Connective tissue macrophages (histiocytes) belong to the group of free-floating (migrating) phagocytes. Their protective function includes phagocytosis and intracellular digestion of absorbed substances, as well as presenting antigenic data to immunocompetent cells. They also exhibit an upregulating action concerning other cells of the defence system in the body. The ultrastructure of macrophages differs depending on their functional activity.

TEM image. 4,000X. During phagocytosis, phagosomes #1, #2 are formed on the surface of macrophages. These are vacuoles with dense particles. Also, pinocytic vacuoles #1, #2 with a liquid content may be formed. The cytoplasm of macrophages contains mitochondria, short cisterns of rough endoplasmic reticulum, Golgi apparatus, and many lysosomes (primary lysosomes). You may also see a lipid droplet and a centriole of the cytocenter.

TEM image, 3,000X. Intracellular digestion in a macrophage occurs in phagolysosomes #1, #2, #3, #4 (secondary lysosomes, heterophagosomes, digestive vacuoles). They are formed upon fusoin of primary lysosomes #1, #2 and phagocytic or pinocytic vacuoles. Hydrolytic products of organic substances are transferred as monomers via the phagolysosome membrane into the cytoplasm for recycling. If a phagolysosome accumulates undigested products, it becomes a residual body (telolysosome).
The cytoplasm of the macrophage contains mitochondria, cisterns of endoplasmic reticulum, lysosomes. There are an idle fibroblast and a bundle section of collagen fibrils near the macrophage.

TEM image, 3,000X. In an inflammatory area, macrophages actively devour cell debris. These macrophages can contain a lot of phagolysosomes #1, #2, #3, #4 and residual bodies #1, #2 of different shape and structure. In such a macrophage the functional activity is often maintained at a high level. this is indicated by a presence of large nucleolus and euchromatin in the nucleus, as well as of primary lysosomes and lipid inclusions( in the cytoplasm, and numerous projections and invaginations on cell surface.