Also called bone-lining cells, the osteoprogenitor cells are active when bone growth and regeneration are required, like after a fracture or during childhood. In healthy adults, the osteoprogenitor cells are inactive and are found in the external surface of the bones, as well as covering the internal bone surface, which faces the bone marrow.
Osteoblasts are cells that originate from the osteoprogenitor cells, during bone growth or regeneration. Osteoblasts are rich in the enzyme alkaline phosphatase, important in producing substances released in the extracellular matrix, which make bones hard. These cells are oval and have a large nucleus. The extracellular matrix surrounding osteoblasts is called prebone, because it is not totally rigid or calcified.
Osteocytes are star-shaped mature bone cells that form the larger part of adult bones. They develop from osteoblasts that eventually became trapped in the rigid extracellular matrix that they produced, during bone growth or regeneration. Osteocytes have long structures to connect with each other, as well as to absorb nutrients and eliminate wastes. Unlike osteoprogenitor cells, osteocytes cannot perform cellular division. With age, these cells die and bones can become more brittle.
The largest cells in the connective bone tissue are osteoclasts. They are multinuclear, having up to 50 nuclei. Osteoclasts perform a very important function in the maintenance of the bone tissue. Generally present in the outer layers of bones, osteoclasts are mobile cells and involved in bone resorption, which is a process of calcium release from old tissue formations.