The macrophage is the final stage of development in the monocyte lineage. It is a phagocyte whose roles include the removal of dead and dying tissue and the destruction and ingestion of invading organisms. Macrophages (histiocytes) act as immune modulators as they will present antigens from ingested pathogens to helper T-cells.
Their primary role in the bone marrow is the removal of cellular debris, including old red blood cells (RBCs). As a result, they become a source of iron for maturing RBC precursors.
A histiocyte is a less phagocytic form of a macrophage with fewer lysosomal granules. Histiocytes may form clusters, or even fuse together into mulitnucleated giant cells. These giant cells are particularly evident on bone marrow biopsy from a patient with a marrow granuloma.
The top image on the right shows the early transformation of a monocyte into a macrophage (see red arrow). Notice the increase in the amount of cytoplasm present as the cell begins to ingest debris in the bone marrow. This is demonstrated by the increasing vacuolization present in the cytoplasm. The larger the debris ingested, the larger the vacuoles will be.
The lower image on the right shows a macrophage with large vacuoles (red arrow) adjacent to an RBC cluster (blue arrow). This is a common placement, since the macrophage is the iron source for these developing RBCs in the bone marrow.