Ultrastructural studies on splenic microcirculation of the rat.
In an attempt to establish or supplement the three dimensional features of splenic microvasculature vascular corrosion casts and freeze-cracked tissue blocks of the rat spleen were studied by electron microscopy (SEM & TEM) with special reference on the filtration mechanism of blood cells (RBCs) in the spleen. Both open and closed circulation were observed. TEM investigation showed that macrophages were often found to be located outside the abluminal surface and between two venous sinus branches. Some of them occasionally extend their filopodia and emerge through intercellular slits (IES) out to the luminal surface of the venous sinus. In many cases discontinuity of the junctional complex of the endothelial cells is occasionally identified; this may behave as a detour to prolong RBC passage through the reticular space, thus facilitating their contact with phagocytic cells. SEM observation on vascular corrosion casts have provided clear evidence of direct arteriovenous connection in the rat spleen. Observation on the freeze-cracked tissue block showed a consistent evidence that the macrophages occasionally extend their filopodia and emerge through IES out to the luminal surface of the venous sinus. It indicated that a great majority of the RBCs passing through meshwork of the reticular space of the red pulp, i.e., open circulation would be detected by the macrophages dwelling in the reticular space. The RBCs passing through the fast component. i.e., closed circulation were also subjected to detection and phagocytosis either by macrophages dwelling in the reticular space while their passage through the discontinuity of the junctional complex to the reticular space or by macrophages emerging through IES while their passage along the venous sinus wall. Ultrastructural studies provide an evidence that the spleen performs it filtration function by a multifiltration mechanism.