The arrow in this slide is pointing to a monocyte. The nucleus has an open chromatin pattern which gives it a spongy appearance. There is another monocyte in the lower right corner of the field. The other two cells could be classified as macrophages (histiocytes) because the nucleus 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.