Another example of a cell rarely seen in the bone marrow is an osteoblast. Osteoblasts are cells which are similar in appearance but somewhat larger than plasma cells or tumor cells. The nucleus is eccentric and the "hoff" area is sometimes located away from the nucleus. The cytoplasm appears rather foamy when compared to a plasma cell. The size of an osteoblast is 20-25 microns. Osteoblast produce bone.