Phylogeny and ontogeny of the lymphatic stomata connecting the pleural and peritoneal cavities with the lymphatic system--a review.

This paper reviews the phylogeny and ontogeny of "lymphatic stomata" through which fluids and cells in the pleural and peritoneal cavities enter the lymphatic system. In amphibians, the pleuroperitoneal cavity is connected through numerous pores with the wide subvertebral lymphatic sinus corresponding to the thoracic duct in mammals. In reptiles, direct connections of the pleural and peritoneal cavities with the lymphatic system have been reported. In mammals, the pleural and peritoneal cavities are directly connected with lymphatics through lymphatic stomata. How do lymphatic stomata develop in mammals? In the rat, distinct lymphatics are noted in the subpleural space of the diaphragm periphery in 16 days old embryo. With age, the supleural lymphatics increase and form a polygonal network. They show a tubular appearance and possess many valves. Some of them become endowed with smooth muscle cells. In 19 days old embryos, some lymphatics appear in the subperitoneal space of the diaphragm. They extend centripetally and form many lateral projections that later elongate and connect with those from adjacent lymphatics, thus forming a lattice-like network or "lymphatic lacunae". During early postnatal days, the lymphatic lacunae project many bulges that subsequently come into contact with the pores among mesothelial cells lining the diaphragmatic peritoneum, thus forming lymphatic stomata. They increase until postnatal week 10. The lymphatic stomata in the costal pleura also develop during early postnatal days.