Bone marrow neoplastic

Bone marrow - neoplastic myeloid

AML not otherwise specified


Topic Completed: 1 February 2013

Minor changes: 27 August 2021

Copyright: 2001-2021,, Inc.

PubMed Search: Acute monoblastic leukemia [title] M5a

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Cite this page: Mihova, D. M5a. website. Accessed December 8th, 2021.
Definition / general
  • Acute monoblastic leukemia (M5a)
  • 5 - 8% of AML
  • Children and young adults
  • 80%+ of monocyte lineage cells are monoblasts
Case reports
  • 66 year old man with erythropoietin dependent transformation of refractory anemia with ringed sideroblasts into acute monoblastic leukemia (Blood 2001;98:3492)
  • 73 year old woman with coexisting mantle cell lymphoma (Leuk Lymphoma 2005;46:1813)
  • 82 year old man with acute monoblastic leukemia following granular lymphocyte proliferative disorder (Rinsho Ketsueki 2011;52:1870)
Microscopic (histologic) description
  • Hypercellular marrow with large number of monoblasts
  • Monoblasts are large with moderately abundant intensely basophilic cytoplasm, variably basophilic and delicate azurophilic granules but no / rare Auer rods
  • May have pseudopods or vacuoles
  • Have round nuclei and lacy chromatin with one or more prominent nucleoli but no folds
  • Promonocytes have abundant less basophilic cytoplasm with obvious azurophilic granules and nuclei have delicate folds
Microscopic (histologic) images

Blood smear (Wright-Giemsa):
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Two monoblasts (left) have abundant cytoplasm with numerous
azurophilic granules, promonocyte (right) has abundant cytoplasm
with fine azurophilic granules, nuclei have delicate folds

Bone marrow smear (Wright-Giemsa):
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Monoblasts are large with abundant cytoplasm
containing numerous fine azurophilic granules,
nuclei lack folds but have slightly
coarse chromatin and 2 - 3 distinct nucleoli

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Some monoblasts also
show pseudopods

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Monoblasts have variable cytoplasm
with mild to intense basophilia,
some vacuoles but no granules

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

Bone marrow biopsy:
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Marrow completely replaced by monoblasts
- large cells with abundant cytoplasm,
round / oval nuclei with prominent nucleoli

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Monoblasts are large with abundant
pale cytoplasm, numerous vacuoles and
prominent pseudopods, nucleoli are distinct

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Monoblasts are large with abundant
cytoplasm, some nuclei are irregular,
many blasts have single prominent nucleoli

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Monoblasts are intensely positive
for nonspecific esterase

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Monoblasts are positive for CD68 #1 (KP-1)

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Buffy coat smear-vacuoles are PAS+

Positive stains
Negative stains
Molecular / cytogenetics description
  • 75% have cytogenetics abnormalities, including 11q23 in 30% (these cases should be classified as a recurrent genetic abnormality)
  • FLT3 mutations in 7%
Electron microscopy images

Images hosted on PathOut server:
Missing Image

Monoblast stained for nonspecific
esterase shows scattered electron
dense deposits indicating enzyme activity

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Cytoplasm contains focal area of glycogen
deposition, also scattered dense granules
and a paranuclear fibrillar array

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