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

100Abnormalities of the White Cell SeriesAcute Monocytic Leukemia (FAB Classification Types M 5 a+b). Two morphologicallydistinct forms of acute monocytic leukemias exist, monoblasticand monocytic. In the monoblastic variant M 5a , blasts predominate in theblood and bone marrow. The blast nuclei show a delicate chromatin structurewith several nucleoli. Often, only the faintly grayish-blue stained cytoplasmhints at their derivation.In monocytic leukemia (type M 5b ), the bone marrow contains promonocytes,which are similar to the blasts in monocytic leukemia, but their nucleiare polymorphic and show ridges and lobes. Some promonocytesshow faintly stained azurophilic granules. The peripheral blood containsmonocytoid cells in different stages of maturation which cannot be distinguishedwith certainty from normal monocytes. Both types are characterizedby strong positive esterase reactions in over 80% of the blasts,whereas the peroxidase reactivity is usually negative, or positive in only afew cells.Acute Erythroleukemia (FAB Classification Type M 6 )Erythroleukemia is a malignant disorder of both cell series. It is suspectedwhen mature granulocytes are virtually absent, but blasts (myeloblasts)are present in addition to nucleated erythrocyte precursors, usuallyerythroblasts (for morphology, see p. 33). The bone marrow is completelyoverwhelmed by myeloblasts and erythroblasts (more than 50% of cells inthe process of erythropoiesis). Bone marrow cytology and cytochemistryconfirm the diagnosis. Sporadically, some cases show granulopenia,erythroblasts, and severely dedifferentiated blasts, which correspond toimmature red cell precursors (proerythroblasts and macroblasts).The differential diagnosis in cases of cytopenia with red blood cell precursorsfound in the CBC must include bone marrow carcinosis, in whichthe bone marrow–blood barrier is destroyed and immature red cells (andsometimes white cells) appear in the bloodstream. Bone marrow cytologyand/or bone marrow histology clarifies the diagnosis. Hemolysis with hypersplenismcan also show this constellation of signs.Fig. 33 Acute leukemia M 5 and M 6. a In monoblastic leukemia M 5 a, blasts with a fine nuclear structure and wide cytoplasm dominate the CBC. b Seemingly maturemonocytes in monocytic leukemia M 5 b. c Homogeneous infiltration of thebone marrow by monoblasts (M 5 a). Only residual granulopoiesis (arrow). d Sameas c but after esterase staining. The stage M 5 a blasts show a clear positive reaction(red stain). There is a nonspecific-esterase (NSE)-negative promyelocyte.