Lymph nodes form during the 11th week of gestation from a condensation of capillaries and mesenchymal cells, which then bulge into a sac. Granulopoiesis and erythropoiesis within this sac during the 12th to 14th weeks of gestation lead to the formation of lymphoid cells, including T and B cells (Markgraf et al. 1982). Lymphatic vessels were thought to originate from adjacent veins. However, recent studies have questioned this paradigm; some mesenteric lymph channels arise separately from the mesenteric vein (Semo et al. 2016). Regardless their precise origin, lymphatic vessels can be categorized on the basis of size, as arteries or veins can. Lymph capillaries are lined by a single-cell layer of endothelium linked by button-like junctions, which open in response to increased interstitial fluid pressure (Baluk et al. 2007). The larger pre-collecting and collecting vessels contain valves and smooth muscle cells, preventing backflow (Butler et al. 2009).