77 Lymphology 33 (2000) 77-80 EDITORIAL LYMPH AND LYMPHANGIOGENESIS: FUNCTIONAL ASPECTS The studies of Wilting and cohorts in this CAM induces proliferation of endothelial issue ofthe Journal (1) on "Avian Models in cells and formation of huge lymphatic Studies of Embryonic Lymphangiogenesis" sinuses. Several of these sinuses later become, are superb, timely and worthy of comment in or become confluent with, afferent and/or relation to human, as well as avian physiology. afferent lymphatics of well-organized Thus: lymphopoietic tissues, such as the thymus, avian bursa of Fabricius, gut-associated 1. Their descriptive studies show that both lymphatic tissues (GALT), as well as the intra- and extra-embryonic blood vessels of definitive regional lymph nodes in most chick and quail embryos are accompanied by species of mammals (4,5). lymphatics. This observation may not be a new generic one, but it signals the importance 4. They noted that exceptions to the of lymphatics in relation to the component differential histochemical identification of cells of extra-embryonic tissues upon which lymphatic endothelium from common blood developing embryos of all vertebrates depend vascular endothelium by means of receptors for nutrition, as well as tissue oxygenation for VEGFR-2 and -3 occur during the early during natural embryonic development (2). development of blood vessels in the extra­ embryonic membranes and blood vessels in 2. The authors stress that lymphatics of the the lungs just before hatching in chick chorioallantoic membrane (CAM) are drained embryos. This might be explained by the by lymphatic trunks of the umbilicus, and are considerations that development of an connected to the posterior avian lymph arteriovenous (A-V) circulation normally hearts. The posterior lymph hearts, like the precedes the development of lymphatics in anterior lymph hearts in the lower neck, most, if not all, species of lung-breathing speed access of lymph effluent from the CAM vertebrates; and that intraluminal hydrostatic toward the developing blood vascular system pressure in pulmonary blood vessels is at a crucial time in early embryonic develop­ normally less than that in systemic blood ment before other means of lymph propulsion vessels of each species. become established with differential tissue growth and customary physical activity (2,3). 5. By combining modern histochemical technologies for identifying receptors for 3. They found that lymphatic endothelial differing vascular endothelial growth factors cells are characterized by the expression of with methods for identifying the differing Vascular Endothelial Growth Factor genetic constitutions of chick and quail Receptors (VEGFR) -2 and -3; and that the endothelial cells, they showed by grafting the local application of VEGF-C, the ligand of wing buds of 3-5 day old chick embryos to the these two receptors, on the differentiated stumps of excised counterparts in 3-5 day old Permission granted for single print for individual use. Reproduction not permitted without permission of Journal LYMPHOLOGY . 78 quail embryos that the wing lymph vessels throughout the general body mesenchyme of extend from the body trunk by derivation birds and mammals are formed and coalesce from mesencyhme of new lymph vessels of in a progression radiating from the yolk sac quail origin, as well as by replication of into the body mesenchyme until the heart, vascular endothelial cells in both species. gills and gill arch circulation are demarcated (2,8). With the establishment of A-V 6. They therefore conclude that: (a) in the circulation of lymph containing red blood wing buds of embryonic quail, lymphatics cells in the embryo and vitelline circulation, grow and extend by formation of new the mesenchyme lining the blood puddles "lymphangioblasts" of mesenchymal origin, flattens to form endothelium, basement as well as by replication of differentiated membrane and surrounding smooth muscle lymphatic endothelial cells-more or less as more or less in proportion to the rate of blood suggested by Kampmeier (5) as opposed to flow established in each defined blood vessel the classic teachings of Sabin (6), who (2,9). Notable exceptions to this general rule advocated that lymphatics arose primarily by are found in capillary beds where A-V blood sprouting from endothelium of veins. (b) flow remains intermittent, as in the sinusoids vascular endothelial growth factors, such as of the liver and spleen where blood vessels VEGF-C, might prove useful in the treatment remain lined by macrophages, as well as of patients suffering from lymphatic aplasia thickened endothelium supported by fibrils or hypoplasia. intermittently permeable to objects as large as red blood cells (2,10). For lack of a better In critique and