Lung lymphatic anatomy and correlates.

The pulmonary lymphatics do much more than keep the lung dry. They defend the lungs from airborne particles and microbes and allow a local influx of liquid to clear and clean inflamed or damaged tissue. Lymphatic morphology, especially the three-dimensional structure, is best demonstrated by casting the lymphatics, corroding the tissue, and viewing the casts by scanning electron microscopy. With this technique, different lymphatic forms exist. On the pleural surface prelymphatics are simply tissue planes that connect with lymphatic channels. Reservoir lymphatics are another form of initial lymphatics that have blind-ending pouches. They empty into conduit lymphatics. Lymphatics around blood vessels and airways are generally tubular and saccular. In the last decade, lymphatic markers have been discovered that allow the study of lymphatic development in health and disease. Modulators of these pathways could be potential therapeutics for diverse pulmonary problems such as cancer and lung transplantation. The size of the lymphatic system expands manifold in response to an excess fluid load, cancer, or inflammation. Immune cells move through the lymphatics, mature, and become activated there. The lymphatics enable the immune defense system by allowing a sequestered place and close proximity for antigen presentation and lymphocyte maturation.