Muscular" ingrowth from body wall

Septum transversum

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.

originating in the body wall penetrate the membranes to form the muscular part of the diaphragm.

Thus the diaphragm is derived from the 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.

CLINICAL CORRELATES Diaphragmatic Hernias

A congenital diaphragmatic hernia, one of the more common malformations in the newborn (1/2000), is most frequently caused by failure of one or both of the pleuroperitoneal membranes to close the pericardioperitoneal canals. In that case the peritoneal and pleural cavities are continuous with one another along the posterior body wall. This hernia allows abdominal viscera to enter the pleural cavity In 85 to 90% of cases the hernia is on the left side, and intestinal loops, stomach, spleen, and part of the liver may enter the thoracic cavity (Fig. 10.7). The abdominal viscera in the chest push the heart anteriorly and compress the lungs, which are commonly hypoplastic. A large defect is associated with a high rate of mortality (75%) from pulmonary hypoplasia and dysfunction.

Occasionally a small part of the muscular fibers of the diaphragm fails to develop, and a hernia may remain undiscovered until the child is several years old. Such a defect, frequently seen in the anterior portion of the diaphragm, is a parasternal hernia. A small peritoneal sac containing intestinal loops may enter the chest between the sternal and costal portions of the diaphragm (Fig. 10.7 A).

Another type of diaphragmatic hernia, esophageal hernia, is thought to be due to congenital shortness of the esophagus. Upper portions of the stomach are retained in the thorax, and the stomach is constricted at the level of the diaphragm.

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