Skeletal System: Bones and Joints

Functions of the Skeletal System

Skeletal system overview

  • Support: Provides a rigid framework to bear body weight and support soft tissues; cartilage offers flexible support, and ligaments hold bones together.
  • Protection: Hard bone surrounds and shields delicate internal organs (e.g., skull protects the brain, vertebrae protect the spinal cord, and rib cage protects the thoracic organs).
  • Movement: Skeletal muscles attach to bones via tendons; muscle contraction moves bones across joints, which are cushioned by cartilage.
  • Storage: Acts as a reservoir for essential minerals (primarily calcium and phosphorus) and stores energy in the form of adipose tissue (fat) within yellow bone marrow.
  • Blood Cell Production: Cavities within many bones contain red bone marrow, which generates blood cells and platelets.

Connective Tissue Components

  • Collagen: A tough, ropelike protein that provides flexible strength to connective tissues.
  • Proteoglycans: Large polysaccharide-protein complexes that attract and retain water, conferring resiliency and smoothness to cartilage.
  • Hydroxyapatite: Calcium phosphate crystals (Ca10(PO4)6(OH)2\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2) forming the inorganic mineral component of bone matrix, providing weight-bearing compression strength.
  • Tissue Matrix Composition:
    • Tendons and Ligaments: Matrix composed primarily of dense collagen fibers, making them cable-like and strong.
    • Cartilage: Matrix containing collagen and water-retained proteoglycans, acting as a flexible shock absorber.
    • Bone: Reinforced composite matrix containing collagen fibers (flexible strength) and hydroxyapatite minerals (compression strength).

Bone Structure and Features

Structure of a long bone

  • Bone Shapes:
    • Long Bones: Longer than broad (e.g., upper and lower limb bones).
    • Short Bones: Broad as they are long (e.g., carpals of the wrist and tarsals of the ankle).
    • Flat Bones: Thin, flattened shape (e.g., skull bones, ribs, sternum, scapulae).
    • Irregular Bones: Elaborate shapes that do not fit other categories (e.g., vertebrae, facial bones).
  • Long Bone Anatomy:
    • Diaphysis: The central shaft enclosing the central medullary cavity.
    • Epiphysis: The expanded ends of a long bone, covered by articular cartilage.
    • Epiphyseal Plate: Layer of cartilage between diaphysis and epiphysis responsible for growth in length; becomes an epiphyseal line upon bone maturity.
    • Marrow: Red marrow is blood-forming soft tissue located in children's bones and adult axial skeletons; yellow marrow consists mostly of fat.
    • Membranes: Periosteum coats the outer bone surface; endosteum lines the internal medullary cavity. Both contain bone-forming osteoblasts and bone-resorbing osteoclasts.
  • Histological Tissue Types:
    • Compact Bone: Dense outer layer organized into osteons (haversian systems). Each osteon features concentric lamellae surrounding a central canal carrying blood vessels, with osteocytes residing in lacunae connected by canaliculi.
    • Cancellous (Spongy) Bone: Latticework of interconnecting bone rods or plates called trabeculae; contains no central canals, with spaces filled by marrow.

Bone Ossification, Growth, and Repair

Bone formation processes

  • Bone Formation (Ossification):
    • Intramembranous Ossification: Direct bone formation within connective tissue membranes, initiating at ossification centers (primarily forms flat skull bones).
    • Endochondral Ossification: Bone formation within hyaline cartilage models (forms skull base and most remaining skeletal structures).
  • Bone Growth:
    • Appositional Growth: Osteoblasts deposit new bone matrix on the outer surface beneath the periosteum, increasing bone width/diameter.
    • Length Growth: Chondrocytes proliferate, enlarge, calcify, and die at the epiphyseal plate; osteoblasts replace calcified cartilage on the diaphyseal side with bone.
  • Bone Remodeling and Repair:
    • Remodeling: Removal of old bone by osteoclasts and deposition of new bone by osteoblasts to adapt to stress, repair damage, and maintain calcium balance.
    • Fracture Repair: Clot formation →\rightarrow Callus formation (fibrous connective tissue and cartilage bridges fragments) →\rightarrow Callus ossification (cancellous bone formed in 4–6 weeks) →\rightarrow Bone remodeling over several months.

Bone and Calcium Homeostasis

Calcium homeostasis pathways

  • Parathyroid Hormone (PTH): Secreted by parathyroid glands in response to low blood calcium; indirectly stimulates osteoclasts to breakdown bone, increases renal calcium reabsorption, and promotes active vitamin D formation to boost intestinal calcium absorption.
  • Calcitonin: Secreted by the thyroid gland in response to elevated blood calcium; inhibits osteoclast activity to decrease blood calcium levels.

