Microcirculation in rat spleen (sinusal), studied by means of corrosion casts, with particular reference to the intermediate pathways.

Blood vessels and their connections in rat spleen were traced over considerable distances by scanning electron microscopy of microcorrosion casts prepared by injection of minimal amounts of casting material. The periarterial lymphatic sheath and lymphatic nodules are highly developed, containing an abundance of capillaries which terminate in the marginal sinus (MS) and marginal zone (MZ). The MS, which consists of a series of discontinuous flattened vascular spaces interconnected by short capillaries, shows circumferential filling followed by flow radially outward into the MZ. Contrary to the generally accepted view, many venous sinuses begin as open-ended tubes at the MS or MZ, allowing free entry of blood into the venous system, thereby bypassing the reticular meshwork of the red pulp. The majority of arterial capillaries terminate in the reticular meshwork ("open" circulation), but evidence for direct connections between capillaries and venous sinuses ("closed" circulation) was also obtained. Casts of the subcapsular region reveal an elaborate system of venous sinuses in fan-shaped arrays, superimposed on an extensive network of capillaries draining into flattened reticular spaces; such casts provide a three-dimensional map useful in interpreting light microscopic observations of red cell flow in vivo. Sphincter-like constrictions in venous sinuses, at points of connection with larger sinuses, indicate that these are possible sites for control of intrasplenic flow distribution.