Bones, Skeletal Tissues, and Joints: ANP Study Guide

Bones and Skeletal Tissues

Classification of Cartilage and Bones

The skeletal system is composed of bones and cartilage, which provide structure, support, and facilitate movement. Cartilage is a resilient and flexible connective tissue, while bones are rigid organs that form the skeleton.

• Structure of Cartilage: Cartilage contains no blood vessels or nerves and is surrounded by the perichondrium, a dense irregular connective tissue that resists outward expansion.

• Types of Skeletal Cartilage:

  • Hyaline Cartilage: Most abundant; provides support, flexibility, and resilience. Found in articular (ends of long bones), costal (ribs to sternum), respiratory (larynx, air passages), and nasal cartilages.

  • Elastic Cartilage: Similar to hyaline but contains elastic fibers. Located in the external ear and epiglottis.

  • Fibrocartilage: Highly compressed with great tensile strength; contains collagen fibers. Found in menisci of the knee and intervertebral discs.

• Growth of Cartilage:

  • Appositional Growth: Cells in the perichondrium secrete matrix against the external face of existing cartilage.

  • Interstitial Growth: Lacunae-bound chondrocytes divide and secrete new matrix, expanding cartilage from within.

  • Calcification: Occurs during normal bone growth and old age.

• Classification of Bones:

  • Axial Skeleton: Skull, vertebral column, rib cage.

  • Appendicular Skeleton: Upper and lower limbs, shoulder, hip.

• Shape Classes of Bones:

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

  • Short Bones: Cube-shaped (wrist, ankle); includes sesamoid bones (e.g., patella).

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

  • Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

• Functions of Bones:

  • Support

  • Protection

  • Movement

  • Mineral storage (calcium, phosphorus)

  • Blood cell formation (hematopoiesis)

• Bone Markings: Serve as sites for muscle, ligament, and tendon attachment, joint surfaces, and conduits for blood vessels and nerves.

  • Projections: Tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process, head, facet, condyle, ramus.

  • Depressions/Openings: Meatus, sinus, fossa, groove, fissure, foramen.

• Bone Textures:

  • Compact Bone: Dense outer layer.

  • Spongy Bone: Honeycomb of trabeculae filled with yellow bone marrow.

The Structure of Bone

Bones are structured to provide strength and flexibility. The anatomy varies by bone type, but all share common features.

• Structure of Long Bones:

  • Diaphysis: Tubular shaft; composed of compact bone surrounding the medullary cavity (contains yellow bone marrow).

  • Epiphyses: Expanded ends; exterior is compact bone, interior is spongy bone; joint surface covered with articular (hyaline) cartilage; separated from diaphysis by epiphyseal line.

  • Bone Membranes:

    • Periosteum: Double-layered protective membrane; outer fibrous layer (dense regular connective tissue), inner osteogenic layer (osteoblasts, osteoclasts); rich in nerves, blood, lymphatic vessels; attached by Sharpey's fibers.

    • Endosteum: Delicate membrane covering internal surfaces.

• Structure of Short, Irregular, and Flat Bones: Thin plates of periosteum-covered compact bone outside, endosteum-covered spongy bone inside; no diaphysis or epiphyses; bone marrow between trabeculae.

• Location of Hematopoietic Tissue (Red Marrow):

  • Infants: Medullary cavity and all areas of spongy bone.

  • Adults: Diploë of flat bones, head of femur and humerus.

• Microscopic Structure of Compact Bone:

  • Osteon (Haversian System): Structural unit.

  • Lamella: Weight-bearing matrix tubes (mainly collagen).

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

  • Volkmann's Canals: Connect periosteum to Haversian canal.

  • Osteocytes: Mature bone cells in lacunae.

  • Canaliculi: Hairlike canals connecting lacunae and central canal.

• Chemical Composition of Bone:

  • Organic: Osteoblasts (bone-forming), osteocytes (mature), osteoclasts (resorb), osteoid (unmineralized matrix: proteoglycans, glycoproteins, collagen).

  • Inorganic: Hydroxyapatites (mineral salts, mainly calcium phosphates); responsible for hardness and resistance to compression.

Bone Development and Regeneration

Bone formation (osteogenesis/ossification) occurs during embryonic development, growth, and repair. Two main processes are intramembranous and endochondral ossification.

• Intramembranous Ossification: Bone develops from fibrous membrane; forms most flat bones of skull and clavicles; involves mesenchymal cells.

• Endochondral Ossification: Bone forms by replacing hyaline cartilage; begins in second month of development; forms most bones.

• Stages of Endochondral Ossification:

  1. Formation of bone collar

  2. Cavitation of hyaline cartilage

  3. Invasion by periosteal bud, spongy bone formation

  4. Formation of medullary cavity, secondary ossification centers in epiphyses

  5. Ossification of epiphyses; hyaline cartilage remains in epiphyseal plates

• Growth in Length of Long Bones: Occurs at epiphyseal plate; involves growth, transformation, and osteogenic zones.

• Regulation of Bone Growth: Growth hormone stimulates epiphyseal plate activity; testosterone and estrogens promote growth spurts and later induce plate closure.

• Bone Remodeling and Repair:

  • Bone is resorbed and added by appositional growth.

  • Remodeling units: osteoblasts and osteoclasts deposit and resorb bone.

  • Bone deposition requires protein, vitamins, minerals; alkaline phosphatase is essential.

  • Bone resorption by osteoclasts involves lysosomal enzymes and acids.

• Importance of Ionic Calcium: Necessary for nerve transmission, muscle contraction, blood coagulation, gland/nerve cell secretion, cell division.

• Control of Remodeling:

  • Hormonal Mechanism: Maintains calcium homeostasis.

    • High blood Ca2+: thyroid releases calcitonin → stimulates calcium deposit in bone.

