osteoprogenitor cells are the "stem cells" for bone. They have the capacity to divide and proliferate to form osteoblasts, bone forming cells which actively produce bone tissue. They comprise the periosteal and endosteal cells (see slide 11) which cover the bone and line its cavities. In mature bone where growth and remodeling is not occurring these cells are quiescent, but they are thought to function in maintenance and nutritional support of the osteocytes in the underlying matrix, with which they connect by means of gap junctions. Osteoprogenitor cells are derived from mesenchymal cells (a fundamental embryological germ tissue) and have the ability to differentiate into adipose cells, chondroblasts, and fibroblasts and can modify their morphologic (physical) and physiologic characteristics in response to specific stimuli.
	osteoblasts - these are the "bone forming cells" which secrete the collagen and ground substance that constitutes unmineralized bone (osteoid), and subsequently are responsible for calcification of the matrix. These cells also communicate with one another and with osteocytes by gap junctions.
	Osteocytes are mature bone cells which are responsible for maintaining the bone matrix. They can synthesize and resorb (break down) the matrix to maintain homeostasis. Each osteocyte occupies a space, the lacuna, which conforms to the shape of the cell surrounded by matrix secreted when the cell was an osteoblast. Osteocytes extend processes through canaliculi to connect to neighboring cells by means of gap junctions.
	Osteoclasts are large multinucleated cells whose function is to resorb (reabsorb) bone. Osteoclasts dissolve the matrix and osteoid with acids and hydrolytic enzymes. Osteoclasts are phagocytic and are derived from monocytes and not from the same line as the other bone cells.