Figure 10.5 A. Transformation of the pericardioperitoneal canals into the pleural cavities and formation of the pleuropericardial membranes. Note the pleuropericardial folds containing the common cardinal vein and phrenic nerve. Mesenchyme of the body wall splits into the pleuropericardial membranes and definitive body wall. B. The thorax after fusion of the pleuropericardial folds with each other and with the root of the lungs. Note the position of the phrenic nerve, now in the fibrous pericardium. The right common cardinal vein has developed into the superior vena cava.
the sinus venosus shift the common cardinal veins toward the midline, and the pleuropericardial membranes are drawn out in mesentery-like fashion (Fig. 10.5 A). Finally, they fuse with each other and with the root of the lungs, and the thoracic cavity is divided into the definitive pericardial cavity and two pleural cavities (Fig. 10.5B). In the adult, the pleuropericardial membranes form the fibrous pericardium.
Although the pleural cavities are separate from the pericardial cavity, they remain in open communication with the abdominal (peritoneal) cavity, since the diaphragm is incomplete. During further development, the opening between the prospective pleural and peritoneal cavities is closed by crescent-shaped folds, the pleuroperitoneal folds, which project into the caudal end of the pericardioperitoneal canals (Fig. 10.6A). Gradually the folds extend medially and ventrally so that by the seventh week they fuse with the mesentery of the esophagus and with the septum transversum (Fig. 10.6B). Hence the connection between the pleural and peritoneal portions of the body cavity is closed by the pleuroperitoneal membranes. Further expansion of the pleural cavities relative to mesenchyme of the body wall adds a peripheral rim to the pleu-roperitoneal membranes (Fig. 10.6C). Once this rim is established, myoblasts
Figure 10.6 Development of the diaphragm. A. Pleuroperitoneal folds appear at the beginning of the fifth week. B. Pleuroperitoneal folds fuse with the septum transversum and mesentery of the esophagus in the seventh week, separating the thoracic cavity from the abdominal cavity. C. Transverse section at the fourth month of development. An additional rim derived from the body wall forms the most peripheral part of the diaphragm.
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