As a result of radiotherapy to the mouth, radiation-induced stomatitis may develop, which can be associated with mucosal erosions and ulceration. If the salivary glands are irradiated, there may also be xerostomia (dry mouth), making the oral mucosa more vulnerable to frictional damage as the lubricating function of saliva is lost, and mucosal atrophy (thinning), which makes a breach of the epithelium more likely. Radiation to the bones of the jaws causes damage to osteocytes and impairs the blood supply. The affected hard tissues become hypovascular (reduced number of blood vessels), hypocellular (reduced number of cells), and hypoxic (low levels of oxygen). Osteoradionecrosis is the term for when such an area of irradiated bone does not heal from this damage. This usually occurs in the mandible, and causes chronic pain and surface ulceration, sometimes resulting in non-healing bone being exposed through a soft tissue defect. Prevention of osteradionecrosis is part of the reason why all teeth of questionable prognosis are removed before the start of a course of radiotherapy.