Despite their important physiological and pathophysiological functions, lymphatic endothelial cells and lymphatic vessels remain less well studied compared to the blood vascular system. Lymphatic endothelium differentiates from venous blood vascular endothelium after initial arteriovenous differentiation. Only recently by the use of light sheet microscopy, the precise mechanism of separation of the first lymphatic endothelial progenitors from the cardinal vein has been described as delamination followed by mesenchymal cell migration of lymphatic endothelial cells. Dorsolaterally of the embryonic cardinal vein, lymphatic endothelial cells reaggregate to form the first lumenized lymphatic vessels, the dorsal peripheral longitudinal vessel and the more ventrally positioned primordial thoracic duct. Despite this progress in our understanding of the first lymph vessel formation, intravital observation of lymphatic vessel behavior in the intact organism, during development and in the adult, is prerequisite to a precise understanding of this tissue. Transgenic models and two-photon microscopy, in combination with optical windows, have made live intravital imaging possible: however, new imaging modalities and novel approaches promise gentler, more physiological, and longer intravital imaging of lymphatic vessels.
