Lymph Drainage - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 178

4.1 Lymphatics

Lymphatic vessels are associated with lymph nodes, oval-shaped filters enclosed
by a fibrous capsule, generally organized into clusters. Lymphatic capillaries are
thin walled, composed of endothelium with intercellular gaps. 6 The surrounding
basement membrane is small and permeable or absent. The endothelium of terminal
lymphatics lacks a continuous basement membrane and tight junctions.
Lymph sacs are the earliest elements of the lymphatic vasculature out of which
lymphatic vessels grow. Lymph sacs are formed by budding and differentiation to-
ward lymphatic lineage of endothelial cells under the control of Prospero homeobox
gene product Prox1 [ 319 ].
Lymphatic capillaries are similar in size to venules, without a sheath of pericytes
or smooth myocytes. Larger collecting lymphatic vessels have smooth myocytes and
are similar to veins with thinner walls. The wall is surrounded by a basement mem-
brane. Lymphatic vessels have numerous semilunar valves every few millimeters
that prevent back and forth lymph motion.
Lymph from the upper right quarter of the body drains into the right lym-
phatic duct, and subsequently into the right subclavian vein, otherwise into the
thoracic duct, and then into the left subclavian vein. Additional lymphaticovenous
anastomoses occur in renal, hepatic, and adrenal veins, as well as lymph nodes.
Collecting lymphatic vessels convey interstitial fluid at low pressure down to
veins in which luminal pressure is higher. Between valved segments, lymphatic
vessels are made of lymphangions that contain smooth myocytes aimed at cyclically
pumping lymph against a pressure gradient. Using a mathematical model of lym-
phangion, optimal lymphangion length downstream from a symmetrical junction is
equal to 1.26 times upstream lymphangion length [ 320 ].
Lymphatics are able to generate myogenic constriction and dilation in response
to intraluminal pressure changes, especially to prevent overdistension in edema.
Substance-P increases both magnitude and rate of lymphatic vessel constriction,
as well as the pressure range over which constriction occurs [ 321 ].

4.2 Lymphoid Tissues

Several types of lymphoid tissues exist [ 322 ]. Genetically preprogrammed
anatomically distinct lymphoid organs include lymph nodes and spleen.
Prepatterned environment-dependent mucosal-associated lymphoid tissues are
represented by Peyer patches and gut-associated lymphoid tissue, 7 tonsils, adenoids,

6 Lymphatic vessel endothelial hyaluronan receptor-1 (LyVE1) is a marker for lymphatic endothe-
lial cells. Plasmalemmal podoplanin is also assigned to lymphatic endothelial cells.
7 M cells of Peyer's patches of the intestine take up antigen that is then transferred to cells in the
underlying gut-associated lymphoid tissue (macrophages and dendritic cells).