Promyelocytes are generally larger than myeloblasts, measuring approximately 12–20 µm. The nucleus is similar in size to the myeloblast but the cytoplasm is more abundant at this stage. The nucleoli will begin to close and become less prominent than in the blast stage. The chromatin strand texture in promyelocytes tends to become slightly more coarse and clumped than the chromatin pattern present in a myeloblast.
Promyelocyte cytoplasm will have a gritty basophilic color and texture; however, there will also be prominent primary granules. These granules will look like red/purple grains of sand. With careful observation, one can note the cuboid nature of the granules.
In the top image to the right, notice the size of the promyelocyte on the right-hand edge (red arrow) versus the other myeloid cells in the frame. Notice how basophilic the cytoplasm is compared to the more mature myelocytes that are present. Observe the prominent, red primary granules, which stand out against the basophilic background.
In the bottom image on the right, the promyelocyte (blue arrow) has matured a bit more, giving it an appearance closer to an early myelocyte. Though the overall size of the cell has not decreased noticeably (as a general rule for what happens as blood cells mature), the depth of the basophilia is not as prominent, nor are the primary granules as obvious as they were in the cell shown in the top image. While the nucleoli are obvious in both cells, the chromatin texture in the cell indicated by the arrow in the bottom image is a bit more clumped and coarse. Also, notice the clearing/ lighter color in the Golgi (perinuclear) zone of the bottom cell (indicated by the green arrow). This is where the first development of neutrophil secondary granules will become evident as the cell progresses to the next stage of maturation.
