ABSTRACT
The diaphragm develops first as the septum transversum, which is a sheet of mesodermal tissue separating the thoracic cavity and the stalk of the yolk sac. This initially lies opposite the primitive cervical vertebrae and, as a consequence, the phrenic nerves are derived from the third, fourth and fifth cervical segments. The septum, however, is incomplete, with two large posterolateral canals in which the lung buds initially form. The primitive diaphragm then migrates caudally, creating the potential pleural space. The pleuroperitoneal canals are closed by the development of the pleuropericardial folds. The folds develop first as small ridges and, by the seventh week of intrauterine life, fuse with the septum transversum and the mesentery of the abdominal cavity. Primitive muscle cells then migrate in from the body wall to form the muscular part of the diaphragm.
