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| The Surgical Technologist | MARCH 2016 112 noglycans (GAGs), which play an important role in determining mineral size and crystallinity in bone. A family of proteins called bone morphogenic pro- teins (BMPs) have also been found to reside within the extracellular matrix in the bone. These proteins, structurally and functionally related to transform- ing growth factors, are responsible for the migra- tion, proliferation and differentiation of bone-form- ing cells through signal transduction pathways. 4 2. I norganic material. Inorganic materials, mainly ionic compounds, comprise about 70% of the bone’s dry weight. Bone functions not only as a structural and support tissue, but also as a reservoir for ions such as calcium, phosphorous, sodium and magnesium. 1 Most of the calcium and phosphate found in bone is in the form of hydroxyapatite (Ca 10 (PO 4 ) 6 OH 2 ). Hydroxyapatite appears in bone as mineral platelets of organized collagen fibers intercalated by hydroxyapatite crystals. While collagen gives bone soft and ductile properties, the hydroxyapa- tite crystals give it a stiff and brittle character. 5 B. Cellular composition. 1. Osteoblasts. Osteoblasts are better known as bone- forming cells. They comprise about 4-6% of bone cells. These cells are in charge of bone production and min- eralization through the secretion of polysaccharides, collagen fibers and other factors for osteoclast differ- entiation. 6 Osteoblasts differentiate from mesenchy- mal stem cells (MSCs). Differentiation occurs through signal transduction cascades headed by cytokines. 7 2. Osteoclasts. Osteoclasts are the bone resorptive cells. These cells encompass 1-2% of bone cells and work by secreting acids and collagenolytic enzymes. Stimu- lation by the receptor activator of nuclear factor-kB ligand (RANKL) generates a cascade of events in a signal transduction pathway that leads to the prolif- eration of bone marrow macrophages (BMMs), which eventually fuse to each other to become multinucle- ated mature osteoclasts. 8 3. Osteocytes. Osteocytes, better known as mature osteo- blasts, encompass 90-95% of total bone cells. These cells are found dispersed in a sea of mineralized matrix within lacunas connected by dendritic processes that