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Intraembryonic Coelom

The primitive intraembryonic coelom forms in the lateral and cardiogenic mesoderm about the fourth week of development.  The embryo undergoes two foldings and this cavity is eventually divided into the pericardial, pleural, and peritoneal embryonic body cavities.

During the fourth week the septum transversum grows to separate the pericardial cavity from the pleural cavities.  During the sixth week the pleuroperitoneal membranes grow to separate the pleural cavities from the peritoneal cavity.  During the seventh week the pleuropericardial membranes separate the pericardial cavity from the pleural cavities.  In the adult the pleuropericardial membranes form the fibrous pericardium of the heart.

Diaphragm

The diaphragm separates the thoracic and abdominal cavities.  It arises from tissue from four sources:

·    The septum transversum, which forms the central tendon of the diaphragm.

·    The pleuroperitoneal membranes, which contribute only a small amount to the adult diaphragm

·    The dorsal mesentery of the esophagus, which forms the crura and median portion of the diaphragm

·    The body wall, which forms the periphery of the diaphragm

The diaphragm develops initially at the level of cervical somites 3-5 and it “descends” to the level of L1 as the embryo grows.  As it moves it takes along its innervation, which explains why the phrenic nerve arises from cervical roots three, four, and five (“C3-4-5 keeps a man alive.”)

Clinical Correlations

Congenital Diaphragmatic Hernia

Defective formation of the pleuroperitoneal membranes and/or their failure to fuse with the dorsal mesentery of the esophagus and the septum transversum results in a congenital posterolateral defect of the diaphragm. This means that the intestines pass into the thorax, sometimes accompanied by the stomach and spleen. The lungs are often compressed and hypoplastic, impairing the initiation of respiration. The defect is much more likely to occur on the left side of the diaphragm, away from the large embryonic liver.

Esophageal Hiatial Hernia

An abnormally large esophageal hiatus can allow the stomach to herniate into the pleural cavity.  This incapacitates the physiological lower esophageal sphincter, allowing the contents of the stomach to reflux into the esophagus.

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