Myelocytes

MYELOCYTES, immature granular forms of leukocytes that, via metamyelocytes [see separate table (vol. XIV, pp. 531-532), Fig. 5], subsequently develop into mature leukocytes with a segmented nucleus [see separate table (vol. XV, pp. 463-464), Fig. 3, 1]. Size 12-20 µ; they have a large, round, oval or indented, vesicular nucleus rich in chromatin. 2-4 nucleoli are visible in the latter. According to the character of the granulation contained in the protoplasm, neutrophil, eosinophil, and basophil myelocytes are distinguished. When stained by Pappenheim's method, the nucleus of the myelocyte stains violet-red, and in the protoplasm of the neutrophil myelocyte a large number of small, less frequently large, pink-violet-brown granules are discovered; in eosinophil myelocytes the granules are larger, yellowish-red, less frequently dark-red; basophil myelocytes are smaller in size, with violet-black-blue granules. Myelocytes give a positive oxidase reaction. According to the opinion of the majority of researchers, under physiological conditions myelocytes are located only in the bone marrow and are completely absent in the peripheral blood. Myelocytes appear in the latter in various diseases proceeding with leukocytosis; their presence indicates irritation and increased activity of the bone marrow. There are especially many myelocytes in the blood in myeloid leukemia [see separate table: (vol. XV, pp. 515-516), Figs. 1-4a]; in this condition, mitotic figures and degenerative forms are encountered among them. The true transitional stage from the myelocyte to the mature neutrophil is the so-called Pappenheim metamyelocyte, introduced by him in contrast to Ehrlich's transitional forms, which, as has become clear thanks to improved staining methods, contain no specific granulation. Just as in myelocytes, three kinds of metamyelocytes are distinguished according to their granulation: neutrophil, eosinophil, and basophil. The metamyelocyte differs from mature segmented forms only in that its nucleus has not yet divided into segments and has a horseshoe or rod-like shape. They are encountered in various quantities during a shift of the leukocyte blood formula (see) to the left. Lit.—see the corresponding chapters of the main manuals cited in the literature to the article Hematology (especially by Kryukov, Naegeli, Pappenheim, and Schittenhelm).