“In vivo” mechanical compliance of the initial lymphatic vessels in rat diaphragm

Abstract

The lymphatic system originates from the interstitial tissue with initial lymphatic capillaries, whose wall is devoid of smooth muscle cells. In these vessels, lymph formation and progression is sustained by cyclic tissue stresses transmitted to the vessel lumen according to the mechanical properties of the vessel walls, which are at present still unknown. Therefore, we aimed at measuring in vivo the mechanical properties of initial lymphatics belonging to the diaphragmatic network in anesthetized rats. The diaphragm was exposed and a lymphatic vessel was localized via stereomicroscopy. Vessel segments were occluded at both ends and injections of 4.6 nL saline were performed, while measuring lymphatic pressure (Pl) by micropuncture. At each injection, Pl step increase was followed by stress relaxation (τ= 2.9 ± 0.2 (SE) sec). For incremental volume steps < 35 % of initial segment volume (Vo), two vessel populations were identified with mechanical compliance (C) of 40.8 ± 4 nl/cmH2O and 2.8 ± 0.4 nl/cmH2O. For volume steps from 35 to 100 % and greater than 100% of Vo, C was equal in both populations and averaged 4.5 ± 0.6 nl/cmH2O and 62.4 ± 7.4 nl/cmH2O, respectively. In conclusion, the existence of two populations of initial lymphatic vessels with distinct mechanical behaviour suggests a differential recruitment of lymphatic vessels to cope with the varying draining requirements of the tissue