Axial Skeleton

Lateral view of skull

  • Skull (22 Bones):
    • Braincase (8 bones): Parietal (2), temporal (2), frontal (1), occipital (1), sphenoid (1), ethmoid (1).
    • Facial Bones (14 bones): Maxilla (2), zygomatic (2), palatine (2), nasal (2), lacrimal (2), inferior nasal concha (2), mandible (1), vomer (1).
    • Key Cranial Features: Sutures (coronal, squamous, lambdoid), external acoustic meatus, mastoid process, zygomatic arch, sella turcica, foramen magnum, and paranasal sinuses (frontal, maxillary, ethmoidal, sphenoidal).
  • Hyoid Bone: U-shaped bone providing attachment for tongue and neck muscles; has no direct bony articulation with the skull.
  • Vertebral Column (26 Bones):
    • Regions: 7 cervical (C1–C7), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 1 sacrum (5 fused), 1 coccyx (4 fused).
    • Key Vertebrae: Atlas (C1) articulates with occipital condyles for "yes" head motion; Axis (C2) features the dens process for "no" rotation.
    • General Plan: Body, vertebral arch enclosing the vertebral foramen (forming the vertebral canal), spinous process, transverse processes, and intervertebral disks.
    • Curvature Abnormalities: Kyphosis (exaggerated thoracic posterior curve), Lordosis (exaggerated lumbar anterior curve), Scoliosis (abnormal lateral curve).
  • Thoracic Cage:
    • Ribs (12 pairs): True ribs (1–7) attach directly to sternum via costal cartilages; False ribs (8–12) do not attach directly (ribs 8–10 attach via common cartilage; ribs 11–12 are floating ribs).
    • Sternum: Composed of manubrium (with jugular notch), body (junction forms sternal angle at rib 2), and xiphoid process.

Appendicular Skeleton

Comparison of male and female pelvis

  • Pectoral Girdle and Upper Limb:
    • Pectoral Girdle: Scapula (featuring glenoid cavity, spine, acromion process, and coracoid process) and Clavicle.
    • Arm & Forearm: Humerus (head, anatomical/surgical necks, greater/lesser tubercles, deltoid tuberosity), Ulna (medial side, semilunar notch, olecranon process), Radius (lateral side, radial tuberosity).
    • Wrist & Hand: 8 Carpals (scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate), 5 Metacarpals (1–5), 14 Phalanges.
  • Pelvic Girdle and Lower Limb:
    • Pelvic Girdle: Coxae (each formed by fused ilium, ischium, and pubis) joined anteriorly at pubic symphysis and posteriorly to sacrum; contains acetabulum and obturator foramen.
    • Pelvic Differences: Female pelvis is lighter, broader, has a larger pelvic inlet/outlet, and a subpubic angle of 90o90^\text{o} or greater.
    • Thigh & Leg: Femur (head, neck, greater/lesser trochanters, condyles), Patella, Tibia (medial weight-bearing bone, tibial tuberosity, medial malleolus), Fibula (lateral bone, lateral malleolus).
    • Ankle & Foot: 7 Tarsals (talus, calcaneus, cuboid, navicular, medial cuneiform, intermediate cuneiform, lateral cuneiform), 5 Metatarsals, 14 Phalanges.

Articulations and Movement

Types of synovial joints

  • Structural Joint Classes:
    • Fibrous Joints: Bones bound by fibrous tissue with little/no movement (e.g., Sutures with fontanels in newborn skulls, Syndesmoses, Gomphoses).
    • Cartilaginous Joints: Bones united by cartilage with slight movement (e.g., epiphyseal plates, costal cartilages, fibrocartilage intervertebral disks).
    • Synovial Joints: Freely movable joints containing articular cartilage, joint capsule, synovial membrane, and lubricating synovial fluid (may include bursae and tendon sheaths).
  • Types of Synovial Joints:
    • Plane (Gliding): Flat articular surfaces (e.g., articular processes of vertebrae, intercarpal).
    • Saddle: Biaxial movement (e.g., carpometacarpal joint of thumb).
    • Hinge: Uniaxial movement in one plane (e.g., elbow, knee, interphalangeal).
    • Pivot: Uniaxial rotation (e.g., atlantoaxial joint, proximal radioulnar joint).
    • Ball-and-Socket: Multiaxial movement in all directions (e.g., shoulder, hip).
    • Ellipsoid (Condyloid): Modified ball-and-socket, movement in two planes (e.g., atlantooccipital, radiocarpal).
  • Major Joint Movement Pairs:
    • Flexion / Extension (and Hyperextension)
    • Plantar flexion / Dorsiflexion
    • Abduction / Adduction
    • Pronation / Supination
    • Inversion / Eversion
    • Rotation (Medial / Lateral)
    • Protraction / Retraction
    • Elevation / Depression
    • Excursion
    • Opposition / Reposition
    • Circumduction

Aging and Skeletal System Disorders

  • Effects of Aging:
    • Collagen synthesis declines, making bone matrix relatively more mineralized and brittle.
    • Osteoblast activity slows relative to osteoclasts, decreasing overall bone mass after age 35.
    • Articular cartilage wears down, synovial fluid production declines, and surrounding ligaments/tendons stiffen, restricting range of motion.
  • Major Skeletal Disorders:
    • Osteoporosis: Severe reduction of bone tissue quantity leading to porous, fragile bones susceptible to fracture; highly prevalent in postmenopausal women.
    • Osteoarthritis (Degenerative Joint Disease): Gradual wear-and-tear degradation of articular cartilage in weight-bearing joints.
    • Rheumatoid Arthritis: Autoimmune inflammatory condition causing synovial membrane proliferation, pannus formation, cartilage erosion, and joint destruction.
    • Gout: Accumulation of uric acid crystals within joint capsules and kidneys, causing acute inflammation.