They are formed by the fusion of several macrophages. These cells are commonly found, together with other leukocytes, in a type of chronic inflammatory response called a granuloma.
Granulomas are formed in several diseases produced by microorganisms that are difficult for phagocytic cells to eliminate.Examples are tuberculosis and leprosy. They enclose structures that are not easily digested by phagocytes, such as cellulose fibers, mineral particles and certain bacteria and protozoa.
The figure shows a granuloma that developed after a cotton thread, composed mainly of cellulose fibers, was inserted into the skin of a laboratory animal. The granuloma contains numerous multinucleated giant cells surrounding the cellulose fibers of the thread.
The cellulose fibers are not stained because they do not take up the hematoxylin and eosin (H&E) dyes used to prepare the section. They therefore appear as unstained or very pale structures.
