Mechanisms causing initial lymphatics to expand and compress to promote lymph flow

Microlymphatics can be devided into two segments, initial lymphatics which are made up of irregular tissue crevices lined by a continuous attenuated endothelium, and collecting lymphatics with a smooth muscle media and the ability for spontaneous contractility. Virtually the entire array of mammalian organs with lymphatic drainage have initial lymphatics which are drained by collecting lymphatics, but in organs like skeletal muscle and intestine almost all lymphatics are of the initial type, and the muscular collecting lymphatics arise only outside the organs per se. How can interstitial fluid find its way into the sparely positioned initial lymphatics? Initial lymphatics exhibit no detectable contractile activity. Their endothelium shows incomplete attachment between neighbouring cells, providing a mechanism to open and close lymphatic endothelial microvalves along the walls of the initial lymphatics. Current evidence suggests that lymph fluid formation in the initial lymphatics requires periodic expansion and compression of the initial lymphatics. Expansion of the initial lymphatics causes filling by percolation of interstitial fluid across the open endothelial microvalves. Compression causes closure of the endothelial microvalves and outflow along the lumen of the microlymphatics with eventual transport into collecting lymphatics, towards the nodes and into the thoracic ducts. Reflow towards the initial lymphatics is prevented by valves. Expansion and compression of the initial lymphatics depend on deformation of the tissue in which they are embedded. In skeletal muscle, lymphatics are usually paired with arterioles so that vasomotion and arterial pulsations as well as muscle contraction may lead to periodic expansion and compression. In other organs alternative mechanisms are expected to operate.(ABSTRACT TRUNCATED AT 250 WORDS)