6 CAP TODAY | JANUARY 2018 Monocyte, Immature (Promonocyte, Monoblast) Here’s a look inside the new Color Atlas lated or has delicate folding or creasing of the SYNONYMS of Hematology: An Illustrated Field Guide nuclear membrane. Nucleoli are present but often Monoblast, promonocyte Based on Proficiency Testing, from the sec- not as distinct as in a monoblast. Nuclear chro­ tion on granulocytic and monocytic cells. matin is more immature and delicate than that of VITAL STATISTICS Immature monocytes are usually not seen in any a mature monocyte. The cytoplasm contains Size ������������������������15-25 µm N:C ratio ��������������7:1 to 3:1 significant number except in malignancies in­ uniformly distributed, fine, azurophilic granules Cell shape �����������round to oval volving the monocytic cell line. The monoblast with generally few cytoplasmic vacuoles. Nuclear shape ���round or indented is a large cell with relatively more cytoplasm than Some monoblasts cannot be distinguished Chromatin ����������reticular, lacy a myeloblast. The monoblast nucleus is round or morphologically from other blast forms, hence Nucleoli ���������������one to two present Cytoplasm ����������gray to cloudy blue, few red granules oval and has finely dispersed chromatin with the need for using other means (e.g., cytochem­ distinct nucleoli. The cytoplasm is blue to gray- istry and flow cytometry) before assigning a KEY DIFFERENTIATING FEATURES blue and may contain small, scattered azurophilic particular lineage to a blast cell. For purposes of High N:C ratio, nucleoli, indented nucleus granules, but Auer rods are rare. Monoblasts proficiency testing, selection of the response Enzyme cytochemical stain: nonspecific esterase positive differentiate into promonocytes (see A Closer “monocyte, immature (promonocyte, mono­ Immunophenotype: CD4, HLA-DR, CD13, CD33, Look At . . . Life Cycle of Monocytes); promono­ blast)” should be reserved for malignant cells in CD36, CD38, CD64, CD11b, CD45 positive; cytes are considered to be monoblast equivalents myeloid neoplasms (see A Closer Look ­variable CD14, CD15 for classification determination. Promonocytes At . . . Acute Myeloid Leukemia and Related Neo- have nuclear and cytoplasmic characteristics that plasms). A spectrum of immature, non-neoplastic OTHER FEATURES May appear as undefined “blast cell” are between those of monoblasts and mature monocytes may circulate in small numbers in Vacuoles may be seen in cytoplasm monocytes. The nucleus is more irregular than reactive conditions, including cells with enlarged that of a monoblast and is often indented or lobu­ nuclei and small nucleoli. POTENTIAL LOOK-ALIKES Other blasts (myeloblast, lymphoblast, megakaryo- Monoblast Promonocyte blast) Blastic plasmacytoid dendritic cell neoplasm Natural killer cell Mature monocyte Dysplastic myeloid or erythroid precursors nucleoli Atypical lymphocyte (i.e., Epstein-Barr virus infection, rarely) Lymphoma cell or prolymphocyte coarse nuclear chromatin Carcinoma

ASSOCIATED DISEASE STATES AND CONDITIONS Acute monocytic/monoblastic leukemia Acute myelomonocytic leukemia normal RBC Chronic myelomonocytic leukemia gray-blue cytoplasm containing a few red granules Myelodysplastic/myeloproliferative neoplasm lacy chromatin and prominent nucleolus 7 µm

BCK/BCP-17, 2002 (Blood, WG, X400) EHE1-09, 2013 (Blood, WG, X400) BCK/BCP-14/XHP-20, 2009 (Blood, WG, X360) Identification Referee % Participant % Identification Referee % Participant % Identification Referee % Participant % Monocyte, immature 80.0 71.1 Monocyte, immature - 47.0 Immature monocyte 66.7 59.7 Monocyte 10.0 14.8 Neutrophil, promyelocyte - 13.7 Monoblast 28.2 17.5 Neutrophil, myelocyte - 13.1 A promonocyte is typically large like a monoblast with The cell indicated by the arrow is an immature mono- Nucleated red cell, normal or abn. - 10.7 a moderate amount of cytoplasm. The chromatin con- cyte. These cells differ from mature monocytes in that tains a similar fine pattern; however, the nucleus dem- The arrowed cells are immature monocytes, which in- their chromatin pattern is finer than the clumped pat- onstrates a delicate folded or grooved appearance. The clude monoblasts and promonocytes. These cells may tern of a mature monocyte. Also the nucleus lacks the nucleolus is faint to indistinct. The cytoplasmic granu- have overlapping morphologic features. The arrowed more prominent indentation of a mature monocyte and lation is also apparent. Occasional vacuoles may be cells in this image have round to oval nuclear contours nucleoli are present. There is a high N:C ratio and the present, but this is not a typical feature. These cells are with fine, lacy chromatin and distinct nucleoli, best re- cytoplasm is gray–blue. The other nucleated cells in the part of a leukemic population and therefore should not sembling monoblasts. Monoblasts are usually large field have similar nuclear and cytoplasmic character- be classified as benign monocytes. It can sometimes and may appear as a typical myeloblast or may have istics and are also examples of immature monocytes. be difficult to distinguish benign from leukemic mono- increased basophilic cytoplasm and lower N:C ratios, cytes in peripheral blood. In this instance, the leukemic as seen in this image. monocytes have morphologically distinctive immature features. Virtual Peripheral Blood Smear http://capatholo.gy/H16_151-09-1