Precollectors are structures of different length between initial lymphatics and lymph collectors. Their irregular diameter can be from 20 to 100 mm, and often they are in close contact with arterial vessels.
Two shapes of endothelial cells are present. In cells of the distal part an oak-leaf pattern dominates, whereas proximal cells are more stretched in the flow direction of the lymph and are preferentially rhombic, similar to the venous endothelium. Open-junction formations can regularly be seen in the more distal part, acting as inlet valves. A specialty of the precollector endothelium is the development of trabecular and valvelike structures, giving the lumen a labyrinthine structure, looking like an eel basket. The building of these structures is managed by spindle-shaped modified endothelial cells based in the endothelium and running through the lumen of the precollector. Distal valves are more incomplete structures like ridges in the luminon; proximal to the collector they become sufficient and more are visible. Here, the distance between these valves is about 130 mm. This valve system ensures a more proximal flow direction of the lymph. Additionally to the incomplete valves, the tra-becular system generates turbulences of the lymph stream so that all substances within the lymph (molecules, particles, cells) contact the endothelial surface, which possesses immunological and resorptive properties. In this way, there exists a controlling system for the prenodal lymph, proofing this fluid, and with the capacity of unspecific as well as specific immunological reactions (Figure 10).
The nuclei of the precollector endothelium are oval and oriented longitudinally. Their diameter is about 1.2 to 1.9 mm. Numerous vesicles, particularly near the cell borders, are typical. The endothelium is rich in pinocytotic vesicles, and the luminal surface of the endothelium often possesses some microvilli, indicating active transport processes. The cytoskeleton of the precollector endothe-lium, in contrast to the initial lymphatic endothelium, is directed more along the length of the cells. Partially interen-dothelial caveolae are visible, including especially multil-amellar bodies, perhaps relics of destroyed cells or material. A thin discontinuous basement membrane is regularly visible, as are anchoring filaments. Sometimes smooth muscle cells adjacent to the basement membrane can be seen. Their occurrence is not related to the site of valves. Because of the lack of a continuous muscular wall like the collectors, they use different forces for the propulsion of lymph fluid. Often precollectors are arranged between two arterial vessels or an arterial vessel is arranged via a mesangion in the lumen of the precollectors. So, because of the pulsation of the bloodstream, the rhythmical compression displaces the lymph fluid, which can flow only in the proximal direction because of the valvelike structures or complete valves. The typical fiber system of the connective tissue surrounds the precol-lector. Thus, the precollector endothelium is a particular structure, neither initial nor collector endothelium, involved in the fluid absorption and lymph propulsion processes.
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