Friday, June 19, 2009
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 pleuroperitoneal
membranes (Fig. 10.6C ). Once this rim is established, myoblasts
membranes (Fig. 10.6C ). Once this rim is established, myoblasts
originating in the body wall penetrate the membranes to form the muscular part of the diaphragm.Thus the diaphragm is derived fromthe following structures: (a) the septum transversum, which forms the central tendon of the diaphragm; (b) the two pleuroperitoneal membranes; (c) muscular components from the lateral and dorsal body walls; and (d) the mesentery of the esophagus, in which the crura of the diaphragm develop (Fig. 10.6C ).
Initially the septum transversum lies opposite cervical somites, and nerve components of the third, fourth, and fifth cervical segments of the spinal cord grow into the septum. At first the nerves, known as phrenic nerves, pass into the septum through the pleuropericardial folds (Fig. 10.4B). This explains why further expansion of the lungs and descent of the septum shift the phrenic nerves that innervate the diaphragm into the fibrous pericardium (Fig. 10.5). Although the septum transversum lies opposite cervical segments during the fourth week, by the sixth week the developing diaphragm is at the level of thoracic somites. The repositioning of the diaphragm is caused by rapid growth of the dorsal part of the embryo (vertebral column), compared with that of the ventral part. By the beginning of the third month some of the dorsal bands of the diaphragm originate at the level of the first lumbar vertebra. The phrenic nerves supply the diaphragm with its motor and sensory innervation. Since the most peripheral part of the diaphragm is derived from mesenchyme of the thoracic wall, it is generally accepted that some of the lower intercostal (thoracic) nerves contribute sensory fibers to the peripheral
part of the diaphragm.
Initially the septum transversum lies opposite cervical somites, and nerve components of the third, fourth, and fifth cervical segments of the spinal cord grow into the septum. At first the nerves, known as phrenic nerves, pass into the septum through the pleuropericardial folds (Fig. 10.4B). This explains why further expansion of the lungs and descent of the septum shift the phrenic nerves that innervate the diaphragm into the fibrous pericardium (Fig. 10.5). Although the septum transversum lies opposite cervical segments during the fourth week, by the sixth week the developing diaphragm is at the level of thoracic somites. The repositioning of the diaphragm is caused by rapid growth of the dorsal part of the embryo (vertebral column), compared with that of the ventral part. By the beginning of the third month some of the dorsal bands of the diaphragm originate at the level of the first lumbar vertebra. The phrenic nerves supply the diaphragm with its motor and sensory innervation. Since the most peripheral part of the diaphragm is derived from mesenchyme of the thoracic wall, it is generally accepted that some of the lower intercostal (thoracic) nerves contribute sensory fibers to the peripheral
part of the diaphragm.
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