Skeletal System

The Skeletal System is part of the Anatomy and Physiology section which provides High Yield information for the MCAT exam needed for Medical School.

Table Of Contents
1 Functions of bone
2 Types of Bones
3 Basic structure of a long bone
4 Histology
5 Ossification (formation of bone)
6 Bone Growth
7 Bone Remodeling
8 Repair of Fractures
9 Skeletal Anatomy
10 Skull Bones

Functions of bone

  • – Support soft tissues
  • – Protection of internal organs
  • – Movement
    • 1. points of muscle attachment
    • 2. form joints
  • – Mineral storage
    • 1. mainly calcium and phosphorous
  • – Production of blood cells
    • 1. red marrow in some bones
  • – Energy storage
    • 1. yellow marrow is mostly fat

Types of Bones

  • – Long bones
    • 1. longer than they are wide (e.g., most bones of limbs)
  • – Short bones
    • 1. roughly cube shaped (e.g., bones of wrist and ankle)
    • 2. sesamoid bones- in tendons, provide support (e.g., patella)
  • – Flat bones
    • 1. flat and curved (e.g., cranial bones, ribs, sternum, scapula)
  • – Irregular bones
    • 1. complex shapes (e.g., vertebrae and hip bones)

Basic structure of a long bone

  • – Diaphysis
    • 1. long part of bone (shaft)
  • – Epiphyses
    • 1. ends of the bone
  • – Articular cartilage
    • 1. hyaline cartilage covering ends of bone
    • 2. decreases friction and absorbs shock
  • – Periosteum
    • 1. white fibrous membrane covering surfaces not covered by articular cartilage
      • a. outer fibrous layer
        • 1) dense irregular CT
        • 2) blood and lymphatic vessels, nerves
      • b. inner osteogenic layer
        • 1) elastic fibers, blood vessels, bone cells
    • 2. functions
      • a. bone growth in thickness
      • b. repair of fractures
      • c. protects and nourishes
      • d. point of attachment for ligaments and tendons
  • – Medullary cavity
    • 1. cavity in diaphysis containing yellow marrow
  • – Endosteum
    • 1. lining of marrow cavities
    • 2. delicate CT with bone cells

Histology

  • – Cells
    • 1. osteoblasts
      • a. secrete collagen and other parts of bone tissue
      • b. on inner and outer bone surfaces
      • c. become osteocytes
    • 2. osteocytes
      • a. main cells of bone tissue
      • b. in cavities within bone called lacunae
      • c. exchange nutrients and wastes with blood
    • 3. osteoclasts
      • a. may come from circulating WBCs
      • b. on inner and outer bone surfaces
      • c. break down bone (resorption)
  • – Matrix
    • 1. 25% water
    • 2. 25% protein fibers
      • a. collagen gives bones flexibility and strength
    • 3. 50% mineral salts
      • a. mainly calcium phosphate and calcium carbonate (hydroxyapatite = mineral salts)
      • b. gives bone hardness
  • – Compact bone
    • 1. makes up outer portion of all bones and diaphyses of long bones
    • 2. made up of osteons with interstitial lamellae in between
      • a. blood and lymphatic vessels and nerves run through canals
      • b. matrix arranged in lamella
      • c. osteocytes in lacunae
      • d. lacunae connected to one another by canaliculi (filled with ECF)
  • – Spongy bone
    • 1. makes up most of bone tissue in short, flat and irregular bones, and epiphyses of long bones
      • a. site of red marrow in adults (axial skeleton, girdles, proximal epiphyses of humerus and
        femur)
    • 2. no true osteons
    • 3. lamellae arranged in trabeculae

Ossification (formation of bone)

  • Intramembranous
    • 1. bone forms within a CT membrane
      • a. most bones of skull, clavicles
    • 2. basic steps
      • a. mesenchyme (embryonic CT) develops into osteoblasts at center of ossification, matrix is secreted
    • b. osteoblasts are now osteocytes in lacunae
    • c. matrix hardens as minerals deposited
    • d. trabeculae develop (woven bone)
    • e. outer layers replaced by compact bone
  • – Endochondral
    • 1. bone replaces cartilage (most bone formed this way)
    • 2. basic steps
      • a. hyaline cartilage model surrounded by perichondrium
      • b. periosteum develops, bone collar forms
      • c. cartilage in center of diaphysis calcifies
      • d. primary ossification center forms
      • e. secondary ossification centers form in epiphyses

Bone Growth

  • – Growth in length of long bones
    • 1. cartilage at epiphyseal plate grows toward epiphyseal end
    • 2. other side of epiphyseal plate ossifies
    • 3. continues until growth completed in early adulthood
      • a. eventually the whole plate ossifies and becomes epiphyseal line
  • – Appositional growth (growth in diameter)
    • 1. osteoclasts in endosteum destroy inner portion of bone
    • 2. osteoblasts in periosteum produce new bone on outer surface

Bone Remodeling

  • – Bone is constantly being broken down and reformed
    • 1. particularly in areas where bone is stressed

Repair of Fractures

  • – Hematoma formation
    • 1. due to broken blood vessels, hematoma forms (mass of clotted blood)
    • 2. bone cells deprived of nutrition die
    • 3. fracture site become swollen and painful
    • 4. capillaries grow into hematoma, osteoclasts and macrophages remove dead tissue and debris
  • – Fibrocartilage callus
    • 1. fibroblasts and osteoblasts migrate from periosteal and endosteal membranes
    • 2. fibroblasts make collagen that connects the broken bone ends
    • 3. chondroblasts secrete cartilage matrix
    • 4. osteoblasts form spongy bone
    • 5. this callus “splints” the bone
  • – Bony callus
    • 1. osteoclasts and osteoblasts break down fibrocartilage callus and form bony callus
  • – Remodeling occurs until bone completely healed

Skeletal Anatomy

Skull Bones