Pleuroperitoneal canal development and closure were studied with light microscopy and transmission and scanning electron microscopy in 12.75- to 16-day fetuses. The major chronological events described in this paper are (1) the caudal tips of the lung buds projecting to the pleuroperitoneal canal (12.75 through 13.50 days); (2) the caudal tips of the lungs becoming situated medial to the canal areas at 14 days; and (3) both canals becoming crescent shaped with a uniform diameter until closure. Concurrently, the developing diaphragm and associated pleuroperitoneal folds assume more caudal positions. Both canal regions are bordered by the liver, lung, gonadal ridge, and suprarenal glands. In addition, on the left side, the stomach and mesogastrium also border the early canal. The right canal closes before the left (right, 14.75–15 days; left, 15–15.25 days).
The results suggest that the pleuroperitoneal folds are pushed together, thereby closing the canals. This may be accomplished by one or a combination of the following: (1) enlargement of the liver pushing the ventral fold dorsad and a molding of the liver to the dorsal body wall caudal to the canal; (2) liver and thorax enlargement which appears to pull the dorsal fold taut against the central fold; and (3) a change in the orientation of the canal near the time of closure. Each canal is fully closed by the mergence of the dorsal and ventral fold mesothelia and mesenchyme. This study provides a basis for relating pleuroperitoneal canal development and closure to the surrounding organs and tissues.