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Myelocytes: Eosinophils and Basophils

As stated previously, the myelocyte stage is the first stage where cell-specific maturation appears in the granulocyte lineage. When you look at a promyelocyte, you cannot yet identify if it will mature into a neutrophil, eosinophil, or basophil. It is only when secondary granules are produced that the endpoint of maturation can be identified. The size and shape of these initial secondary granules help with this identification. While the cytoplasm and granules may look different depending on the lineage of the cell, the progression of nuclear maturation is the same for all granulocytes.

When eosinophil secondary granules are first produced, they may not show the same bright orange color found in the mature eosinophil. It is the larger, spherical shape of the granules that identifies the early eosinophil myelocyte, not necessarily the eosinophilic color. In fact, early eosinophil granules may appear somewhat basophilic in color. The larger, three-dimensional, spherical shapes of the granules help to identify cells as eosinophil precursors (see red arrows in top image).

Immature basophils (see blue arrow in lower image) can be hard to distinguish from promyelocytes as well. It is important to note the differences in color and shape between primary neutrophil granules and basophil secondary granules. The primary granules in promyelocytes appear as red/purple grains of sand - think of red/purple cubes with defined edges. Basophil granules have a purple /black color and look more like splinters - think of a purple/black sheet of glass that is shattered into the cytoplasm of a cell. The cytoplasmic color in an early basophil myelocyte may be similar to a promyelocyte since there are no fine granules in the basophilic cytoplasm. Remember that mature basophils are not as fully granulated as neutrophils and have a clear/uncolored cytoplasmic background rather than the pink/tan background of neutrophils.