Color Atlas of Hematology - Practical Microscopic and Clinical ...

34Normal Cells of the Blood and Hematopoietic OrgansImmature White Cell Precursors: Myeloblasts andPromyelocytesMyeloblasts are the least mature cells in the granulocyte lineage. Mononuclear,round-to-ovoid cells, they may be distinguished from proerythroblastsby the finer, “grainy” reticular structure of their nuclei and thefaintly basophilic cytoplasm. On first impression, they may look like largeor even small lymphocytes (micromyeloblasts), but the delicate structureof their nuclei always gives them away as myeloblasts. In some areas, condensedchromatin may start to look like nucleoli. Sporadically, the cytoplasmcontains azurophilic granules.Promyelocytes are the product of myeloblast division, and usually growlarger than their progenitor cells. During maturation, their nuclei show anincreasingly coarse chromatin structure. The nucleus is eccentric; thelighter zone over its bay-like indentation corresponds to the Golgi apparatus.The wide layer of basophilic cytoplasm contains copious largeazurophilic granules containing peroxidases, hydrolases, and otherenzymes. These granulations also exist scattered all around the nucleus, asmay be seen by focusing on different planes of the preparation using themicrometer adjustment on the microscope.Diagnostic Implications. Ordinarily, both cell types are encountered onlyin the bone marrow, where they are the most actively dividing cells andmain progenitors of granulocytes. In times of increased granulocyte production,promyelocytes and (in rare cases) myeloblasts may be releasedinto the blood stream (pathological left shift, see p. 112). Under strong regenerationpressure from the erythrocyte series, too—e.g., during thecompensation phase following various anemias—immature white cellprecursors, like the red cell precursors, may be swept into the peripheralblood. Bone marrow involvement by tumor metastases also increases thepermeability of the blood–bone marrow barrier for immature white cellprecursors (for an overview, see p. 112 ff.).In some acute forms of leukemia, myeloblasts (and also, rarely, promyelocytes)dominate the blood analysis (p. 97).