    • Low blood Ca2+: parathyroid releases PTH → signals osteoclasts to release Ca2+ from bone.

  • Mechanical/Gravitational Forces: Wolff's law: bone grows/remodels in response to forces.

Fractures and Homeostatic Imbalance

Bone fractures are classified by position, completeness, orientation, and skin penetration. Bone repair involves several stages, and certain diseases affect bone health.

• Classification of Bone Fractures:

  • Position: Nondisplaced (normal position), displaced (out of alignment).

  • Completeness: Complete (all the way through), incomplete.

  • Orientation: Linear (parallel), transverse (perpendicular).

  • Skin Penetration: Compound (open), simple (closed).

• Common Types of Fractures: Comminuted, spiral, depressed, compression, epiphyseal, greenstick.

• Stages of Bone Healing:

  1. Hematoma formation

  2. Fibrocartilaginous callus formation

  3. Bony callus formation

  4. Bone remodeling

• Bone Disorders:

  • Osteomalacia: Softened, weakened bones; pain; caused by insufficient calcium or vitamin D.

  • Rickets: In children; similar to osteomalacia; bowed legs, deformities; caused by vitamin D deficiency.

  • Osteoporosis: Bone resorption outpaces deposit; fragile bones; common in postmenopausal women; treated with supplements, exercise, hormone therapy.

  • Paget's Disease: Excessive bone formation/breakdown; spotty weakening; localized; treated with drugs.

Joints

Classification and Movement

Joints (articulations) are sites where two or more bones meet. They provide mobility and hold the skeleton together. Joints are classified structurally and functionally.

• Structural Classification:

  • Fibrous: Bones joined by fibrous tissue; no joint cavity; mostly immovable (sutures, syndesmoses, gomphoses).

  • Cartilaginous: Bones united by cartilage; lack joint cavity (synchondroses, symphyses).

  • Synovial: Bones separated by fluid-containing joint cavity; freely movable.

• Functional Classification:

  • Synarthroses: Immovable

  • Amphiarthroses: Slightly movable

  • Diarthroses: Freely movable

Fibrous Joints

• Sutures: Skull bones; interlocking junctions; allow growth during youth; fuse in middle age (synostoses).

• Syndesmoses: Bones connected by ligament; movement varies; e.g., tibia-fibula, radius-ulna.

• Gomphoses: Peg-in-socket joint; tooth in alveolar socket; periodontal ligament.

Cartilaginous Joints

• Synchondroses: Hyaline cartilage unites bones; synarthrotic; e.g., epiphyseal plates, first rib-sternum.

• Symphyses: Hyaline cartilage covers bone, fused to fibrocartilage pad; amphiarthrotic; e.g., intervertebral joints, pubic symphysis.

Synovial Joints

Synovial joints are the most common and freely movable joints, characterized by a fluid-filled cavity.

• General Structure: Articular cartilage, synovial cavity, articular capsule, synovial fluid, reinforcing ligaments.

• Friction-Reducing Structures: Bursae (fibrous sacs with synovial fluid), tendon sheaths (elongated bursae around tendons).

• Stability Factors: Articular surfaces, ligaments, muscle tone (tendons kept tight by muscle tone).

• Movement Types:

  • Gliding: Flat bone surfaces glide over each other (e.g., intercarpal joints).

  • Angular: Flexion, extension, dorsiflexion, plantar flexion, abduction, adduction, circumduction.

  • Rotation: Bone turns around its axis (e.g., atlas-axis, hip, shoulder).

  • Special Movements: Supination/pronation, inversion/eversion, protraction/retraction, elevation/depression, opposition.

Types of Synovial Joints

Type	Structure	Movement	Example
Plane	Flat articular surfaces	Slipping/gliding (nonaxial)	Intercarpal joints
Hinge	Cylindrical projection fits into trough	Flexion/extension (uniaxial)	Elbow, interphalangeal joints
Pivot	Rounded end in ring	Rotation (uniaxial)	Atlas-axis, proximal radioulnar
Condyloid	Oval surface fits depression	Angular (biaxial)	Wrist, knuckle joints
Saddle	Concave and convex surfaces	Greater movement (biaxial)	Thumb (carpometacarpal)
Ball-and-Socket	Spherical head in socket	All movements (multiaxial)	Shoulder, hip

Joint Injuries and Pathologies

• Sprains: Ligaments stretched or torn; partial tears repair slowly, complete tears require surgery.

• Cartilage Injuries: Overstressed cartilage; common in aerobics; repaired with arthroscopic surgery.

• Dislocations: Bones forced out of alignment; accompanied by sprains, inflammation; caused by falls, sports; subluxation is partial dislocation.

Inflammatory and Degenerative Conditions

• Bursitis: Inflammation of bursa; pain, swelling; treated with anti-inflammatory drugs.

• Tendonitis: Inflammation of tendon sheaths; similar symptoms and treatment as bursitis.

• Arthritis: Over 100 types; pain, stiffness, swelling; acute forms caused by bacteria, treated with antibiotics.

  • Osteoarthritis (OA): Most common; "wear-and-tear"; affects aged; cartilage destroyed faster than replaced; bone ends thicken, form spurs, restrict movement; slow, irreversible; treated with pain relievers, activity, magnetic therapy, glucosamine sulfate.

  • Rheumatoid Arthritis (RA): Chronic, autoimmune; onset 40-50 years; joint tenderness, anemia, osteoporosis, muscle atrophy, cardiovascular problems; exacerbations/remissions; begins with synovitis, pannus formation, cartilage erosion, scar tissue, ankylosis (deformed fingers); treated with aspirin, antibiotics, physical therapy, anti-inflammatory drugs, immunosuppressants, Enbrel (biological modifier).