DEVELOPMENT OF DIAPHRAGMATIC LYMPHATICS - THE PROCESS OF THEIR DIRECTCONNECTION TO THE PERITONEAL-CAVITY

The development of the lymphatic system in the rat diaphragm was studi ed from embryonic day 16 to 25 weeks after birth by histochemistry for 5'-nucleotidase, scanning electron microscopy of KOH-treated or intac t tissues, and transmission electron microscopy of thin sections. On e mbryonic day 16, distinct lymphatics were noted in the subpleural spac e of the diaphragm periphery. The endothelial cells at this stage cont ained an abundance of rough endoplasmic reticulum, a developed Golgi a pparatus and mitochondria, and fewer pinocytotic vesicles than those i n adults. The subpleural lymphatics subsequently increased and formed a polygonal network. They possessed many valves, and by postnatal week 6, some thick collecting lymphatics became endowed with smooth muscle cells. On embryonic day 19, some lymphatics appeared in the subperito neal space. They extended centripetally and had many lateral projectio ns that subsequently became elongated and connected with those from ad jacent lymphatics, thus forming a lattice-like network. During the ear ly postnatal days, the subperitoneal lymphatics projected many bulges that subsequently became elongated, and came into contact with the por es among the mesothelial cells, thus forming lymphatic stomata connect ing the lymphatic lacunae to the peritoneal cavity. The lymphatic stom ata increased until postnatal week 10. The results show that lymphatic s appear as early as embryonic day 16 in the subpleural space of the d iaphragm periphery, and develop with age by sprouting to form networks in both the subpleural and the subperitoneal spaces, and that the dir ect connection of the lymphatic lacunae to the peritoneal cavity is fr omed after birth.