The mature cells of bone are called:

The mature bone cell is called an osteocyte. It sits in a space, called a lacuna. An osteoblast is an immature bone cell. The osteoblast is the bone forming cell.

Osteocyte

Explanation:

Osteocyte, a cell that lies within the substance of fully formed bone. It occupies a small chamber called a lacuna, which is contained in the calcified matrix of bone. Osteocytes derive from osteoblasts, or bone-forming cells, and are essentially osteoblasts surrounded by the products they secreted.