Integration of CD45-positive leukocytes into newly forming lymphatics of adult mice.

The embryonic origin of lymphatic endothelial cells (LECs) has been a matter of controversy since more than a century. However, recent studies in mice have supported the concept that embryonic lymphangiogenesis is a complex process consisting of growth of lymphatics from specific venous segments as well as the integration of lymphangioblasts into the lymphatic networks. Similarly, the mechanisms of adult lymphangiogenesis are poorly understood and have rarely been studied. We have recently shown that endothelial progenitor cells isolated from the lung of adult mice have the capacity to form both blood vessels and lymphatics when grafted with Matrigel plugs into the skin of syngeneic mice. Here, we followed up on these experiments and studied the behavior of host leukocytes during lymphangiogenesis in the Matrigel plugs. We observed a striking co-localization of CD45(+) leukocytes with the developing lymphatics. Numerous CD45(+) cells expressed the LEC marker podoplanin and were obviously integrated into the lining of lymphatic capillaries. This indicates that, similar to inflammation-induced lymphangiogenesis in man, circulating CD45(+) cells of adult mice are capable of initiating lymphangiogenesis and of adopting a lymphvasculogenic cellular differentiation program. The data are discussed in the context of embryonic and inflammation-induced lymphangiogenesis.

Lymphatics and inflammation.

Inflammation is a local or systemic tissue reaction caused by external or internal stimuli with the objective to remove the noxa, inhibit its further dissemination and eventually repair damaged tissue. Blood vessels and perivascular connective tissue are important regulators of the inflammatory process. After a short initial ischemic phase, inflamed tissue is characterized by hyperaemia and increased permeability of capillaries. Therefore, blood vessels have been in the focus of inflammation research for quite some time, whereas lymphatic vessels have been neglected. Their reactivity is not immediately obvious, and, their identification within the tissue has hardly been possible until lymphatic endothelial cell (LEC)-specific molecules have been identified a few years ago. This has opened up the possibility to study lymphatics in normal and diseased tissues, and to isolate LECs for transcriptome and proteome analyses. Initial studies now provide evidence that lymphatics are not just a passive route for circulating lymphocytes, but seem to be directly involved in both the induction and the resolution of inflammation. This review provides a summary on the basics of inflammation, the structure of lymphatics and their molecular markers, human inflammation-associated diseases and their relation to lymphatics, animal models to study the interaction of lymphatics and inflammation, and finally inflammation-associated molecules expressed in LECs. The integration of lymphatics into inflammation research opens up an exciting new field with great clinical potential.

Effects of acute normovolemic hemodilution on splanchnic oxygenation and on hepatic histology and metabolism in anesthetized pigs.

Perioperative hemodilution (HD) has become an accepted means of reducing transfusion requirements. Therefore, the effects of limited (decrease in hematocrit [Hct] from 30 to 20%, "HD1") and severe (decrease in Hct from 20 to 14%, "HD2") acute normovolemic HD with 6% hydroxyethyl starch on splanchnic blood flows (electromagnetic flow probes), O2 uptakes and deliveries, surface O2 tensions (PO2) (Clark-type electrode), hepatic metabolism (organic acids), and hepatic histology (liver biopsies) were studied in nine pigs anesthetized and paralyzed with ketamine/flunitrazepam and pancuronium. HD1 caused significant (P less than 0.05) increases in cardiac output and all splanchnic flows. Only hepatic arterial blood flow increased twice as much as did cardiac output. Except for hepatic arterial O2 delivery, all splanchnic O2 deliveries decreased. Splanchnic O2 extractions increased, and O2 uptakes remained unchanged. There were no changes in mean surface PO2 values or in surface PO2 histograms of liver and small intestine; in portal or hepatic venous pH; and in hepatic uptake of pyruvate and lactate. In contrast, during HD2 (despite further increases in flows and O2 extractions) portal and hepatic venous pH decreased; mean surface PO2 of liver and small intestine decreased; and the liver surface PO2 histogram showed broadening and a shift to the left. However, hepatic uptake of lactate and pyruvate, and splanchnic O2 uptake remained unchanged, and histologic examination did not reveal significant cell injury. These data indicate that in this experimental model limited acute normovolemic HD was well tolerated by the splanchnic organs. After severe HD, gross liver function remained intact, but there was evidence that compensatory mechanisms (increases in flow and O2 extractions) were no longer fully able to counteract the decrease in splanchnic O2 delivery.