The arrow in this slide is pointing to a monocyte. The nucleus has an open chromatin pattern which gives it a spongy appearance. The other two nucleated cells could be classified as macrophages (histiocytes) because the nucleus in each cell is oval or kidney bean-shaped and the cytoplasm is very irregular. After circulating in the blood for one to three days, monocytes enter the tissues. The tissue form of the monocyte is called a macrophage or histiocyte.