Primary Hematopoietic and Lymphoid Tissue ›› Bone Marrow ›› Normal Monocyte* Click Here for Full Size Additional Image 1 › 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: Click and drag for direct comparison › Misc: Monocytes leave the blood and enter tissue to become macrophages/histiocytes