Digestive and Respiratory system

• The body folding process at the cranial, caudal and lateral edges of the embryonic disc during the 4th week results in the fusion of the endodermal, mesodermal and ectodermal layers of the embryonic disc along the ventral midline and transforms the flat embryonic endoderm into the gut tube
• Initially, this gut tube consists of a blind-ended cranial foregut and a blind-ended caudal hindgut. Both are separated by the future midgut which is, at that moment, still open to the yolk sac through the vitelline duct.
• Finally, when the edges of the ectoderm fuse along the ventral midline, the space embedded in the lateral plate mesoderm (also called the extra-embryonic coelom) is partially enclosed in the embryo. This enclosed cavity becomes the intra-embryonic coelom and is now completely separated from the extra-embryonic coelom.
• The somatic mesoderm coats the outer surface and the splanchnic mesoderm covers the inner surface of the intra-embryonic coelom with the serous membrane that lines the whole cavity.
• Further differentiation of the splanchnic mesoderm ensheathing the gut tube will result in the formation of connective tissue, blood vessels and smooth muscles and permits also the infiltration of migrating neural crest cells that will form the enteric nervous system.
• By the 5th week, the abdominal portion of the foregut divides into the esophagus, the stomach and the proximal duodenum.
• a budding process in the proximal duodenum forms the hepatic, cystic and dorsal and ventral pancreatic diverticula that grow further into the mesogastrium and will give rise, respectively, to the liver, gallbladder, cystic duct and pancreas
• The development of the spleen is not well understood. It is formed by the mesenchym in the dorsal mesogastrium but does not belong to the digestive system.
• During rotation of the stomach, the dorsal mesogastrium moves with, leading to formation of a pouchlike structure, the omental bursa (also known as the lesser sac). The dorsal mesogastrium subsequently undergoes voluminous expansion giving rise to the greater omentum that covers the inferior abdominal viscera and embeds the developing spleen and the tail of the pancreas. Meanwhile, the rapidly enlarging liver occupies an increasing large portion of the ventral mesentery. 
• Further differentiation of the midgut results in the formation of the distal duodenum, and the jejunum, ileum, cecum, the ascending colon as well as the proximal two thirds of the transverse colon.
• This anteroposterior hairpin fold herniates into the umbilical cord during the 6th week and rotates 900 counterclockwise around its long axis. As a result, the future ileum will situate in the right abdomen and the future large intestine will lie in the left abdomen. In the same period, the cecum and appendix differentiate and the jejunum and ileum continue to elongate.
• The hindgut differentiates into the distal one third of the transverse colon, the descending and sigmoid colon as well as into the rectum. It forms also the cloaca, just superior to the cloacal membrane. The cloaca is divided by a coronal urorectal septum during the 4th to the 6th week and splits into the urogenital sinus, which will give rise to urogenital structures, and into a dorsal anorectal canal. The distal one third of this canal forms an ectodermal invagination, the anal pit.
• The lumen of the gut tube is gradually recanalized between the 6th and 8th week and the developing epithelium layer together with mesodermal extensions projects into the lumen of the digestive tube to form the villi of the intestines. Cytodifferentiation of the epithelium depends on interactions with the underlying mesoderm and is regionally specified based on the cranial-caudal and radial axis.
• Between the 10th and 12th week, the intestinal loop is retracted into the abdominal cavity and turns 180 degrees through an additional counterclockwise rotation to produce the definitive configuration of the small and large intestine
• which three structures originate in the ventral mesentary → falciform ligament, omentum minor, visceral peritoneum
• Retraction of the intestinal loop into the abdominal cavity of the human embryo occurs at the end of the 3rd month. What are the reasons for this retraction:

  • degeneration of the pronephros
  • Retarded growth of the liver
  • enlargement of the abdominal cavity
• The lung bud (also called the respiratory diverticulum) is the first primordium of the lungs and develops on the 22th day as a ventral evagination of the foregut
• The lung bud grows further into the splanchnopleuric mesoderm, which will give rise to the lung vasculature and to the connective tissue, cartilage and smooth muscle around the bronchi.
• Between the 26th and 28th day, the growing lung bud bifurcates into the left and right primary bronchial buds, which initiate the formation of the two lungs. Both primary bronchial buds form further branches in the 5th week giving rise to three secondary bronchial buds, one on the right and two on the left side. These three further buds are the primordia of the future lung lobes.
• By the 28th week, the branching process generates the terminal bronchioles, which subsequently divide into two or more respiratory bronchioles. These respiratory bronchioles become invested with capillaries in the 36th week and form the primitive alveoli terminal bronchioles which mature between the 36th week and birth. Additional alveoli continue to be produced throughout early childhood
• Which organs originate from the caudal part of the foregut:

  • duodenum
  • gall bladder
  • liver
  • esophagus
  • pancreas
  • stomach
• The ductus cysticus and gall bladder originate from the cystic duct
• The ductus vitelline connects the open midgut with the yolk sac
• What evokes herniation of the primary intestinal loop → fast growth in lengt of the midgut and the mesentary
• What is the name of the septum that splits the cloaca into the urogenital sinus and the dorsal anorectal canal → septum urorectale
• Which organs develop consecutively in the primordium of the respiratory diverticulum? → trachea and lungs
• What phenomenon induces the formation of the curvatura major and minor of the stomach? → the back of the stomach grows faster than the initial front
• What does bursa omentum mean? → pocket from bulge of the peritoneum
• where is the bursa omentum situated? → between the stomach and pancreas
• The ligamentum falciforme is located between the ventral abdominal wall and liver
• What is the differens between the right and left lung? → the right lung has 3 lobes and the left lung has 2 lobes
• Which cavity surrounds the developing lungs? → pleural cavity
• Which digestive gland is responsible for the intraembryonic process of hematopoiesis? → liver