Skeletal System

Skeletal System

1. Axial Skeleton

   • 80 Bones
     • Skull, Vertebral column, rib cage

2. Appendicular Skeleton

   • 126 Bones
     • Limbs, Shoulders, Hips, Clavical

3. Functions of the Skeletal System

   • Support
   • Protection
   • Movement
   • Mineral Storage/Release
   • Energy Storage
     • Adipose tissue is found in the yellow marrow
   • Blood Cell Production
     • Red Marrow produces red blood cells, white blood cells and plasma. Process is called HEMATOPOIESIS (Hemopoiesis).

4. Classification of Bones Based on Shape

   • Long Bones
     • Longer than they are wide. Example: Femus, Tibia, Finger bones.
     • Long Bone Structure
       • Consists of a shaft (diaphysis) and two expanded extremities (epiphysis).
       • Diaphysis
         • Consists of a think collar of compact bone surrounding a medullary cavity
         • Medullary Cavity
           • Contains yellow bone marrow
           • Yellow Bone Marrow
             • Fat (energy source)
       • Epiphysis
         • Consists of a thin layer of compact bone covering an interior of spongy bone.
         • Metaphysis
           • Portion of a long bone between the diaphysis and epiphysis. Contains the epiphyseal plate
           • Epiphyseal Plate (Growth Plate)
             • Composed of hyaline cartilage and found in long bones that are still growing in length. Becomes the epiphyseal line (disk) when bone growth stops.
             • Epiphyseal Line (Disk)
               • Cartilage of the epiphyseal plate is replaced by bone.
       • Articular Cartilage
         • A thin layer of hyaline cartilage that covers the ends of the epiphyses.
         • Located in areas where one bone articulates with another bone.
         • Functions: Reduces friction and absorbs shock during movement.
       • Periosteum
         • Tough sheath that covers the external surface except for the articular cartilage.
         • Functions: Essential for bone growth in diameter and repair. Point of attachment for ligaments and tendons.
       • Endosteum
         • Thin membrane that lines internal surfaces of bone. Essential for bone repair.
   • Short Bones
     • About as long as they are wide (cube shaped) Example: Carpals, Tarsals
   • Flat Bones
     • Thin and Flat Example: Scapulae (shoulder blade), sternum, ribs, most bones of the skull. Wormian Bones: Suture bones found between the flat skull bones.
   • Irregular Bones
     • Have unusual shapes that fit none of the 3 previous classes. Example: Vertebrae and hip bones
   • Sesamoid Bones
     • Round bones- embedded in tendons. Example: Knee cap

5. Bone Marrow

   • General term for the soft tissue occupying the medullary cavity of a long bone and the spaces amid the trabeculae of spongy bone.
   • Yellow Bone Marrow
     • Consists mostly of fat (energy source).
   • Red Bone Marrow
     • Blood cell forming tissue= hematopoietic tissue.

6. Bone Cell Types

   • Osteogenic Cells
     • Stem cells that divide and develop into osteoblasts.
     • Located within the periosteum, endosteum, and the central canal.
     • Osteoblasts
       • Bone- building cells.
       • Synthesize and secrete collagen fibers and other organic components of bone matrix.
       • Initiate the process of calcification.
       • Osteocytes
         • Mature bone cells.
         • Osteoblasts that have become trapped by the secretion of matrix become osteosytes.
         • No longer secrete matrix.
         • Responsible for maintaining the bone tissue.
         • Osteoclasts
           • Large cells derived from the fusion of as many as 50 monocytes (a type of white blood cell).
           • Cells that digest bone matrix- this process is called bone resorption and is part of normal bone growth, development, maintenance, and repair.
           • Concentrated in the periosteum and endosteum.
           • On the side of the cell that faces the bone surface, the plasma membrane is deeply folded into a ruffled border. Here, the osteoclast secretes digestive enzymes to resorb the bone matrix. It also pumps out hydrogen ions to create an acid environment that eats away at the matrix.

