9.15.11+-+Division+of+the+Primitive+Body+Cavity

=Division of the Primitive Body Cavity: Formation of the Diaphragm; Establishment of the Major Systems and Structures=


 * 1) Define**
 * **Epicardium (visceral serous pericardium)** - A layer of visceral serous pericardium formed of splanchnic mesoderm that forms the inside of the pericardial cavity.
 * **Fibrous pericardium** - The outermost layer (most superficial layer) of the pericardium. It is formed from pleuropericardial membranes.
 * **Parietal Serous pericardium** - It is the layer of pericardium that is immediately superficial to the pericardial cavity, but it is deep to the fibrous pericardium. The parietal serous pericardium is derived from somatic mesoderm.
 * **Pleuropericardial membranes** - The pleuropericardial membranes fuse dorsally to the heart tubes to form the fibrous pericardium, thus separating the pleural cavities from the pericardial cavity. They also fuse the mesenteries of the esophagus to the septum transversum.
 * **Pleuroperitoneal membranes** - They separate the pleural cavities from the peritoneal cavity
 * **Visceral pleura and p**arietal pleura - These are the inner and outer layers of the pleura, which forms the pleural cavity around the pulmonary cavity. The visceral pleura is formed from splanchnic mesoderm, while the parietal pleura is formed from somatic mesoderm.
 * **Visceral Peritoneum and Parietal Peritoneum**
 * **Nephrogenic Cords and Urogenital Ridges** - The nephrogenic cords are formed on either side of the notochord from intermediate mesoderm. The urogenital ridges are formed when the nephrogenic cords bulge down into the peritoneal cavity carrying more intermediate mesoderm with them.
 * **Mediastinum (splanchnic mesoderm mass separating the lungs)-** The mass includes the superior and inferior mediastinum (inferior is further divided into A/M/P). Highlights include heart, pericardium, esophagus, major vessels, bronchii/trachea, phrenic and vagus nerves.
 * **Falciform ligament-** The ligament dividing the liver into the right and left lobes (inferior portion called the round ligament of the abdomen). Connected to the diaphragm, posterior and anterior walls of the abdomen. Embryologically suspends the liver from the ventral wall. Derived from the septum transversum.
 * **Lesser Omentum -** Mesenteric connection between lesser angle of the stomach and duodenum to the liver. Two ligaments make up lesser omentum, the hepatoduodenal ligament and the hepatogastric ligament. Embryologically connects the ventrally suspended liver to the foregut. Derived from the septum transversum.
 * **Central Tendon and "bare area" of the liver** - The bare area of the liver is a region of liver not covered with a mesentery. It is attached to the central tendon of the diaphragm, the tendonous aponeurosis that forms the middle of the diaphragm.

At this stage, the heart is going from two heart tubes to one. As the heart matures, the heart tubes develop into the endocardium lining of the vessels of the heart (the inner lining). The cells that condense into the endocardial heart tubes are prochordal mesoderm, a specialized region of the lateral mesoderm, ventral to the IEC, so also splanchnic mesoderm. To boil it down: lateral splanchnic prochordal mesoderm condenses into two parallel heart tubes by day 21.
 * 2) Describe the origins of the heart: endocardial heart tubes**

Each heart tube is connected to: 1 Dorsal Aorta 1 Sinous Vinosus

Each Dorsal Aorta is connected to: 1 Umbilical Artery 1 Vitaline (Yolky) Artery 3 Intersegmental Arteries

Each Sinous Vinosus is connected to: 1 Umbilical Vein 1 Vitelline Vein 1 Cardinal Vein (branches into anterior and posterior)

The myocardium is derived from splanchnic mesoderm. It is the thickest layer of the heart and makes up most of its mass. It forms on the lateral sides of the heart tubes as they fuse into one. It becomes associated with the heart tubes The visceral pericardium is formed from the cells lining the outside of the splanchnic mesoderm wrapping of the heart tubes as they migrate into the IEC. At the base of the heart it is continuous with the parietal pericardium which lines the inner edge of the pericardium. The mass of the pericardium is formed by the fibrous pericardium. The fibrous pericardium develops from the pleuropericardial membranes, outgrowths of the somatic mesoderm that meet at the mediastinum with splanchnic mesoderm. The fibrous pericardium is lined on its superficial surface by parietal pleura. The fibrous pericardium carries with it the bilateral phrenic nerves (laterally) and cardinal veins (medially).
 * 3) Describe the myocardium, the visceral pericardium and the Fibrous pericardium**

