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Lecture 6

Lecture 6 - Bone

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Barbara Ballyk

ANA300Y1 September 26, 2011 Lecture 6 – Bone  Bone is used for physical support, leverage, protection of internal organs (like the brain), storage of minerals (calcium phosphate) and lipids), and the protection of blood cells (hematopoiesis)  Bone connective tissue is mostly matrix. A third of your bone is organic matter, it’s not all calcium. There’s osteoid and it’s maid mostly of collagen type I (eosinophilic and it’ll stain very pink). There are also components of glycoproteins and proteoglycans. These molecules are much smaller compared to collagen, meaning there is less tissue fluid. There is tissue fluid (fluid between blood and cells). The organic component of the bone matrix resists pull (provides tensile strength) and it endows flexibility. Bones will bend before it breaks. Without the organic component, it’ll be very fragile.  Two thirds of the bone is inorganic. It is mainly calcium and phosphate (hydroxyapatite; hydroxyl molecule). This component of the bone provides compressional strength and mineral storage (where calcium phosphate is stored).  There are four cell types (osteoprogenitor, osteoblasts and osteocytes are the same cells but are at different stages:  Osteoprogenitor cells come from embryonic mesenchyme, and are capable of differentiating into active osteoid producing cells (into osteoblasts).  Osteoblasts are located on the surface of the bone matrix and secrete osteoids into their surroundings, calcified extracellularly. The appearance of the osteoblast range from being cuboidal to squamous. They are purple-blue in color.  Osteoblasts mature into osteocytes. It’s a lot like cartilage. They are living in a little space inside the matrix, called the lacunar (space occupied by cell); there is no calcium deposition in it (halo of unmineralized matrix). Osteocytes have all kinds of growths extending through the matrix (matrix is solid, so there has to be little tunnels in the matrix called canaliculi). Osteocytes form this network where they can communicate with each other via gap junctions. The job of the osteocyte is to maintain surrounding bone tissue. They sense mechanical stresses and they are remodeled or reorganized. Bone is more dynamic and is constantly being remodeled, reorganized or improved (for ex. to maintain optimal blood calcium levels). If bone experiences constant pressure, it will resorb (ex. getting braces: constant pressure forcing teeth a certain direction will cause the bone socket [gums] to resorb). Constant tension (ex. muscle attachment to bone via tendon: if there is force to generate a greater amount of tension) will deposit more bone, making it stronger. Bone from males have more bulk and can generate more power (bones are larger), bones from females are smaller thus generate less power and are lighter.  We get a new femur every few years because of the matrix turnover.  Osteoclasts are not from the same original cell as osteoprogenitor cells. Osteoclasts are completely different cell types. They are large multinuclear cells from the macrophage cell lineage. Osteoclasts are found always at the surface of bone matrix, in Howship’s (resorption) lacunae. It secretes acids and enzymes for a process called osteolysis which releases calcium and phosphate, which is added to tissue fluid and blood (bone breakdown). Osteoclasts have a ruffled border. Osteoclasts form from multiple monocytes, fusing together. They are full of lysosomes (sacks of enzymes) which can be released onto the matrix to dissolve the bone. Howship’s lacuna is a pivot where the osteoclast lies. Ruffled border increases surface area to facilitate processes.  Osteoblasts are being left behind and left into the matrix and will mature into the o
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