Osteoblasts are the cells responsible for the production and deposition of bone. They may not be apparent in normal cellular bone marrow, since they appear in low frequency. In situations where the total bone marrow cellularity is decreased, they become more visible.
Osteoblasts are individual cells but tend to travel in small groups or clusters. They are quite large compared to the normal background blood cells and resemble giant plasma cells. They are oval-shaped cells and tend to have quite basophilic cytoplasm. An osteoblast has a single round nucleus with a fairly open chromatin texture.
Notice in the images to the right how the nucleus of the osteoblast is eccentrically placed. On some smears it will almost appear as if the nuclei are in the process of being extruded from the cells. This effect is more commonly seen on extremely hypocellular bone marrows and is less pronounced in bone marrows with a higher cellularity. Notice the large size of the osteoblasts in comparison to the background bone marrow elements.