Description of a new population of fixed macrophages in the splenic cords of pigs.

The ultrastructure of porcine splenic cords was analysed by light and electron microscopy after both perfusion and immersion fixation. The external aspect of the splenic cords was found to be composed of a network of smooth muscle cells arranged between the trabeculae and the venous sinus walls. Macrophages frequently attached to smooth muscle cells surrounded the whole of their surfaces and were attached by slender cytoplasmic projections. Intercellular junctions in the form of electron-dense areas 200-350 nm in length and 45-55 nm in thickness were observed in the macrophage membrane close to the muscle cells. In the splenic cords of nonperfused animals, in addition to the macrophage population adhering to smooth muscle cells, numerous macrophages without cell junctions were also seen and identified as free macrophages.