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Bone marrow - nonneoplastic

Normal

Monocytes


Reviewers: Dragos Luca, M.D. (see Reviewers page)
Revised: 17 October 2012, last major update September 2012
Copyright: (c) 2001-2012, PathologyOutlines.com, Inc.

General
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● Monocytes usually < 1% of bone marrow cells
● Develop from myeloid stem cell, to monoblast to promonocyte to monocyte (bone marrow) to monocyte (peripheral blood) to macrophage (tissue)
● Same progenitor cell as neutrophils, under the influence of M-CSF
● Difficult to identify monoblasts and promonocytes in normal bone marrow
● Gradual nuclear folding and acquisition of cytoplasmic granules similar to those in neutrophils, but fewer and smaller
● Monocytes are also precursors of dendritic cells
● Produce numerous regulatory cytokines in addition to their traditional phagocytic role
● Part of the innate immune system: replenish macrophages and dendritic cells and move quickly to the site of infection (8-12 hours)
● A significant proportion (~50%) are stored in the spleen; they spend 1-3 days in the bloodstream before moving into tissue

Clinical features
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Causes of monocytosis in bone marrow: reactive / accumulation disorders (high cell turnover states, granulomatous processes, hemophagocytic lymphohistiocytosis, post-chemotherapy, fat necrosis, post cytokine therapy, storage diseases, Rosai-Dorfman disease); neoplastic disorders (Langerhans cell histiocytosis, chronic myelomonocytic leukemia, juvenile myelomonocytic leukemia, myeloproliferative diseases, acute myeloid leukemia, malignant histiocytosis, histiocytic sarcoma)
Causes of monocytopenia in peripheral blood: acute infections, stress, glucocorticoids, aplastic anemia, hairy cell leukemia, acute myeloid leukemia, myelotoxic drugs, genetic syndromes

Micro description
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Monoblast: 12-20 microns, moderate basophilic cytoplasm without granules, may show pseudopod formation, often intense staining on periphery and with perinuclear zone, round / oval nuclei with fine chromatin and 1-4 large prominent nucleoli; nucleus may show indentations or folding
Promonocyte: features intermediate between monoblast and monocyte, more irregular nuclear contour, less basophilic and more granulated cytoplasm
Monocyte: largest of leukocytes (12-20 microns); round with smooth margins or pseudopod-like cytoplasmic extensions; abundant light-blue cytoplasm with fine pink azurophilic granules; may have vacuoles or phagocytized material; large bilobed, kidney shaped or U shaped nucleus with moderately clumped chromatin; no nucleolus; N/C ratio is 65-80%

Micro images
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Monoblast

       
Promonocyte

           
Monocyte

Bone marrow in acute monocytic leukemia (AML M4):

Monoblasts are larger cells with abundant cytoplasm and round nuclei, and promonocytes are two cells with folded and creased nuclei next to monoblast in center

Positive stains
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● Mainly CD14
● Also CD4, CD7, CD11a, CD11b, CD11c, CD11d, CD12, CD13, CD15 (variable), CD15u, CD17, CD18, CD23 (activated), CD29, CD30, CD32, CD33, CD36, CD37, CD38, CD39, CD40, CD43, CD44R, CD45, CD45RB, CD45RC, CD45RO, CD48, CD49a, CD49b, CD49d, CD49e, CD49f, CD51, CD52, CD54, CD61, CD62L, CD64, CD65, CD65s, CD68, CD83 (transient), CD84, CD85A, CD85B, CD85D, CD85E, CD85F, CD85I, CD85J, CD85K, CD85M, CD86, CD87, CD88, CD89, CD91, CD92, CD93, CD97, CD101, CD102, CD105 (activated), CD111, CD112, CD114, CD116, CD122, CD123 (plasmacytoid), CD126, CD127, CD128, CD132, CD137, CD139, CD141, CD148, CD156, CD157, CD163, CD165, CD166 (activated), CD171, CD180, CD210, CD226 and CD227

Negative stains
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● CD24, CD56, CD57, CD60 (usually) and CD231

Electron microscopy images
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Monocyte-TEM and SEM

Additional references
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Wikipedia

End of Bone Marrow - nonneoplastic > Normal > Monocytes


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