Distribution of lymphatic stomata on the pleural surface of the thoracic cavity and the surface topography of the pleural mesothelium in the golden hamster.

BACKGROUND: The distribution of lymphatic stomata that open to the pleural cavity is unclear. The distribution and the surface topography of the pleural and visceral pleurae are key factors in the turnover of pleural fluid and respiration physiology.
METHODS: Nine golden hamsters (Mesocricetus auratus) from 26 to 33 weeks of age were used for the study. The gross anatomy of the thorax and the arterial supply to the lung were studied in four hamsters. Five thoracic hemispheres, three diaphragms, and tissue blocks of the heart and lung were prepared from the remaining five hamsters. The thoracic hemispheres were fixed in 2.5% glutaraldehyde and the muscular bands at each intercostal space were carefully cut along the costae. The intercostal bands were processed for scanning electron microscopy (SEM) and the localization and the number of lymphatic stomata were recorded. The diaphragms and blocks of the lung and heart were also processed for SEM and the surface topography was observed.
RESULTS: The right and left superior lobes of the lung were supplied by the bronchial artery that originated from the right costocervical trunk and left internal thoracic artery, respectively. Lymphatic stomata and mesothelial discontinuities (pores and gaps) were predominantly located in areas lined with cuboidal cells. The areas of cuboidal cells occupied approximately 4.6 mm2, namely, 1% of the total area of the thoracic hemisphere. There were about 1,000 lymphatic stomata per thoracic hemisphere. About 15% of lymphatic stomata were distributed in the ventro-cranial regions of the thoracic wall, with about 85% in the dorsocaudal region. In the former region, lymphatic stomata were found along the costal margins. In the latter, they were predominantly located in the pre- and paravertebral fatty tissue. There were also areas of cuboidal cells on the pleural surface of the diaphragm. Some mesothelial pores and gaps were found, but no lymphatic stomata opened on the pleural surface of the diaphragm. The pleural surface of the lung and that of the heart were lined with flattened polygonal cells. The topography of the surface varied, but there were no mesothelial discontinuities of the type commonly found in the parietal pleura.
CONCLUSIONS: 1) The parietal pleura has a surface structure that is more permeable and absorptive for fluid and particulate matter than the visceral pleura. 2) The distribution of lymphatic stomata does not correspond directly to the pleural liquid pressures that have been reported. 3) The functions of lymphatic stomata should be considered not only in terms of fluid turnover but also in terms of self-defense mechanisms. 4) The presence or absence of lymphatic stomata on the diaphragmatic pleura should be re-examined and determined in a variety of animal species.