Osteogenic (stem cells) found on periosteum and endosteum, bone forming osteoblast. Osteoblast: active produce and secrete the organic component, create extracellular matrix. Osteocyte: when osteoblast no longer producing new osteoid, to keep the bone matrix healthy. Osteoclast: fusion of several leukocytes, has mutineucleate, large, mobile, phagocyte, microvilli (secrete enzymes that break down bone)