Under normal circumstances, the segmented neutrophil is the most common nucleated cell in the peripheral blood. These bacterial-infection-fighting cells are produced in the bone marrow and arise from their precursor cell, the myeloblast.
The myeloblast is the youngest cell in the myeloid lineage. It is approximately 12-20 microns in size with very basophilic cytoplasm. The nucleus takes up around 2/3 of the total cell volume with a soft, finely stranded chromatin with very little clumping. The nucleus is eccentrically placed and ovoid, but can also be slightly flattened. Myeloblasts will typically have two or more nucleoli with well defined nucleolar membranes. In a well-stained preparation, you should be able to observe the outline and blue color of the nucleoli.
The myeloblast's cytoplasm is basophilic and can have a hint of background "ground glass" graininess. This graininess is separate from any primary granules that develop as the cell progresses toward the progranulocyte stage. The cytoplasmic membrane tends to be regular without much denting, bumps, pseudopods, or shredding. Within the cytoplasm of myeloblasts, Auer rods may be present. Auer rods are needle-like cytoplasmic inclusions which result from an abnormal fusion of the primary (azurophilic) granules.
The cell in the first image on the right shows the relative size, nucleus, and gritty basophilic cytoplasm of a classic myeloblast. Note that there is a small cluster of red primary granules present which, in addition to its other features, help to identify this cell as a myeloblast.
The second image shows a myeloblast (blue arrow) at a later stage that is not quite a promyelocyte but is very close. The nucleoli are still prominent, the size has not changed much, and the cytoplasm is still only about 1/3 of the cell. There are a few more primary granules but they are not prominent enough to consider this cell a progranulocyte. (Please note that some laboratories may only count agranular immature cells blasts, and consider immature cells containing primary granules as promyelocytes. Please check your laboratory's procedures and policy.)
While the myeloid sequence tends to be the predominant cell type found in normal bone marrows, myeloblasts should make up less than 5% of the bone marrow's nucleated cells.