Skeletal 2

• Osteogenesis


  Making Bone

  
• Cleared and stained newborn mouse skeleton

  • Bone stains red, cartilage stains blue
• Bone is a dynamic tissue that is constantly growing and changing

  
• Bone is a dynamic tissue

  
• Humerus (the bone toward the top) in the elbow joint of a non-tennis playing elbow (left) and a tennis playing elbow (right)


  • Difference in size and density
• Bone organ dynamically adjusting to repetitive stresses in the sport

  Causes the difference in size and density

  
• Specialized cells responsible for modeling, remodeling, and repair

  • Osteoblasts: bone depositors
  • Osteoclasts: bone removers

  
• Osteoblasts

  Bone depositors

  
• Osteoclasts

  Bone removers
• Bone is a living tissue. It is dynamically repaired and remodeled throughout your lifetime

  
• Osteocytes

  Osteoblasts that have become entrapped by bone deposition; situated within voids called lacunae

  
• Bone matrix permeated by canaliculi - containing interconnecting processes of osteocytes

  
• Lacuna formation

  1. Surface Osteoblast - early stage

  2. Further development of lacuna; collagen fibrils

  3. Osteoblast depositing apatite and becoming incorporated into an enclosed lacuna
• Periosteum

  Membrane surrounding bones (except at the joints of long bones), consisting of dense connective tissue divided into an outer "fibrous layer" and inner "osteogenic layer"

  
• Fibrous layer of periosteum

  • Contains fibroblasts, which secrete various components of extra-cellular matrix (mostly collagen), also possesses an extensive nerve network
• Osteogenic layer of periosteum


  • Contains progenitor osteoblasts, which serve to increase overall bone size and are essential to healing after fracture
• Compact bone, cancellous/trabecular bone, and fat tissue (marrow) in a human femoral head

  
• Majority of compact bone tissue is organized into osteons, but surfaces have longitudinally-oriented layering known as "circumferential lamellae"

  
• Continuous remodeling is occurring (resorption cavities)
• Haversian system (Osteon)


  Unit of Haversian bone

  
• Osteoclast 'front' - Channel eroded through existing bone. Osteoblasts follow behind depositing new bone in concentric rings

  
• This secondary bone replaces former bone from earlier in development
• Osteon


  • Forms characteristic concentric rings of bone cells and layers of bone matrix surrounding a tube, with a central canal containing blood capillary and nerve

  
• Crack-stopping of Haversian bone

  Haversian bone functions to prevent bone breakages; deflecting cracks around the outer cement layer

  
• Haversian bone in the longbones of a theropod dinosaur

  
• Bone is continuously being remodeled and repaired to respond to encountered stresses
• Within the first year of life, approximately the entire human skeleton is replaced with new bone

  
• In adult humans, remodeling rates are about 10% per year
• Vitamin D


  Required for proper Ca2+ and PO4 3- uptake in the gut
• Osteomalacia (or "Rickets" in children)


  Softening of bone because of insufficient calcium uptake (usually low calcium and/or vitamin D supply)
• Femoral buckling and reduced mineralization of long bones typical of Rickets

  
• No Haversian systems in actinopterygians (ray-finned fishes: >50% of vertebrate biodiversity)
• Not all mammal bone is Haversian

  
• Earliest 'complete' fossil fish Astraspids - Late Ordovician ~450 mya Colorado
• Acellular bone


  Bone formed if the osteoblasts migrate away faster than bone matrix is laid down, therefore they do not become enclosed within it

  
• Acellular bone is a common tissue type among teleost fishes

  
• Endochondral Bone Growth


  1. Cartilage cells stacked up in the Zone of Proliferation

  2. Osteoblasts establish growth center, calcified cartilage overlain with bone

  3. Osteoclasts remodel, & trabeculae grow as composites of new bone and 'old' calcified cartilage

  
• Cancellous bone


  Porous at the macroscopic level, composed of fine struts called trabeculae

  
• Actual bone volume in cancellous bone is only a fraction of total organ volume: from 5 – 60%

  
• Trabecula (basic unit of cancellous bone): a rod, spicule or plate of hydroxyapatite, with no central canal, nerve, or blood vessel