Direct arteriovenous connections and the intermediate circulation in dog spleen, studied by scanning electron microscopy of microcorrosion casts.

The exact nature of the circulatory pathways in dog spleen, particularly with reference to the intermediate circulation and the possible existence of direct arteriovenous connections, has been studied by scanning electron microscopy (SEM) of microcorrosion casts. A new casting procedure was developed in which minimal amounts of material were injected into contracted spleens, thus filling preferentially the faster channels for flow. Extensive filling of the red pulp was thereby avoided, leaving an open view of blood vessels and their connections. The depth of focus of the SEM, incomparably greater than those of transmission electron or light microscopes, enabled vascular pathways to be traced over considerable distances. Using this approach, we have obtained clear evidence for abundant connections between arterial capillaries and venous sinuses (i.e., "closed" circulation). Typically, the terminal arteriole bifurcates repeatedly, in quick succession, giving rise to as many as twelve short capillaries, each leading directly to at least one sinus. However, an "open" circulation also exists, inasmuch as the majority of all capillaries end in the marginal zone around lymphatic nodules. In the dilated spleen, direct connections to sinuses are rarely visible but endings in the red pulp are found, in addition to those going to the marginal zone.