Bone

• Bone is a connective tissue, with an ossified extracellular matrix, that supports the body, protects internal organs, forms blood cells in bone marrow and the body's reservoir for calcium and phosphate
• Osteoblasts secrete matrix which harders by calcification, trapping differentiating cells (osteocytes) in individual lacunae
• Osteocytes are mature bone cells
• Osteocytes maintain the calcified matrix and receive nutrients from vessels in the central canals of the osteons via canaliculi
• Canaliculi are tiny channels that connect the lacunae
• Osteoclasts are monocyte-derived cells that remodel bone
• 1. Osteoblasts differentiate from osteoprogenitor cells and cover the surface of bone matrix and secrete osteoid
  2. As osteoid calcified, it entraps some osteoblasts which differentiate as osteocytes by the process of ossification, occupying lacunae
  3. Osteoclasts, produced by the fusion of blood monocytes, erode the matrix during bone remodeling
• Ossification is the formation of the bone
• Calcification is the deposition of calcium salts into the osteoid
• Ossification is the remodeling of the bone
• 50% of dry bone weight is inorganic matter
• Hydroxyapatite is the most abundant inorganic compound
• Chondroitin sulfate provides most of the resistance tocompression in cartilage
• hydroxyapatite crystals growth gradually calcifies the entire matrix
• Osteoclasts are large multinucleated cells which arederived by the fusion of several monocytes (type of WBC).
• Osteoclasts are a type of macrophage
• 90% of calcified matrix in organic matter is collagen and a small part of proteoglycans and glycoproteins
• Periosteum and endosteum connective tissuecover the external and internal surfaces,respectively, of all bones
• The periosteum is like the perichondrium of cartilage, withan outer fibrous layer of dense connective tissue, containingmostly type I collagen, fibroblasts, and blood vessels
• The very thin endosteum covers small trabeculae of bonymatrix that project into the marrow cavities
• Woven bone is nonlamellar, with random arrangement ofcollagen fibers. Think of fiberboard
• Woven bone is the primary aka the first bone to appear
• Lamellar is most adult bone, whether compact or cancellous
• Lamellar is fully ossified and parallel, alternating collagenfibers add to the strength.
• Compact (cortical) bone: 80% of the total bone mass.
• Cancellous (trabecular) bone: 20% of total bone mass
• epiphyses are composed of cancellous bonecovered by a thin layer of compact (cortical) bone
• the diaphysis, is almost totally densecompact bone, with a thin region of cancellous bone on the inner surface around the central marrow cavity
• Because it contains osteoprogenitor stem cells in the periosteum, endosteum, and marrow, and is very well vascularized, bone normally has an excellent capacity for repair
• Osteoblasts differentiate directly from mesenchyme and secrete osteoid.
• Endochondral ossification: A preexisting matrix of hyaline cartilage is eroded and invaded by osteoblasts, which produce osteoid
• A preexisting matrix of hyaline cartilage is eroded and invaded by osteoblasts, which produce osteoid
• Appositional growth occurs by formation of a periosteal bone collar around the diaphysis
• The primary ossification center in long bone diaphyses and the secondary ossification centers in epiphyses are separated by a plate of cartilage called the epiphyseal plate
• Fracture repair: Torn blood vessels release blood that clots to produce a large hematoma. The hematoma is gradually removed by macrophages; the periosteum and the endosteum produce by a soft fibrocartilage-like mass called procallus tissue. The procallus is invaded by regenerating blood vessels and proliferating osteoblasts. In the next few weeks the fibrocartilage is gradually replaced by woven bone which forms a hard callus throughout the area of fracture. The woven bone is then remodeled as compact and cancellous bone and fully functional vasculature is reestablished.
• Parathyroid hormone (PTH) from the parathyroid glands raises low blood calcium levels by stimulating osteoclasts and osteocytes to resorb bone matrix and release Ca
• Calcitonin, produced by thyroid gland, reduces blood calcium levels by opposing the effects of PTH. It directly targets osteoclasts to slow bone matrix resorption
• The skeleton is often the site of secondary, metastatic tumors, when cancer cells move into bones via the circulatory system from malignancies in other organs, such as the breast, lung, prostate gland, kidney, or thyroid gland
• In a long bone, the primary ossification center is in the diaphysis
• Cancellous bone is also known as trabecular bone