According to Encyclopædia Britannica, a mature bone cell is called an osteocyte. These cells, which are found in fully formed bone, develop from the bone-forming cells known as osteoblasts.
An osteocyte is, in the simplest terms, an osteoblast with the products it secreted enveloping it. The secreted products eventually calcify, forming mature bone and trapping the cell. The space the cell is trapped in is known as a lacuna. Encyclopædia Britannica states that osteocytes are involved in bone deposition, remodeling and resorption. Osteocytes can also help remove calcium from bone if calcium levels in the body drop to low levels.