The diaphragm develops from four sources: the septum transversum, the pleuroperitoneal membranes, muscle tissue from the body wall, and mesenchyme from the region of the oesophagus.
At S12, the primordium of the septum transversum is initially at the level of C4 and is disposed almost frontally. It is shifted caudally with the descent of the heart (and the liver). At S19, the septum transversum already lies at the level of Th12 and is disposed transversally (due to changes in position and size of the heart and liver). The part of the septum transversum located directly beneath the heart gives rise to the sternal part, the costal part (together with the material coming from the body wall), and a part of the central tendon. As the septum transversum initially lies at the level of the cervical somites, it is innervated from fibres of the 3rd, 4th and 5th spinal nerves that together form the phrenic nerve. The phrenic nerve initially runs in the pleuropericardial membrane.
The pleuroperitoneal membrane grows from dorsal and lateral directions towards the septum transversum and fuses together with it. It becomes elongated in ventral and lateral directions due to the extension of both the lungs and the pleural cavities. It contributes to a small part of the lumbocostal triangle.
Muscle tissue from the body wall penetrates into the lengthened pleuroperitoneal membrane and, together with the septum transversum, gives rise to the costal part of the diaphragm.
The crura of the diaphragm develop from the mesenchyme around the oesophagus (lumbar part with right and left crura).