Structure of the ellipsoid sheath in the spleen of the armadillo (Dasypus novemcinctus). A light and electron microscopic study

The spleen of the armadillo is invested by a capsule composed predominantly of smooth muscle. Inward extensions from the capsule contribute to an extensive system of trabeculae of the non‐vascular type. The white pulp presents a typical follicular arrangement and the red pulp is permeated by an anastomosing series of venous sinsuses. The capillary segment of the penicillar artery is invested by an ellipsoid sheath. The spleen of the armadillo may be included in the class of mammalian spleens characterized as sinusal in nature and possessing ellipsoid sheaths. Electron micrographs of thin sections of the sheathed capillaries demonstrate a high‐type of endothelium. A characteristic feature of this endothelium is the presence of a distinct intracytoplasmic fibrous component. The reticular cells comprising the sheath proper exhibit various sized vacuoles and inclusion bodies suggestive of their potentially phagocytic nature. In both light and electron microscopy, venous sinuses lie in close proximity to the peripheral cells of the ellipsoid sheaths. Within the wall of the venous sinuses, patent openings are appearent between the lining cells. If such an arrangement exists in the functional state of the organ, it could provide for an additional means of exposure of both plasma and cellular elements to the adjacent phagocytic cells of the sheath proper.