Organization of the blood and lymphatic microvasculature of the gallbladder in the guinea pig: a scanning electron microscopic study.

The organization of the blood and lymphatic microvessels of the gallbladder in the guinea pig is demonstrated by scanning electron microscopy (SEM) of vascular corrosion casts, and SEM of KOH-macerated tissues. In the lamina propria of the gallbladder, there is a dense network of subepithelial capillaries. The network is supplied by the arterioles that come off the arterial plexus located deep in the lamina propria. The network gathers into the postcapillary venules continuous with the collecting venular plexus located immediately below the subepithelial capillary network. The precapillary arterioles are sparsely surrounded by a single layer of circularly oriented extensions of smooth muscle cells. The terminal arterioles are endowed with circularly oriented fusiform smooth muscle cells. The nervous plexus is also noticed along the terminal arterioles. The capillaries are embraced by flat prolongations of pericytes. The postcapillary venules are sparsely surrounded by stellate pericytes and the collecting venules are sparsely surrounded by elongated or branched spindle-shaped, primitive smooth muscle cells which extend their long process in various directions along the vascular wall. The lymphatics are mostly located in the subserosal layer. The tips of the initial lymphatics are closed by endothelial cells, although there are frequently some gaps between them. The thin flaps of the lymphatic endothelial cells overlap or interdigitate with each other. The luminar surfaces of the lymphatics show oval nuclear protrusions, while the abluminal surfaces showed numerous microfolds except for the oval and flat nuclear portions. The lymphatics possess neither smooth muscle cells nor pericytes.