The lymphatic system is comprised of a network of vessels interrelated with lymphoid tissue, which has the holistic function to maintain the local physiologic environment for every cell in all tissues of the body. The lymphatic system maintains extracellular fluid homeostasis favorable for optimal tissue function, removing substances that arise due to metabolism or cell death, and optimizing immunity against bacteria, viruses, parasites, and other antigens. This article provides a comprehensive review of important findings over the past century along with recent advances in the understanding of the anatomy and physiology of lymphatic vessels, including tissue/organ specificity, development, mechanisms of lymph formation and transport, lymphangiogenesis, and the roles of lymphatics in disease.

Therapies aimed at manipulating the microcirculation require the ability to control angiogenesis, defined as the sprouting of new capillaries from existing vessels. Blocking angiogenesis would be beneficial in many pathologies (e.g. cancer, retinopathies and rheumatoid arthritis). In others (e.g. myocardial infarction, stroke and hypertension), promoting angiogenesis would be desirable. We know that vascular pericytes elongate around endothelial cells (ECs) and are functionally associated with regulating vessel stabilization, vessel diameter and EC proliferation. During angiogenesis, bidirectional pericyte-EC signaling is critical for capillary sprout formation. Observations of pericytes leading capillary sprouts also implicate their role in EC guidance. As such, pericytes have recently emerged as a therapeutic target to promote or inhibit angiogenesis. Advancing our basic understanding of pericytes and developing pericyte-related therapies are challenged, like in many other fields, by questions regarding cell identity. This review article discusses what we know about pericyte phenotypes and the opportunity to advance our understanding by defining the specific pericyte cell populations involved in capillary sprouting.

FLT3 mutations are prevalent in AML patients and confer poor prognosis. Crenolanib, a potent type I pan-FLT3 inhibitor, is effective against both internal tandem duplications and resistance-conferring tyrosine kinase domain mutations. While crenolanib monotherapy has demonstrated clinical benefit in heavily pretreated relapsed/refractory AML patients, responses are transient and relapse eventually occurs. Here, to investigate the mechanisms of crenolanib resistance, we perform whole exome sequencing of AML patient samples before and after crenolanib treatment. Unlike other FLT3 inhibitors, crenolanib does not induce FLT3 secondary mutations, and mutations of the FLT3 gatekeeper residue are infrequent. Instead, mutations of NRAS and IDH2 arise, mostly as FLT3-independent subclones, while TET2 and IDH1 predominantly co-occur with FLT3-mutant clones and are enriched in crenolanib poor-responders. The remaining patients exhibit post-crenolanib expansion of mutations associated with epigenetic regulators, transcription factors, and cohesion factors, suggesting diverse genetic/epigenetic mechanisms of crenolanib resistance. Drug combinations in experimental models restore crenolanib sensitivity.

Background: Lymphatic and blood microvascular systems play a coordinated role in the regulation of interstitial fluid balance and immune cell trafficking during inflammation. The objective of this study was to characterize the temporal and spatial relationships between lymphatic and blood vessel growth in the adult rat mesentery following an inflammatory stimulus. Methods and Results: Mesenteric tissues were harvested from unstimulated adult male Wistar rats and at 3, 10, and 30 days post compound 48/80 stimulation. Tissues were immunolabeled for PECAM, LYVE-1, Prox1, podoplanin, CD11b, and class III b-tubulin. Vascular area, capillary blind end density, and vascular length density were quantified for each vessel system per time point. Blood vascular area increased compared to unstimulated tissues by day 10 and remained increased at day 30. Following the peak in blood capillary sprouting at day 3, blood vascular area and density increased at day 10. The number of blind-ended lymphatic vessels and lymphatic density did not significantly increase until day 10, and lymphatic vascular area was not increased compared to the unstimulated level until day 30. Lymphangiogenesis correlated with the upregulation of class III b-tubulin expression by endothelial cells along lymphatic blind-ended vessels and increased lymphatic/blood endothelial cell connections. In local tissue regions containing both blood and lymphatic vessels, the presence of lymphatics attenuated blood capillary sprouting. Conclusions: Our work suggests that lymphangiogenesis lags angiogenesis during inflammation and motivates the need for future investigations aimed at understanding lymphatic/blood endothelial cell interactions. The results also indicate that lymphatic endothelial cells undergo phenotypic changes during lymphangiogenesis.

Objective Lymphatic and blood microvascular systems are critical for tissue function. Insights into the coordination of both systems can be gained by investigating the relationships between lymphangiogenesis and angiogenesis. Recently, our laboratory established the rat mesentery culture model as a novel tool to investigate multicellular interactions during angiogenesis in an intact microvascular network scenario. The objective of this study was to determine whether the rat mesentery culture model can be used to study lymphangiogenesis. Methods Mesenteric tissue windows were harvested from adult male Wistar rats and cultured for 3 or 5 days in either serum-free minimum essential media (MEM) or MEM supplemented with VEGF-C. Tissues were immunolabeled for PECAM and LYVE-1 to identify blood and lymphatic endothelial cells, respectively. Tissues selected randomly from those containing vascular networks were quantified for angiogenesis and lymphangiogenesis. Results VEGF-C treatment resulted in an increase in the density of blood vessel sprouting compared to controls by day 3. By day 5, lymphatic sprouting was increased compared to controls. Conclusions These results are consistent with in vivo findings that lymphangiogenesis lags angiogenesis after chronic stimulation and establish a tool for investigating the interrelationships between lymphangiogenesis and angiogenesis in a multi-system microvascular environment.