Lymphatic drainage starts with the central lacteal, which drains into the submucosal lymphatic plexus (Figs. 6.14 and 6.17). CD 38, a type II transmembrane glycoprotein involved in signaling and adhesion, is a novel marker of small intestinal lacteals (36). The broad proximal villi contain from two to five lacteals, whereas more distal thin villi contain only one. Lacteals measure 5 to 15 μm in diameter and run parallel to one another in the longitudinal direction of the villus. The endothelial lining contains gaps and has overlapping areas with adjacent endothelial cells. Chylomicrons and fatty droplets can pass through the gaps (Fig. 6.18) (37). The wall of the lacteal consists of endothelium and a reticulin fiber sheath to which smooth muscle fibers attach (Figs. 6.17 and 6.19) (38). Villi intermittently contract and shorten due to the activity of the smooth muscle cells. These contractions force lymph from the central lacteal into the basal lymphatics. The

central lacteal is also completely surrounded by the subepithelial blood capillary network (Fig. 6.17). Lacteals anastomose with each other, which forms an expanded sinus. In the fasted state, lacteals are difficult to see.