DefinitionThe diaphragm is a dome-shaped musculo-fibrous septum that separates the thoracic and abdominal cavities. The word “diaphragm” is Greek and means a wall of separation or closure or cutting (Mosby's, 2008). This muscolofibrous structure is very important, if not the most important muscle in the body with regard to its main function, which is the involuntary action of breathing. Embryonic Origins of the Diaphragm The diaphragm is a composite structure derived from several embryonic components. It is formed between the 4th and 10th week of embryonic life and develops from four embryonic components, namely: the septum transversum, pleuroperitoneal membranes, dorsal mesentery and body wall (Figure 1). The septum transversum forms the central tendon of the diaphragm and separates the developing pericardium (ventrally) from the developing intestine (dorsally). The dorsal mesentery is a double layer of peritoneum that forms the middle part of the diaphragm. The branches of the diaphragm develop from muscle fibers that grow into the dorsal mesentery during the ninth to twelfth week (Moore, 2008). The pleuroperitoneal membranes form the posterolateral elements of the diaphragm (peripheral muscular part of the diaphragm). Although the pleuroperitoneal membranes make up large parts of the primary diaphragm, they represent small parts of the definitive diaphragm. The fourth embryonic component, the body wall, constitutes the peripheral part of the diaphragm. This occurs because, as the lungs and pleural cavities enlarge and invade the body wall, the tissue is divided into an outer layer which becomes the body wall and an inner layer which forms the peripheral part of the diaphragm. The process...... middle of article...... essential role of Gab1 for c-Met receptor signaling in vivo. J Cell Biol 150:1375-1384. Brohmann et al. 2000. The role of Lbx1 in muscle precursor cell migration. Curr Opin Cell Biol 12:725-730. Dietrich S et al. 1999. The role of SF/HGF and c-Met in skeletal muscle development. Development 126:1621-1629. Ackerman KG, Greer JJ. 2007. Development of the diaphragm and mouse genetic models of diaphragmatic defects. Am J Med Genet Part C Semin Med Genet 145c: 109-116. Ackerman KG, Herron BJ, Vargas SO, Huang H, Tevosian SG, et al. (2005) Fog2 is required for normal development of the diaphragm and lungs in mice and humans. PLoS Genet 1(1): e10. doi:10.1371/journal.pgen.0010010Clugston et al. 2008. Gene expression in the developing diaphragm: importance for congenital diaphragmatic hernia. AJP-Lung Physiol vol.294. n°4 L665-L675.