Background: Fluid and free cells in the peritoneal cavity enter the lymphatics through the lymphatic stomata which are channels connecting the lymphatics in the peritoneal side of the diaphragm with the peritoneal cavity. While the stomata thus play a very important in the physiology of the peritoneal cavity, it is unclear when they appear or how they are distributed on the diaphragm. We therefore conducted an embryological study of the process and timing of mouse lymphatic stomata development in the peritoneal surface of the diaphragm.
Methods: The mouse diaphragm, at stage ranging from embryonic day 18 (ED18) to postnatal week 10 (PW10), was observed by scanning electron microscopy, and the number of lymphatic stomata was counted on each observation day. A map of the data was constructed to illustrate the process of appearance of lymphatic stomata.
Results: Lymphatic stomata were not found on ED18. They were first found on PD0 and their number increased exponentially until PW10. Lymphatic stomata were usually located in cuboidal cell areas but not in the areas lined with flattened cells. The cuboidal cell area with several lymphatic stomata was first found in the retroparasternal region on PD0, followed by in the muscular portion, as "ridges" on PD4 and "bands" on PD6 or up to a few days later. The long axis of the ridges and bands was oriented from the center to periphery of the diaphragm. Subsequently, cuboidal cell areas with lymphatic stomata formed along the border between the central tendon and the muscular portion, most frequently on PD10. Another cuboidal cell area with lymphatic stomata appeared rather suddenly ventral to the inferior vena cava on PD10. This was the full complement of cuboidal cell areas seen in the adult of PW10.
Conclusion: These results verified that the course of the change of the shape and distribution of cuboidal cell areas parallels that of the underlying lymphatic lacunae, suggesting the delivery of some stimuli from the lymphatic lacunae to the overlying mesothelial cells that results in alternation of their cell shape.