Synthesis, degradation, and bone maintenance are carried out by three types of cells.
Osteoblasts (figures D and E) (figures D and E) are cells involved in bone matrix synthesis and are responsible for growth and bone remodeling. They synthesize new bone on the external surface of the bone (see figure E), where they are located. Osteoblasts are rounded in shape and contain a well visible nucleus located away from the surface of the bone matrix. These cells are unable to divide and therefore all of them come from bone progenitor cells. As they synthesize extracellular matrix they become completely surrounded by bone matrix and locked up in bone cavities called bone lacunae. Subsequently, they differentiate into osteocytes.
Osteocytes are the most abundant type of cell in the mature bone. They are located in cavities, known as bone lacunae, in the bone matrix. As shown in figures A and B, they look like spiders with long legs. Those legs are channels, called canaliculi, that extend through the bone matrix. Osteocyte processes run inside these channels allowing the communication of nearby osteocytes. Substances from blood vessels can reach all the osteocytes by traveling through this net of canaliculi. The primary role of the osteocytes is the maintenance of bone matrix, by resorption and synthesizing bone matrix. During this activity, osteocytes are able to detect mechanical forces and respond by producing stronger bone. Osteocytes can release calcium from bone matrix and this is how they are involved in body calcium homeostasis.
Osteoclasts are responsible for removing mineralized and organic bone matrix, through a process called resorption. As shown in figures C, D and F, osteoclasts are very large, multinucleated cells. In figure C, it can be observed the waved shape surface of the osteoclast, with numerous and densely packed folds, facing the bone surface. This arrangement of the osteoclast surface appears during bone resorption and contains many enzymes that degrade bone matrix.