The development of the diaphragm and thoracic cavity explained in 15 minutes.
If you are completely new to embryology and you want to understand it quickly, this should be the first video you watch:
- kzfaq.info/get/bejne/opuXiKSDzb67hI0.html
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SUMMARY OF THE VIDEO FOR YOUR NOTES
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Thoracic cavity
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Septum transversum is a thick plate of mesoderm that lies between the thoracic cavity and the stalk of the yolk sac.
The septum transversum comes from visceral lateral plate mesoderm. Its position changes as the fetus grows, curving into the fetal position
This septum isn’t enough to form the diaphragm, as it leaves an open communication between the thoracic and the peritoneal cavity. The canals are called the pericardioperitoneal canals. There is one canal on each side of the GIT.
When the lungs begin to grow, from their humble beginnings of “buds”, they fill the thoracic cavity by growing laterally, and inferiorly, into these canals.
The pleuropericardial folds then appear, as growths into the thoracic cavity. Then, mesoderm of the body wall contribute to the formation of the pleuropericardial membranes. The pleuropericardial membranes are extensions of the pleuropericardial folds, which now contain the common cardinal veins and the phrenic nerves.
As the heart develops, shifting downward, the common cardinal veins move medially, and the pleuropericardial membranes finally fuse with each other and with the root of the lungs. Thus the thoracic cavity is now divided into the pericardial cavity and the two pleural cavities
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Diaphragm
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The diaphragm is made from 4 parts
1. Septum transversum - central tendon
2. Pleuroperitoneal membranes x2
3. C3,c4,c5 somites to form the muscles of the diaphragm
4. Dorsal mesentery of oesophagus in which crura of diaphragm develop
The pleural cavities have now separated from the pericardial cavity, but it still is in open communication with the peritoneal cavity via the pericardioperitoneal canal. These canals are closed by the pleuroperitoneal folds, which grow downwards into the canals. They keep growing towards the middle and the front of the body, and by week 7 they fuse with the dorsal mesentery of the oesophagus and the septum transversum. The division between pleural and peritoneal cavities are completed by the pleuroperitoneal membranes. As the pleural cavity expands, the mesoderm of the body wall adds a muscular component to the pleuroperitoneal membranes. This muscular component is derived from myoblasts, originating from somites at c3-5. This is the muscular part of the diaphragm.
In week 4, septum transversum lies opposite to cervical segments of the spinal cord, therefore the nervous innervation of the septum transversum / diaphragm is from these levels, of C3-5. These are the phrenic nerves. The phrenic nerves pass into the septum transversum via the pleuropericardial folds. This explains why the diaphragm is innervated by cervical nerves, even though its final position will be opposite the thoracic level of the spine, in the fibrous pericardium. The phrenic nerves provide sensory and motor control to diaphragm. However, since the most peripheral components of the diaphragm are developed from thoracic tissue, the intercostal/thoracic nerves contribute some sensory fibres.