Bone marrow nonneoplastic



Last author update: 1 September 2012
Last staff update: 14 May 2021

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PubMed Search: Bone marrow [title] monocytes [title]

Dragos C. Luca, M.D.
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Cite this page: Luca DC. Monocytes. website. Accessed June 3rd, 2023.
Definition / general
  • 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
  • Causes of monocytosis in bone marrow: reactive / accumulation disorders (high cell turnover states, granulomatous processes, hemophagocytic lymphohistiocytosis, postchemotherapy, 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
Microscopic (histologic) description
  • 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%
Microscopic (histologic) images

AFIP images


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

Images hosted on other servers:


Positive stains
Negative stains
Electron microscopy images

Images hosted on other servers:

Monocyte - TEM and SEM

Additional references
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