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The objective of this study was twofold: first to investigate the ultrastructure of the lymphatic drainage unit on the costal pleura of rats by electron microscopy, and secondly to examine the effect of nitric oxide on the pleural lymphatic stomata and fluid absorption from the pleural cavity. The lymphatic drainage unit of the rat costal pleura is composed of three special components: the lymphatic stomata between the mesothelial cells, the initial part of the lymphatic vessels and the underlying connective tissue containing many foramina. The unit is the main passage to drainage fluid, particles and cells in the pleural space. To investigate the regulator of the lymph drainage, nitric oxide synthase inhibitor and nitric oxide donor were injected into the peritoneal cavity of the rats, respectively. Trypan blue was used as tracer. The ultrastructural changes of pleural lymphatic stomata were observed under scanning electron microscope and analyzed by a computer image processing system. It turned out that the area and density of the pleural lymphatic stomata were positively correlated with the nitric oxide quantity (p < 0.05). After the tracer was injected into the pleural cavity, the nitric oxide donor group exhibited a higher trypan blue concentration than the control group (p < 0.05). The ability of the pleura to absorb trypan blue was enhanced because of the larger opening of the lymphatic stomata (p < 0.05). It is suggested that nitric oxide can increase lymphatic absorption of the pleura by opening pleural lymphatic stomata.

The generation and development of the peritoneal lymphatic stomata (PLS) and lymphatic vessels of the diaphragm were studied in mice at gestational ages from the embryonic to the postnatal period with TEM, SEM and enzyme histochemistry and the PLS data were quantitatively analyzed with computer-assisted image processing technology (Elescope image analysis software). The results showed that the diaphragmatic mesothelium was covered only by flattened mesothelial cells (FMC) at the 13th embryonic day (ED 13). At ED 15, some cuboidal mesothelial cells (CMC) and immature lymphatic stomata (NLS) were found scattered on the diaphragmatic mesothelium. The sub-peritoneal lymphatic capillaries did not appear until ED 18. However, no absorptive function was observed in NLS when trypan blue granules were injected into the peritoneal cavity. At postnatal day 1 (PND 1), the endothelial cytoplasm processes of the diaphragm lymphatic capillaries span the connective tissue fibers and the basal membrane of CMC to form the subperitoneal channels. These channels were connected with NLS and serve as the absorptive route between the peritoneal cavity and the sub-peritoneal lymphatic vessels. The trypan blue absorption test demonstrated that postnatal PLS possessed an absorptive function and had transformed to mature lymphatic stomata (MLS) by PND 1. Thus, NLS were renamed of MLS. At PND 5, the cuboidal mesothelial cell ridge (CMCR) appeared with increased CMC areas. At PND 10, CMCR were fused to form the band-like CMC area with much more MLS distributed in the muscular portion of the diaphragm. With distribution area and density of PLS increasing and growth of lymphatic vessels, an increased absorptive function from the peritoneal cavity was observed in the experiment.

The lymphatic stomata in the pelvic peritoneum of human fetuses and mature mice were initially observed and studied quantitatively by using computer image processing (C.I.P.) attached to a scanning electron microscope (SEM). Two types of mesothelial cells were found in the pelvic peritoneum of human fetuses and mature mice, i.e. flattened and cuboidal cells. The lymphatic stomata, arranged in clusters, were only found irregularly distributed among the cuboidal cells. The divergence of stoma area in the pelvic peritoneum of human fetuses varied greatly, ranging from 0.8 micron2 to 43.4 microns2. The average area of the lymphatic stomata in human fetuses was 10.00 +/- 9.44 microns2. The variation coefficient was 94.40. The standard deviations and standard errors were 9.44 and 0.98 respectively. Most of the lymphatic stomata in human fetuses were between 1.34 microns2 and 32.11 microns2 in size (accounting for 90%), with maximum and minimum values of 43.4 microns2 and 0.8 micron2. The average distribution density of the lymphatic stomata in human fetuses was 7.2% and the maximum density was 11.6%, which means that the average and the maximum absorption rates of the human pelvic peritoneum from the peritoneal cavity were 7.2% and 11.6% respectively. Therefore, it is suggested that the lymphatic stomata in pelvic peritoneum play an important role in draining materials from the peritoneal cavity, and that the absorption effect of the pelvic peritoneum is similar to that of the diaphragmatic peritoneum.

