Treatment also includes surgical drainage of abscesses and excision of necrotic tissue. Alveolar macrophages are also involved in the phagocytosis of apoptotic and necrotic cells. When prolapsed tissues are edematous and necrotic, amputation is performed. Without nickel, toxic levels of urea accumulate, leading to the formation of necrotic lesions. The method cannot distinguish between necrotic and apoptotic cells. More marked cases are characterized by a foul odor and necrotic infected tissue. Fibrosis is the formation of excess tissue in replacement of necrotic or extensively damaged tissue. Their gross appearance is solid, friable, and yellow, with necrotic and hemorrhagic areas. Attracted to oxygen-starved (hypoxic) and necrotic tumor cells they promote chronic inflammation. Affected tissue is replaced by scarring and cavities filled with caseous necrotic material. The outer leaves are trimmed, and any diseased, damaged, or necrotic leaves are removed. The symptoms are chlorosis and curling of the affected tissues with necrotic spots. Chemical enzymes are fast acting products that slough off necrotic tissue. They feed on the dead or necrotic tissue, leaving sound tissue largely unharmed. Myonecrosis is a condition of necrotic damage, specific to muscle tissue.