7. Bone Matrix

   • Is the nonliving structural part and consists of:  Water (~25%) Collagen fibers (~25%) Crystalized mineral salts (50%)
   • Collagen Fibers
     • Provide the bone with flexibility- gives it resilience and the ability to resist stretching and twisting.
     • Frame work for calcification and mineral crystallization.
   • Crystalized Mineral Salts
     • Mainly hydroxyapatite (interaction of calcium phosphate and calcium carbonate).
     • Trace amounts of magnesium, fluoride, and sulfate.
     • Minerals are deposited among the collagen fibers and crystallize giving bone its characteristic hardness.
   • Mineral Salts
     • Are like the concrete and collagen fibers are like the metal robs in concrete.
     • Without the collagen fibers, bone would be very brittle.

8. Compact and Spongy Bone

   • All 206 bones are a combination of compact and spongy bone.
   • Compact Bone
     • The dense, solid, outer layer of the bone.
   • Spongy (Cancellous) Bone
     • Contains lots of spaces in a honeycomb of needle-like projections called trabeculae.
     • Surrounded by compact bone and contains red marrow.

9. Organization of Compact Bone

   • Osteons (Harversian Systems)
     • Are the fundamental units of compact bone which are multiple cylindrical structural units.
     • Imagine these osteons as weight-bearing pillars that are arranged parallel to one another along the long axis of a compact bone.
     • Osteon Components
       • Each osteon consists of a single longitudinal central (Haversian) canal, surrounded by concentric layers of calcified bone matrix called lamellae.
       • Central Canals
         • Allow the passage of blood vessels, lymphatic vessels, and nerve fibers.
   • Perforating (Volkmann's) Canals
     • Connect the blood and nerve supply between the periosteum, central (Haversian) canals and the medullary cavity.
   • Osteocytes and Canaliculi
     • Osteocytes
       • Occupy cavities known as lacunae at the junctions of the lamellae. 
     • Canaliculi
       • Canals that connect the lacunae to each other and to the central canal.
       • Allow osteocytes to exchange nutrients, wastes, and chemical signals to each other.

10. Organization of Spongy Bone

    • Spongy (Cancellous) Bone
      • Does not contain true osteons (no central canals)
      • Trabeculae
        • Consists of lamellae arranged in an irregular latticework of thin beams of bone.

11. Bone Scan

    • A nuclear scanning test that identifies new areas of bone growth or breakdown.
    • Used to evaluate damage (fractures, infection, trauma) to bones and detect cancer.
    • Procedure
      • A Radioactive (bone-seeking) tracer is injected into a vein and images are collected (gamma camera).
      • Normal bone tissue will appear gray throughout- uniform uptake.
      • "Cold Spots"
        • Areas that absorb little or no tracer- decreased metabolism
        • Degenerative bone disease or lack of blood supply to the bone (bone infraction).
        • "Hot Spots"
          • Areas that absorb a lot of tracer- rapid bone growth or repair.
          • Fracture that is healing, bone cancer, a bone infection (osteomyelitis), arthritis, or a disease of abnormal bone metabolism (such as Paget's disease).

12. Bone Formation (Osteogenesis)

    • Ossification
      • Process of bone formation.
    • Calcification
      • Process of depositing calcium salts within a tissue.

13. Formation of the Bony Skeleton

    • Before week 7, the human embryotic skeleton is made of stem cells that form fibrous membranes and hyaline cartilage in the shape of the future bones.
    • After week 7, bone tissue starts replacing the fibrous membranes and hyaline cartilage by 2 methods.
    • Intramembaneous Ossification
      • Development of bone from a fibrous membrane.
    • Endochondral Ossification
      • Development of bone from hyaline cartilage.
    • Bone Growth
      • Bones grow in length by the addition of bone material by the epiphyseal plate and increase in thickness by appositional growth.