A more complex and recent representation of the bone remodeling cycle adds a reversal phase between the resorption and formation phases (Figure ). Before osteoblasts rebuild bone, it is suggested that the resorbed bone cavity is cleared of debris by reversal cells, which are currently not considered part of the BMU . The origin of these cells is unclear, but they express markers of osteoblastic lineage . Bone lining cells are another cell not canonically considered part of the BMU. However, osteoblasts can also become bone lining cells at the end of the bone formation phase, forming a protective layer on the bone surface that prevents osteoclasts from interacting with bone where remodeling should not occur .