extension of this study, term, such endothelial cells were formerly the following points are cogent: designated as reticuloendothelial cells with an extraordinary capacity to engulf and digest • As observed by Sabin (7) and acid colloids, as well as foreign organisms Kampmeier (5), development of vascular noxious to a given species (2,10,11). endothelium is predicated on the formation In invertebrates, a blood vascular system of liquid lymph by dissolution of mono­ does not develop, because the mesenchyme nuclear mesenchymal cell cytoplasm to form does not produce red blood cells (2,12). In isolated "lakes" or "puddles" containing warm-blooded birds and mammals, unable to formed elements. The microscopically visible carry enough dissolved oxygen in circulating "formed elements" are mostly nucleated red lymph, the development of the lymphatic blood cells and lymphocytes in embryonic system awaits arterial circulation of blood blood vessels and puddles devoid of formed cells through the gill arches (2,8,12). structures except small lymphocytes in Lymphatic development then proceeds by embryonic lymphatics. formation of separate puddles of lymph • The formation of lymph, lymphocytes, whose formation from mesenchyme and erythrocytes and other "formed" elements progressive coalescence closely parallels the such as granulocytes and thrombocytes, in growth and extension of the aorta and its warm-blooded birds and mammals requires intra-embryonic and extra-embryonic substrates absorbed into the mesenchyme branches (5). The first lymph puddles are from the entodermal cells lining the yolk sac formed as jugular lymph sacs in close relation and quantities of oxygen carried by erythro­ to the jugular or subelavian veins below the cytes from points of diffusion through the gills in the lower neck, and they establish a egg-shell in birds or through the placenta in lumen-to-Iumen connection with these veins mammals (2). The puddles of blood which (5). The subsequent extension of lymphatics develop, first, in the mesenchyme supporting continues by formation of progressive the yolk-laden entodermal cells and, later, coalescence of puddles in peri-aortic and Permission granted for single print for individual use. Reproduction not permitted without permission of Journal LYMPHOLOGY . 79 periarterial mesenchyme to form the central mesothelial cells lining these sinuses are more lymph ducts and branches which accompany differentiated than reticuloendothelium lining development of the arterial system through­ A-V sinusoids or lymph nodal sinuses, and out the body (2,5). Progressive lymphatic less differentiated than common vascular development depends primarily on diffusion endothelial cells lining arteries and veins. of oxygen and essential substrates from These lymph-filled sinuses surround the nearby arterioles, and partly on the output of heart, lungs and abdominal digestive organs cells in diverse segmental A-V capillary beds such that expansion, contraction and other (2,9). As in the blood vascular system, the movements of contained parts are minimally flattening of mesenchyme to form vascular impeded by other body structures during endothelial cells, basal membrane and smooth customary homeostatic activities. Small muscle coats is more or less proportional to collections of organized lymphoid tissues in local flow rate (2,4,9). However, in the the form of "milky spots" variably develop in periphery and in the organized lymphopoeitic the lining of the pericardial, pleural and tissues, the most distal rami of the lymphatic peritoneal lymph sacs for engulfing and system do not become completely lined by disposing offoreign material (17). endothelium, but rather remain open into the • Another essential feature of lymphatic ground substance formed by mesenchymal development is the formation of myriad connective tissue cells (2,4,9,13-15) As a intralymphatic valves which normally result, relatively large dissolved molecules prevent the backflow of lymph, especially in absorbed from local parenchymal cells or lymphatics gravitationally inferior to the emanating from regional cells during outflow tracts of the heart (3). oxidative metabolism essential to cellular • Birds differ from mammals in that well­ nucleotide synthesis and neoformation of organized regional lymph nodes do not water after aerobic oxidation of glucose are develop to interrupt the flow of lymph effluent collected without passage by diffusion or from the periphery before flow into the pinocytosis through relatively impermeable thoracic ducts in synchrony with pulmonary endothelium (2,16). inspiration (3,13). Instead, all species of birds • Although the lymphatic system are unique in that they develop a cloacal gradually develops in this manner paralleling pouch densely