The peritoneal stomata, lymphatic drainage units and subperitoneal terminal lymphatics, called lymphatic lacunae, form a specialized drainage system in the diaphragm, by which absorption of fluid in bulk, particles and cells is carried out in the peritoneal cavity. The aim of this study is to elucidate the three-dimensional organization and function of the subperitoneal lymphatic lacunae and lymphatic drainage units by using lymphatic casts in the scanning electron microscope (SEM), ODO (OsO4-DMSO-OsO4) freeze fracture, conventional SEM and the transmission electron microscope (TEM). The subperitoneal lymphatic lacuna is unique for its large size and its multiple morphology and can be recognized by its broad, flattened enlargement and the blind-ends of lymphatic vessels, from which extend numerous main lymphatic vessels and side branches. These lymphatic vessels communicate with each other and form a rich lymphatic plexus under the diaphragmatic peritoneum. Two layers of lymphatic networks, i.e. the subperitoneal plexus and the deeper plexus are found in the muscular portion. Only one layer is present in the tendinous portion of the human diaphragm. The lymphatic plexus is denser in the tendinous portion than that in the muscular portion. The lymphatic lacunae occur exclusively in the muscular portion of the human diaphragm. The lumina of lymphatic lacunae are separated from the peritoneal cavity by a barrier consisting of cuboidal mesothelial cells, endothelial cells of the lymphatic lacunae and intervening connective tissue forming a lymphatic drainage unit. All these three components of the lymphatic drainage unit abut upon each other, but are not linked by specialized junctions. The cuboidal mesothelial cells frequently extend valve-like cytoplasmic processes that bridge the subperitoneal channel and make give it a tortuous course. The fibrous layer of the connective tissue is arranged in fiber bundles and gives a three-dimensional network forming the floor of the peritoneal stomata and the roof of the lymphatic lacunae. Via the fibrous network, the cuboidal mesothelial cells and the endothelial cells of the lacunae come into close contact with each other and form short subperitoneal channels which connect the peritoneal cavity with the subperitoneal lymphatic lacunae. The lymphatic drainage units may regulate the material absorption of the peritoneal stomata from the peritoneal cavity. It is suggested that the peritoneal stomata together with the subperitoneal channels, lymphatic drainage units and lymphatic lacunae comprise an important diaphragmatic lymphatic drainage system which plays an important role in the absorption of materials from the peritoneal cavity.

In 16 human specimens the mesothelial cells of the diaphragmatic peritoneum and the pelvic parietal peritoneum were observed by transmission electron microscopy and freeze fracture replica. There were two types of cells i. e. the vesicle-containing cells and ER-cells. The former were found only in the diaphragmatic peritoneum and contained numerous cytoplasmic vesicles. These vesicles could be alone or in clusters. The clustered vesicles were often fused to form large vacuoles which opened on the free surface of the cell, into the intercellular space, or through the basement membrane, or sometimes became secretory particles on the free surface of the cells. Some vesicles were also observed in the microvilli. The latter were only found in the pelvic parietal peritoneum and contained abundant endoplasmic reticulum. The vesicle-containing cells seemed to absorb the material from the peritoneal cavity. The ER-cells contained abundant endoplasmic reticulum and Golgi apparatus, and showed high synthetic activity, possibly related to the production of peritoneal fluid.

The human dura mater and pia mater were studied by using a scanning electron microscope and a computer image processing system (C.I.P.). The human cerebral meningeal stomata are located between the mesothelial cells of the cerebral meninges. They are round or oval in shape with diameters of 0.33-2.98 microns. The cerebral meningeal stomata are stable structures, scattered or clustered together. Their density in the dura mater is greater than in the pia mater (P0.01), and they are regularly distributed. The statistical analysis showed that the stomata diameter and distribution density in the dura mater are 1.34 microns and 381.55/0.1 mm2; while in the pia mater they are 0.88 micron and 195.06/0.1 mm2 respectively. The cerebral meningeal stomata are probably part of the cerebral prelymphatic capillary system, which undertakes the cerebral lymph drainage because there are no lymphatic vessels in the brain although yet there is lymph drainage. Thus, we believe that the cerebral meningeal stomata are involved in maintaining the physiological function of the brain as part of the cerebrospinal fluid (CSF) which absorbs the cerebral interstitial fluid.