The study by Van Furth and Cohn investigates the origin and turnover of mononuclear phagocytes in mice. Mononuclear phagocytes include circulating monocytes and tissue macrophages in organs such as the spleen, liver, lungs, and peritoneum. Tissue macrophages are involved in clearing bacteria and foreign materials, while the function of monocytes in circulation is less understood. The study shows that monocytes can differentiate into tissue macrophages in vitro and in vivo. The aim was to determine the relationship between free and fixed mononuclear phagocytes under normal conditions. The study used labeled thymidine to track the turnover of monocytes and macrophages. Peritoneal macrophages and blood monocytes were labeled in vivo and in vitro. Results showed that peritoneal macrophages have a low turnover rate, while blood monocytes have a higher turnover. The study also found that monocytes can rapidly enter the peritoneal cavity after an inflammatory response. Splenectomy and X-irradiation experiments indicated that the spleen is not a major source of monocytes, and that monocytes originate from the bone marrow. Bone marrow cultures showed that mononuclear phagocytes in the bone marrow are rapidly dividing cells, and that promonocytes in the bone marrow are the progenitor cells of monocytes. The study also found that monocytes leave the circulation randomly, with a half-life of 22 hours. The average blood transit time of monocytes was calculated to be 32 hours. The turnover rate of peritoneal macrophages was low, estimated at about 0.1% per hour. The study concluded that the life history of mouse mononuclear phagocytes is: promonocytes in the bone marrow → monocytes in the peripheral blood → macrophages in the tissue. The study was supported by grants from the Netherlands organization for the advancement of pure research and the United States Public Health Service. The findings suggest that monocytes give rise to peritoneal macrophages, and that mononuclear phagocytes do not originate from lymphocytes.The study by Van Furth and Cohn investigates the origin and turnover of mononuclear phagocytes in mice. Mononuclear phagocytes include circulating monocytes and tissue macrophages in organs such as the spleen, liver, lungs, and peritoneum. Tissue macrophages are involved in clearing bacteria and foreign materials, while the function of monocytes in circulation is less understood. The study shows that monocytes can differentiate into tissue macrophages in vitro and in vivo. The aim was to determine the relationship between free and fixed mononuclear phagocytes under normal conditions. The study used labeled thymidine to track the turnover of monocytes and macrophages. Peritoneal macrophages and blood monocytes were labeled in vivo and in vitro. Results showed that peritoneal macrophages have a low turnover rate, while blood monocytes have a higher turnover. The study also found that monocytes can rapidly enter the peritoneal cavity after an inflammatory response. Splenectomy and X-irradiation experiments indicated that the spleen is not a major source of monocytes, and that monocytes originate from the bone marrow. Bone marrow cultures showed that mononuclear phagocytes in the bone marrow are rapidly dividing cells, and that promonocytes in the bone marrow are the progenitor cells of monocytes. The study also found that monocytes leave the circulation randomly, with a half-life of 22 hours. The average blood transit time of monocytes was calculated to be 32 hours. The turnover rate of peritoneal macrophages was low, estimated at about 0.1% per hour. The study concluded that the life history of mouse mononuclear phagocytes is: promonocytes in the bone marrow → monocytes in the peripheral blood → macrophages in the tissue. The study was supported by grants from the Netherlands organization for the advancement of pure research and the United States Public Health Service. The findings suggest that monocytes give rise to peritoneal macrophages, and that mononuclear phagocytes do not originate from lymphocytes.