growth.

• Width Growth (Appositional Growth): Osteoblasts add new bone to the periosteum, increasing diameter; osteoclasts resorb bone from the endosteum to maintain proper thickness.

Bone Remodeling

Bone is continuously renewed through remodeling, balancing formation and resorption.

• Purpose: Maintains bone strength, repairs microdamage, and regulates calcium levels.

• Osteoblasts: Build new bone matrix.

• Osteoclasts: Break down bone matrix.

Calcium Homeostasis

The body tightly regulates blood calcium levels through hormones.

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption (via vitamin D), and reducing renal excretion.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts and promoting calcium deposition in bone.

• Vitamin D (Calcitriol): Increases calcium absorption from the gut.

When blood calcium increases: Calcitonin is released. When blood calcium decreases: PTH is released.

Fracture Repair

Bone healing occurs in four main stages:

1. Hematoma Formation: Blood clot forms at the fracture site.

2. Fibrocartilaginous (Soft) Callus: Collagen and cartilage bridge the broken bone ends.

3. Bony (Hard) Callus: Spongy bone replaces the soft callus.

4. Bone Remodeling: Compact bone replaces spongy bone; bone regains original shape.

Bone Disorders

• Osteoporosis: Decreased bone mass and density, increasing fracture risk. Often due to aging, hormonal changes, or calcium deficiency.

• Osteopenia: Mild reduction in bone mass, precursor to osteoporosis.

Joints (Articulations)

Functions of Joints

Joints connect bones, allowing movement and providing stability.

• Hold bones together

• Allow movement

• Provide stability

Structural Classification of Joints

Joints are classified by the material binding bones and the presence of a joint cavity.

• Fibrous Joints: Bones joined by dense connective tissue; no joint cavity. Examples: Sutures (skull), syndesmoses (distal tibiofibular joint), gomphoses (teeth in sockets).

• Cartilaginous Joints: Bones joined by cartilage; no joint cavity. Examples: Synchondroses (epiphyseal plate), symphyses (pubic symphysis).

• Synovial Joints: Bones separated by a fluid-filled joint cavity; most movable type.

Functional Classification of Joints

Joints are also classified by the amount of movement they allow.

• Synarthrosis: Immovable joints (e.g., sutures).

• Amphiarthrosis: Slightly movable joints (e.g., symphysis).

• Diarthrosis: Freely movable joints (all synovial joints).

Synovial Joint Anatomy

Synovial joints have a complex structure allowing free movement.

• Articular Cartilage: Covers bone surfaces; reduces friction.

• Joint Cavity: Space containing synovial fluid.

• Fibrous Capsule: Outer layer; provides strength.

• Synovial Membrane: Inner layer; secretes synovial fluid.

• Synovial Fluid: Lubricates, nourishes, and cushions the joint.

• Ligaments: Reinforce and stabilize joints.

• Accessory Structures:

  • Bursae: Fluid-filled sacs reducing friction between tissues.

  • Tendon: Connects muscle to bone; stabilizes joint.

  • Menisci: Pads of fibrocartilage improving fit and cushioning.

  • Fat Pads: Cushion and protect joint structures.

Types of Synovial Joints

Synovial joints are classified by the shapes of their articulating surfaces and the movements they allow.

Type	Movement	Example
Plane	Gliding	Intercarpal joints
Hinge	Flexion/Extension	Elbow, knee
Pivot	Rotation	Atlantoaxial joint (neck)
Condylar	Flexion/Extension, Abduction/Adduction	Wrist
Saddle	Flexion/Extension, Abduction/Adduction	Thumb (carpometacarpal joint)
Ball-and-socket	All movements	Shoulder, hip

Joint Movements

Joints allow a variety of movements, classified as angular, rotational, or special.

• Angular Movements:

  • Flexion: Decreases angle between bones (e.g., bending elbow).

  • Extension: Increases angle (e.g., straightening elbow).

  • Hyperextension: Extension beyond anatomical position.

  • Abduction: Movement away from midline (e.g., raising arm sideways).

  • Adduction: Movement toward midline.

  • Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Rotational Movements:

  • Rotation: Bone turns around its own axis (e.g., turning head side to side).

• Special Movements:

  • Supination: Turning palm upward.

  • Pronation: Turning palm downward.

  • Dorsiflexion: Lifting foot upward at ankle.

  • Plantar Flexion: Pointing foot downward.

  • Inversion: Turning sole of foot inward.

  • Eversion: Turning sole of foot outward.

  • Opposition: Thumb touches fingertips.

  • Reposition: Thumb returns to anatomical position.

  • Elevation: Lifting a body part (e.g., shrugging shoulders).

  • Depression: Lowering a body part.

  • Protraction: Moving a body part forward (e.g., jutting jaw).

  • Retraction: Moving a body part backward.

High-Yield Topics and Practice Questions

• Bone cell functions: Osteoblasts build bone; osteoclasts break down bone.

• Long bone anatomy: Know all regions and their functions.

• Compact vs. spongy bone: Compact is dense and strong; spongy is lighter and contains marrow.

• Osteon structure: Central canal, lamellae, lacunae, canaliculi.

• Intramembranous vs. endochondral ossification: Direct bone formation vs. cartilage model replacement.

• Bone growth and remodeling: Length via epiphyseal plate; width via appositional growth.

• Hormonal regulation of calcium: PTH increases, calcitonin decreases blood calcium.

• Stages of fracture repair: Hematoma, soft callus, hard callus, remodeling.

• Structural and functional classifications of joints: Fibrous, cartilaginous, synovial; synarthrosis, amphiarthrosis, diarthrosis.

• Synovial joint anatomy and types: Know all structures and joint types.

• Joint movements: Be able to identify and describe all types.

Example Questions:

• Which bone cell builds bone? Osteoblast

• Which bone cell breaks down bone? Osteoclast

• What is the difference between compact and spongy bone?

• How does bone grow in length? Via the epiphyseal plate

• How does bone grow in width? Appositional growth

• Which hormone increases blood calcium? PTH

• What hormone lowers blood calcium? Calcitonin

• What are the stages of fracture repair?

• What is the difference between a fibrous, cartilaginous, and synovial joint?

• Which joint allows the greatest range of motion? Ball-and-socket

• What movement occurs when turning the palm upward? Supination

• What movement allows the thumb to touch the fingertips? Opposition

• Which joints are freely movable? Diarthrosis (synovial joints)

• Which joints are immovable? Synarthrosis (fibrous joints)