The Postnatal Development of the Mouse Pericardium; the Time and Mechanism of Formation of Pericardial Pores

The mouse pericardium is a continuous serous membrane, up to postnatal day 6 (P6), that consists of pericardial and pleural mesothelia with sparse connective tissue sandwiched in between. This study shows that on P7 the pericardium becomes fenestrated, and the pericardial and pleural cavities become continuous. We found that the number of pores per pericardium increased continuously with the advance of age and reached as many as 3000 on P21. Transmission electron microscopy of the pericardium revealed various structures of the pericardial and pleural mesothelia that may relate to the formation of the pericardial pores. The pericardial and pleural mesothelia were adjoined with cytoplasmic processes that extended toward the base of the opposing mesothelium. As a result of the adjoinment, a pair of U-shaped folds were formed. The two U-shaped folds were connected with tight junctions at their apices, and separation of these junctions may give rise to the pericardial pores.