Primary Hematopoietic and Lymphoid Tissue ›› Bone Marrow ›› Normal

Monocyte*






Microscopic Features:
  • 3-4x larger than a mature RBC
  • Lower nuclear to cytoplasmic ratio because of increased cytoplasm
  • Kidney shaped Indented Nucleus with mature chromatin (clumped)
  • Nucleoli are absent
  • Cytoplasm is abundant, gray to pale blue and with rare to no granules
  • vacuoles are commonly noted in cytoplasm
Normal % blood-PB, marrow-BM, lymphoid tissue-LN:
  • PB: Monocytes are the 3rd most common WBC in the blood (after neutrophils and lymphocytes)
  • BM: Scattered monocytes can be seen in the bone marrow (some in the form of histiocytes/macrophages)
  • LN: Scattered monocytes can be seen in the bone marrow (some in the form of histiocytes/macrophages)
May Resemble:
  • Reactive lymphocyte
  • Myelocyte
  • Metamyelocyte
  • Band Neutrophil
  • Promonocyte
  • Monoblast
Differential Diagnoses:

If increased in blood or increased numbers in bone marrow:
Chronic Myelomonocytic Leukemia
Autoimmune disorders (sometimes)
Chronic infections (e.g. CMV, tuberculosis, etc.)

Classic Immunophenotype:
  • CD45+
  • Intermediate SSC (Side light scatter)
  • CD14+
  • CD4dim+
  • CD64+
Cartoon Image:



Misc:
  • Monocytes leave the blood and enter tissue to become macrophages/histiocytes