Ultrastructural study of macrophages in the rat thymus, with special reference to the cortico-medullary zone.

Electron microscopic study of the normal rat thymus has demonstrated that macrophages with different ultrastructural features are positioned in the thymic cortex, in the cortico-medullary zone and in the medulla. Phagocytic cells, containing necrotic lymphocytes in various stages of degradation, are distributed throughout the thymic cortex. The cortico-medullary zone, in contrast, is populated with macrophages displaying specific ultrastructural features. These cells contain numerous vacuolar inclusions of different size, filled with homogeneous, flocculent material of very low electron density. The dense bodies, occasionally positioned to the inner side of the vacuolar membrane, selectively contrast with silver methenamine and contain polysaccharides, as demonstrated by the thiocarbohydrazide-silver proteinate method. Very rarely, these cells contain phagocytosed lymphocyte remnants. The predominant type of mononuclear phagocytic cells in the thymic medulla are the interdigitating cells.