Skeletal System Lecture Notes

Jul 4, 2024

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Skeletal System Lecture Notes

Overview

  • The skeletal system consists of bones, cartilage, tendons, and ligaments.
  • Functions of the skeletal system include support, protection, movement, storage of minerals and fat, and blood cell formation (hematopoiesis).

Structure and Lifecycle of Bones

  • Bones at Birth and Adulthood: Initially, the human body has around 450 bones at birth that fuse to form 206 bones by adulthood.
  • Components: Bones, cartilage, tendons, ligaments.
    • Ligaments: Connect bones to bones.
    • Tendons: Connect muscles to bones.

Functions of the Skeletal System

  1. Support: Supports soft organs and maintains posture against gravity.
  2. Protection: Protects brain, heart, lungs, and spinal cord via the skull, rib cage, and vertebral column respectively.
  3. Movement: Facilitates movement with muscles attached to bones.
  4. Storage: Stores calcium, phosphate, and other minerals; fat stored in yellow marrow.
  5. Hematopoiesis: Blood cell formation in red marrow.

Classification of Bones

  • Long Bones: Longer than they are wide (e.g., limbs, fingers, and toes but not wrists, ankles, or knees).
  • Short Bones: Cube-shaped (e.g., carpals and tarsals in wrists and ankles).
  • Flat Bones: Thin and flat (e.g., skull, sternum, ribs, and scapulae).
  • Irregular Bones: Various shapes not fitting above categories (e.g., vertebrae, sphenoid, and ethmoid bones).

Anatomy of a Long Bone

  • Diaphysis: The shaft of the bone.
  • Epiphysis: The ends of the bone, includes proximal and distal epiphyses.
  • Articular Cartilage: Covers surfaces of epiphyses; made of hyaline cartilage, crucial for smooth joint movement.
  • Medullary Cavity: Hollow center within the diaphysis; contains red marrow in children and yellow marrow (fat) in adults.
  • Epiphyseal Plate: Growth plate, located between diaphysis and epiphysis, facilitates lengthwise growth during childhood; fuses into epiphyseal line in adulthood.
  • Periosteum: Fibrous tissue covering the bone surface, containing blood vessels and nerves.
  • Endosteum: Lines the medullary cavity; contains osteoclasts for bone remodeling.

Bone Histology and Formation

  • Bone is composed of various tissues: bone tissue, cartilage, vascular tissue, etc.
  • Compact Bone: Dense and solid, found in diaphysis; contains osteons (Haversian systems) with concentric lamellae.
  • Spongy Bone: Porous, found in epiphyses; contains trabeculae.
  • Bone Cells:
    • Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts and osteoclasts.
    • Osteoblasts: Bone-building cells, produce osteoid (bone matrix).
    • Osteocytes: Mature bone cells living in lacuna, derived from osteoblasts.
    • Osteoclasts: Bone-resorbing cells, break down bone matrix.
  • Hydroxyapatite: Hard component of bone, composed of calcium and phosphate crystals.

Bone Remodeling and Growth

  • Bones are constantly remodeled based on mechanical stresses.
  • Bone Growth Regulation: Controlled by hormones such as growth hormone, sex hormones (testosterone and estrogen), and thyroid hormone.
  • Calcium Homeostasis: Maintained by hormones:
    • Parathyroid Hormone (PTH): Increases calcium levels in blood by stimulating osteoclast activity, increasing intestinal absorption, and kidney reabsorption.
    • Calcitonin: Decreases calcium levels in blood by promoting calcium deposition in bones.
    • Calcitriol: Active form of vitamin D, aids in calcium absorption from diet.
  • Vitamin D Synthesis: Requires UV light; helps in calcium absorption.

Bone Diseases and Disorders

  • Osteoarthritis: Degeneration of articular cartilage, leading to bone spurs (osteophytes) and pain.
  • Rickets/Osteomalacia: Bone softening due to calcium or vitamin D deficiency; leading to weak, deforming bones.
  • Osteogenesis Imperfecta: Genetic disorder causing brittle bones due to defective collagen production, leading to frequent fractures.
  • Osteoporosis: Excessive osteoclast activity leading to brittle, easily fractured bones, common in post-menopausal women.

Key Concepts and Important Points

  • Bones are living tissues that continuously undergo remodeling.
  • The skeletal system provides critical roles beyond support, including protection, storage, production of blood cells, and enabling movement.
  • Hormonal balance is crucial for maintaining healthy bones and regulating calcium levels in the blood.
  • Proper nutrition, specifically calcium and vitamin D, is vital for bone health.

Hint for Exams: Pay attention to how bone remodeling works and the different types of bone cells and hormones involved.