leukocyte monocyte A mononuclear phagocytic leukocyte, 13 to 25 mm in diameter, with an ovoid or kidney-shaped nucleus, containing lacy, linear chromatin and abundant gray-blue cytoplasm filled with fine reddish and azurophilic granules. Formed in the bone marrow from promonocytes, monocytes are transported to tissues such as the lung and liver, where they develop into macrophages. BTO:0000876 BrendaTissueOBO mononuclear leukocyte nongranular leukocyte mononuclear phagocyte