The larygotracheal diverticulum (an out pocketing of the foregut) branches into splanchnic mesoderm proximal to the pericardioperitonaeal canals. The lung bud quickly divides into bronchial buds that push into the canals brining splanchnic mesoderm with them. They continue to expand into the canals and are eventually divided from the pericardium by the outgrowing outgrowing pleuropericardial membranes. The embryonic lungs are interiorly lined by foregut derived endoderm and externally lined by splanchnic mesoderm derived visceral serous pleura, which also serves as the inner lining of the pleural cavities. The outer surface of the pleural cavities is somatic mesoderm derived parietal pleura.
 * 4) Describe the origins of the embryonic lungs**

Already described, pleuropericardial membranes --> fibrous pericardium
 * 5) Describe the separation of pericardial cavity from the pleural cavities**

The separation of the pleural cavities (where the IEC folds over the septum transversum) from the peritoneal cavities (the caudal IEC) occurs dorsal to the septum transversum. It is achieved by the outgrowth of the somatic mesoderm derived pleuraperitoneal membranes. Each membrane crosses the IEC moving medially to fuse with the medial splanchnic mesoderm (which contains the now merged descending aorta, esophagus, and inferior vena cava). The cavities are now separated.
 * 6) Describe the separation of the pleural cavities from the peritoneal cavities**

Muscles from the body wall then grow over pleuroperitoneal membrane to create the primitive diaphragm: pleuroperitoneal membrane, septum transversum, body wall muscles, and esophageal mesenteries
 * 7) Describe the formation of the diaphragm (4 tissue types contributing to it)**

The septum transversum is originally located at C3 and so is innervated by the C3-C5 cervical plexus derived phrenic nerve. But as the embryo grows, the ventral side grows less than the dorsal side. This causes the dorsal edge of the diaphragm to descend dramatically (to approximately the L1 vertebrae). The ventral side also descends, just not as dramatically. The innervation is set though, so the phrenic extends to continually innervate its structure, the diaphragm.
 * 8) Describe how the diaphragm changes position and why the phrenic nerve in an adult human is so long**

If the pleuraperitoneal membrane doesn't grow across the IEC fully to merge with the esophageal mesentery, then the muscles of the body wall never extend out to complete the primitive diaphragm. If this occurs, then as the gut develops and the intestines lengthen, they will take what space is given them and expand into the pleural cavity. This leads to underdeveloped lungs.
 * 9) Describe a congenital diaphragmatic hernia**

The original dorsal mesentery persists into maturation. It continues to suspend the gut in the peritoneal cavity. The original ventral mesentery breaks down to form the definitive peritoneal cavity (only 1).
 * 10) What is the fate of the original dorsal and ventral mesenteries of the peritoneal cavity**

It buds from the area of the foregut that will develop into the duodenum as the hepatic diverticulum. It expands directly into the septum transversum. The liver is large due to its important early role in hematopoiesis.
 * 11) What is the origin of the embryonic liver**

The liver becomes so large that it quickly can no longer be contained by the septum transversum and emerges caudally carrying with it a layer of septum transversum splanchnic mesoderm. This bilayer of mesoderm becomes a mesentery, specifically a ventral mesentery (defined as such because ventral to gut) that extends from the ventral side of the gut to the ventral wall and containing the liver. The section of ventral mesentery between ventral wall and liver is called falciform ligament. Section of ventral mesentery between liver and foregut called lesser omentum.
 * 12) Describe the formation of the ventral mesenteries that attach to the liver (falciform ligament, lesser omentum)**

4 Body Cavities
 * 13) What are the origins of the visceral (splanchnic) and parietal (somatic) serous linings of each of the body cavities**

1 Pericardium Cavity Visceral Pericardium - external surface of splanchnic mesoderm surrounding prochordal heart tubes that dive into pericardial cavity of IEC Parietal Pericardium - internal surface of somatic mesoderm extension of pleuropericardial membranes, dividing pleural cavities from pericardial cavity

2 Pleural Cavities Visceral Pleura - external surface of splanchnic mesoderm surrounding laryngotracheal diverticulums diving into pericardioperitoneal canals Parietal Pleura - internal surface of somatic mesoderm of body wall and fibrous pericardium formed by pleuropericardial membranes

1 Peritoneal Cavity Visceral Pleura - external surface of splanchnic mesoderm surrounding gut and diverticulums Parietal Pleura - internal surface of somatic mesoderm lining abdominal cavity

The urogenital system. It forms two long cords called nephrogenic cords that press themselves against the dorsal surface of the peritoneal cavity but do not break the surface.
 * 14) What does the intermediate mesoderm form (general)**


 * 15) What are the derivatives of the endoderm**


 * Most of the quantity of these organs is mesoderm b/c of blood vessels*

1. Thyroid (parenchyma) 2. Lungs (inner lining, respiratory epithelium) 3. Liver (hepatic core to liver) A. Pancreas B. Gallbladder

4. Esophagus 5. Stomach 6. Duodenum 7. Jejunum 8. Ilium 9. Colon (cecum, ascending, transverse, descending, sigmoid) 10. Rectum