Learning Objectives
- Identify the morphological features of intermediate myelocytes (eosinophilic and basophilic).
- Recognize the histological appearance of a plasma cell in the bone marrow.
- Understand the maturation stages of the granulocytic cell line.
- Distinguish between primary and secondary (specific) granules.
Overview of Marrow Precursors
In a normal bone marrow smear at high magnification, we can observe specific stages of leukocyte maturation. At the myelocyte stage, cells begin to develop the specific granules that allow us to differentiate between neutrophil, eosinophil, and basophil lineages.
Characteristic Features
- Eosinophilic Myelocyte: A mid-stage precursor showing a round to oval nucleus and a cytoplasm filled with reddish-orange specific granules.
- Basophilic Myelocyte: A precursor containing large, dark purple-black granules. These are less common than neutrophilic or eosinophilic precursors.
- Plasma Cell: An oval cell with an eccentric nucleus (pushed to one side) and a “clock-face” or “wheel-spoke” chromatin pattern. It typically features a prominent perinuclear clear zone (the Golgi apparatus).
The Myelocyte Stage
- Definition: The myelocyte is the first stage in granulopoiesis where specific (secondary) granules appear, making the cell type identifiable.
- Proliferation: This is the last stage of the granulocytic series capable of mitosis; later stages (metamyelocytes, bands) are purely maturing cells.
Clinical Relevance: Specialized Cells
- Multiple Myeloma: A malignancy characterized by the uncontrolled proliferation of plasma cells in the bone marrow, often exceeding 10% of total marrow cellularity.
- Myelocyte “Bulge”: An increase in myelocytes can be seen in Chronic Myeloid Leukemia (CML) or during recovery from bone marrow suppression (the “rebound” effect).
- Russell Bodies: In some inflammatory states, plasma cells may contain large, pink, globular inclusions of immunoglobulin known as Russell bodies.
