Equine Functional Anatomy: The Skeletal System and Osteogenesis

Functions and Constituents of the Skeletal System

  • Constituents: Bones, cartilage, ligaments, and tendons.
  • Key Functions:     1. Support     2. Locomotion     3. Protection     4. Storage     5. Haemopoiesis

Bone Composition and Classification

  • Living Tissue: Bone is the second hardest substance in the body (after enamel).
  • Classification by Shape:     * Long: E.g., femur.     * Flat: E.g., rib.     * Short: E.g., carpal.     * Irregular: E.g., vertebrae.     * Specialized: Includes Sesamoid (e.g., patella), Splanchnic, and Pneumatic bones.
  • Chemical Composition:     * Inorganic (45%45\%): Primarily Calcium phosphate (2/32/3 of total bone weight) and Calcium hydroxide.     * Organic (30%30\%): Includes cells and Osteoid (collagen fibers, proteoglycans, and glycoproteins produced by osteoblasts).     * Water (25%25\%).

Bone Cell Types and Functions

  • Osteoprogenitor Cells: Precursors to osteoblasts.
  • Osteoblasts: Bone-forming cells that secretes osteoid.
  • Osteocytes: Mature bone cells transformed from osteoblasts; found within a lacuna.
  • Osteoclasts: Large multinucleated cells derived from monocytes; responsible for bone resorption (matrix breakdown) and mineral release.

Intramembranous Ossification

  • Definition: Bone development derived from a fibrous membrane.
  • Examples: Flat bones, mandible, and clavicle (in cats).
  • Heterotopic Bones: Formation in non-skeletal tissues, such as Os cordis (bovine heart) and Os penis.
  • Process: Mesenchymal cells gather and differentiate into osteoblasts and capillaries. Osteoblasts secrete osteoid at the ossification center. Matrix calcification separates osteoblasts, which differentiate into osteocytes.

Endochondral Ossification

  • Definition: Bone development derived from a cartilage model.
  • Examples: Long bones, short bones, vertebrae, pelvis, and ribs.
  • Process:     1. Mesenchymal cells differentiate into chondroblasts to secrete a hyaline cartilage matrix.     2. Growth occurs via interstitial (length) and appositional (width) mechanisms.     3. Perichondrial cells produce osteoblasts to form the periosteum and bone collar.     4. Chondrocytes hypertrophy, synthesize alkaline phosphatase, and the matrix calcifies.     5. Primary Ossification Center: Established in the diaphysis via blood vessel and osteoprogenitor cell migration.     6. Secondary Ossification Centers: Formed in the epiphyses.
  • Residual Cartilage: Only remains in the epiphyseal plates and articular cartilages after ossification is complete.