Osteoclasts are the cells responsible for bone resorption. They work in conjunction with osteoblasts. Under normal circumstances, both cells are constantly in the process of rebuilding/reshaping/repairing bone to ensure strength and function.
Osteoclasts are only infrequently seen in bone marrow aspirates. They become more obvious when the cellularity is depressed.
Osteoclasts are large multinucleate cells somewhat similar in appearance to megakaryocytes, which can cause confusion. Notice in the images to the right that the nuclei of the osteoclasts are flat, even in number, oval or round, uniform in size, and well separated in the cytoplasm. In contrast, megakaryocyte nuclei are segmented and clump in three dimensional clusters. The cytoplasm of an osteoclast is grainy and paler in color than a megakaryocyte. Observe how fluid and irregular the cytoplasmic borders are in the osteoclast.