Ultrastructural identification of specialized endocytic compartments in macrophages of the thymic cortico-medullary zone and germinal centers of peripheral lymphatic organs of the rat.

In this study transmission electron microscopy was used to investigate the ultrastructural features of macrophages which are strategically positioned in the thymic cortico-medullary zone and the dark zone of germinal centers of peripheral lymphatic organs in adult Wistar rats. We show that this, morphologically distinct, type of macrophage displays the entire range of cytoplasmic inclusions, which structurally closely correspond to those of endosomal/MHC-II-enriched compartments of antigen presenting cells. The macrophages of the cortico-medullary zone and germinal centers contain numerous multivesicular bodies, as well as various kinds of cytoplasmic inclusions ranging from single to aggregated multivesicular/multilamellar bodies to large vacuoles. These multilamellar inclusions are composed of elongated, irregularly shaped cisternae, with abundance of internal membrane profiles and dense bodies. Often, cortico-medullary zone and germinal center macrophages contain the typical multilamellar bodies. Polysaccharides are detected by the thiocarbohydrazide-silver proteinate method within the dense bodies of these macrophages. The functional significance of cortico-medullary zone and germinal center macrophages is briefly discussed.