Hydroxyapatite is the amalgamation of calcium in bone. You've heard the expression that calcium builds bone. Hydroxyapatite crystals constitute the bulk of the inorganic matrix of bone. Both these matrices are elaborated by osteoblasts and osteocytes . Osteoblasts begin the elaboration, and when osteoblasts become surrounded by what they've elaborated, the become known as osteocytes.
Osteocytes secrete much less collagen than do osteoblasts.
Osteocytes are the permanent living cells of the bone matrix. The final cell in bone is the osteoclast . Osteoclasts are what remodel bone. They are stimulated by Vitamin D on the surfaces of osteocytes. The image to the left is a typical cross-section of bone stained with the standard hematoxylin and eosin stain.