Examine a smear made from a bone marrow aspirate. Focus on a region near the edge of the preparation where the smear is thinner and cell morphology is relatively well preserved. Note that most of the cells present are mature red and white blood cells. The empty circular areas are spaces left by adipocytes that were disrupted during preparation of the smear. Identifying developing blood cells can be particularly challenging. However, spend some time looking at these cells, focusing mainly on identifying cells in the later stages of development when they begin to acquire features of their mature forms. Also, remember that blood cells develop along a continuum, so their histological features are gradually changing during each stage.

Identify megakaryocytes, which are distinctive because of their large size (up to 150 microns in diameter) and very large, polyploid nuclei. Megakaryocytes produce platelets, which are released from long cytoplasmic extensions as small, round anucleated fragments.

Look for erythrocyte precursors including:

• Proerythroblasts (Pe), which are large cells with lacy chromatin and basophilic cytoplasm
• Basophilic erythroblasts (Be), which are slightly smaller cells with a more condensed nucleus and basophilic cytoplasm
• Polychromatophilic erythroblasts (Pc), which contain regions of basophilia and eosinophilia as hemoglobin is produced
• Orthochromatophilic erythroblast (Oe, normoblasts) with condensing nuclei and increasingly eosinophilic cytoplasm
• Reticulocytes (R) lack nuclei and their cytoplasm transitions from lightly basophilic to strongly eosinophilic

Look for granulocyte precursors (neutrophil (N), eosinophil (E), and basophil (B) precursors), which can be identified based on their emerging bilobed or segmented nuclear morphologies and the appearance of their specific granules:

• Myelocytes (M)
• Metamyelocytes (Mm)
• Band cells (sometimes referred to as stab cells) (Bc)

Clinical note: Leukemias are malignancies arising from the hematopoietic precursors of leukocytes and are classified as lymphocytic leukemia or myelogenous leukemia, depending on whether lymphocytes or granulocytes are involved.
 

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