BONE TISSUE

• Bone tissue


  A complex and dynamic living tissue that continuously engages in a process called bone remodeling - the building of new bone tissue and breaking down of old bone tissue
• When astronauts first returned from space, it was noticed that they experienced loss of bone mass
• Microgravity (virtual absence of gravity) of space flight


  Places only minimal strain on bones, which can result in the loss of 1-2% of bone mass per month, especially bones of the pelvis, backbone, and lower limbs
• Measures taken by astronauts to minimize loss of bone mass


  Physical exercise using space treadmills, stationary bikes, and devices that simulate weight lifting for two and a half hours a day, six days a week
• Athletes subject their bones to great forces, which place significant strain on the bone tissue
• Accomplished athletes show an increase in overall bone density
• Bone tissue


  Continuously growing, remodeling, and repairing itself
• Bone tissue


  Contributes to homeostasis of the body by providing support and protection, producing blood cells, and storing minerals and triglycerides
• More females than males are affected by osteoporosis
• Skeletal system


  • Supports soft tissues and provides attachment points for the tendons of most skeletal muscles
  • Protects the most important internal organs from injury
  • Assists in movement through the contraction of skeletal muscles
  • Stores and releases minerals, especially calcium and phosphorus
  • Produces red blood cells, white blood cells, and platelets within red bone marrow
  • Stores triglycerides within yellow bone marrow
• Bones containing red bone marrow

  • Hip bones
  • Ribs
  • Sternum (breastbone)
  • Vertebrae (backbones)
  • Skull
  • Ends of the humerus (arm bone) and femur (thigh bone)

  
• Red bone marrow


  Consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers
• Yellow bone marrow


  Consists mainly of adipose cells, which store triglycerides
• Parts of a long bone

  • Diaphysis (the long, cylindrical, main portion of the bone)
  • Epiphyses (the proximal and distal ends of the bone)
  • Metaphyses (the regions between the diaphysis and the epiphyses)
  • Articular cartilage (a thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation with another bone)
  • Periosteum (a tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage)
  • Medullary cavity (a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels)
  • Endosteum (a thin membrane that lines the medullary cavity and the internal spaces of spongy bone)

  
• Bone tissue


  Classified as a connective tissue because it contains an abundant extracellular matrix that surrounds widely separated cells
• Cellular composition of bone tissue

  • Osteoprogenitor cells (unspecialized bone stem cells)
  • Osteoblasts (bone-building cells)
  • Osteocytes (mature bone cells)
  • Osteoclasts (cells that break down bone extracellular matrix)
• Calcification


  The process by which mineral salts, such as calcium phosphate, crystallize and harden the extracellular matrix of bone tissue
• Collagen fibers in bone tissue

  Provide tensile strength and flexibility to the bone
• Osteoprogenitor cells


  Unspecialized bone stem cells that can develop into osteoblasts
• Osteoblasts


  Bone-building cells that synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue
• Osteocytes


  Mature bone cells that maintain the daily metabolism of bone tissue
• Osteoclasts


  Cells that break down the protein and mineral components of the bone extracellular matrix, a process called bone resorption
• Osteoclasts help regulate blood calcium level and are target cells for drug therapy used to treat osteoporosis
• Compact bone


  Dense, solid bone tissue that forms the outer layer of all bones

  
• Spongy bone


  Porous bone tissue that is found in the ends of long bones and in the interior of short, flat, and irregular bones

  
• Osteoclast's plasma membrane


  Deeply folded into a ruffled border
• Osteoclast function


  1. Releases powerful lysosomal enzymes and acids

  2. Digests the protein and mineral components of the underlying extracellular bone matrix
• Bone resorption


  Breakdown of bone extracellular matrix
• Bone resorption is part of the normal development, maintenance, and repair of bone
• Suffix -clast


  Cell breaks down extracellular matrix
• Osteoclasts


  Help regulate blood calcium level in response to certain hormones
• Osteoclasts are target cells for drug therapy used to treat osteoporosis
• Mnemonic device


  Memory aid
• Osteooblasts build bone, while osteoclasts carve out bone
• Bone is not completely solid but has many small spaces between its cells and extracellular matrix components

  
• Spaces in bone


  • Serve as channels for blood vessels that supply bone cells with nutrients
  • Act as storage areas for red bone marrow
• Compact bone tissue


  • Contains few spaces
  • Is the strongest type of bone tissue
  • Found beneath the periosteum of all bones
  • Makes up the bulk of the diaphyses of long bones
  • Provides protection and support and resists the strains produced by weight and movement
• Osteons


  Repeating structural units of compact bone tissue
• Osteonic (haversian) canal


  Small network of blood vessels and nerves located at the center of an osteon
• Bone lamellae


  Circular plates of mineralized extracellular matrix of increasing diameter, surrounding the osteonic canal