The mature macrophage also derives from primitive stem cells in the bone marrow, but unlike the lymphocyte, it matures in the tissues. Thus monocytes, the precursors of mature macrophages, circulate for only a few hours before entering the tissues where they live for months as mature macrophages. There is great variety in the tissue macrophages; they are heterogenous in appearance and metabolism. They include mobile alveolar and peritoneal macrophages. There are also fixed cells in the liver called Kupffer cells and skin macrophages called Langerhans cells.