Pleuroperitoneal canal closure and the fetal adrenal gland

Pleuroperitoneal canal (PP canal) closure is generally considered to result from an increase in the height, and subsequent fusion, of the bilateral pleuroperitoneal folds (PP folds). However, the folds develop in the area ventral to the adrenal, in contrast to the final position of the diaphragm, which extends to the dorsal side of the adrenal (the "retro-adrenal" diaphragm). We examined the semiserial histology of 20 human embryos and fetuses (crown-rump length 11-40 mm). We started observations of the canal at the stage through which the lung bud extends far caudally along the dorsal body wall to the level of the future adrenal, and the phrenic nerve has already reached the PP fold. Subsequently, the developing adrenal causes narrowing of the dorsocaudal parts of the canal, and provides the bilateral midsagittal recesses or "false" bottoms of the pleural cavity. However, at this stage, the PP fold mesenchymal cells are still restricted to the ventral side of the adrenal, especially along the liver and esophagus. Thereafter, in accordance with ascent of the lung, possibly due to anchoring of the liver to the adrenal, the PP fold mesenchymal cells seem to migrate laterally along the coelomic mesothelium covering some sheet-like loose mesenchymal tissue behind the adrenal. Final closure of the PP canal by lateral migration to provide the "retro-adrenal" diaphragm is a process quite different from the common dogma. It is likely that the sheet-like loose mesenchymal tissue becomes the caudal part of the pleural cavity through a process involving cell death.