Monocytes progress through maturational stages in a similar fashion to the myeloid series before entering the peripheral blood circulation. The final stage of monocyte maturation into macrophages occurs after they have migrated out of the peripheral blood and into the surrounding tissues via diapedesis. Mature macrophages are also found in the bone marrow. The monocyte lineage does not maintain a maturational pool in the bone marrow as large as the myeloid pool. As a result, the monoblast stage is infrequently noted in most normal bone marrows.
Monoblasts are the largest blasts of all the hematopoeitic cell lines present in the bone marrow. They have a large, round, centrally-placed nucleus with soft, fine-stranded chromatin. They normally have a single, large, prominent nucleolus. The cytoplasm is very generous and has a fine, grainy texture. In the monoblast stage, the cytoplasm will be basophilic, similar to other blasts, but will possess a slightly lighter shade of blue. In the monoblast, the color will shift to blue-gray as the cell matures into a monocyte.
The top image on the right shows a single monoblast. Notice the large, round nucleus, the single large nucleolus and the generous blue, grainy cytoplasm.
The second image shows a group of monocyte precursors. The large cell at the top is a monoblast (see red arrow). Notice the round and flat look of the nucleus in the blast compared to the other stages. Observe the nuclear shape becoming more folded and three-dimensional as the cell matures.