Figure 3. Lymphatic unidirectional valves.

A, transmission electron microphotographs showing the primary unidirectional valves (white arrowhead) in the wall of diaphragmatic initial lymphatics located in the pleural submesothelial interstitium. The valves are formed at the junction between overlapping cytoplasmic extensions of two adjacent lymphatic endothelial cells (d) (reproduced from Grimaldi et al. 2006). B, semi-thin sections of transverse lymphatic ducts, showing a unidirectional intraluminar, or secondary, valve formed by two leaflets (arrowhead) attached at opposite sides of the lymph channel wall. L, lymphatic lumen; b, collagen bundles; M, skeletal muscle fibres; d, lymphatic endothelium (reproduced from Grimaldi et al. 2006).