<P> The bone cells include osteocytes, osteoblasts, osteoclasts, osteogenic cells (stem cells), and lining cells . </P> <P> Osteoclasts are very large multinucleate cells that are responsible for the breakdown of bones . The breakdown of bone is very important in bone health because it allows for bone remodeling . Osteoclasts are formed by the conjoining of many different cells created from the bone marrow that travel in the circulatory system . Osteoclasts are usually found grouped in the small pits on bone surfaces called Howship's lacunae . The pits are sites of bone resorption . After latching on to the site of the bone where it is supposed to resorb it, the osteoclast releases a number of enzymes which break down the bone tissue . The final product of the resorption of the bone is calcium and phosphorus ions . This reabsorption process can sometimes take up to weeks for the osteoclast to complete . The breakdown of bones is controlled by hormones in the bloodstream which instruct the osteoclasts when and where to break down bone tissue . </P> <P> Osteoblasts are bone cells that are responsible for the formation of new bone . Osteoblasts deposit a collagen matrix and release minerals that combine to make the bone mineral . Unlike the much larger osteoclasts, osteoblasts are much smaller; they only have one nucleus . Osteoblasts also group to form new bone . Osteoblasts are important because they allow the bones to be made, remodeled, and repaired . The osteoblasts come from the differentiation of osteogenic cells in the tissue that covers the outside of the bone, or the periosteum and the bone marrow . The osteoblast creates and repairs new bone by actually building around itself . First, the osteoblast puts up collagen fibers . These collagen fibers are used as a framework for the osteoblasts' work . The osteoblast then deposits calcium phosphate which is hardened by hydroxide and bicarbonate ions . The brand new bone created by the osteoblast is called osteoid . Once the osteoblast is finished working it is actually trapped inside of the bone once it hardens . When the osteoblast becomes trapped, it becomes known as an osteocyte . Other osteoblasts remain on the top of the new bone and are used to protect the underlying bone, these become known as lining cells . </P> <P> Osteocytes are osteoblasts which have become trapped inside of the bone matrix . Once an osteoblast creates the new bone around itself, it is trapped and can no longer move or form bone; this is how an osteocyte is created . When the osteoblast is transformed into an osteocyte, the transformation causes the osteocyte to lose a majority of its organelles . What replaces these organelles are large quantities of microfilaments . Osteocytes develop long branches which allow them to contact each other and also contact the bone lining cells . The osteocyte lies within a small chamber called a lacuna, which is within the bone matrix . Osteocytes remain in contact with other cells in the bone through gap junctions--coupled cell processes--which pass through small channels in the bone matrix called the canaliculi . The osteocyte is still a mysterious cell--biologists still have not figured out the true function of the osteocyte . Even though the function of the osteocyte is still under investigation, there are some ideas on what they might do . One function of the osteocyte might be the remodeling of the bone through growths of new arms on the cell . It is also known that osteocytes can secrete growth factors which activate lining cells or stimulate osteoblasts . Finally, it is believed that the osteocyte might compensate for the strain on the bone due to their many arms which extend out to other osteocytes . </P>

Production of new bony matrix is by which cell type