Gastrointestinal Tract (GUT)The gut is endodermal in origin and is derived from yolk sac. Developing gut is divided into:-
- Foregut:- Artery of foregut is coeliac trunk. Foregut derivatives of GIT are esophagus, stomach, duodenum (up to opening of main pancreatic duct), liver and pancreas.
- Midgut:- Artery of midgut is superior mesenteric artery. GIT derivatives of midgut are duodenum (distal to opening of main pancreatic duct), small intestine (jejunum, ileum), appendix, cecum, ascending colon and proximal (right) 2/3 of transverse colon.
- Hindgut:- Artery of hindgut is inferior mesenteric artery. Hindgut derivatives are distal (left) 1/3 of transverse colon, descending colon, proximal part of rectum. Caudal part of hind gut is called cloaca. Distal part of rectum and upper part of canal are developed from endodermal cloaca.
Abdominal part of foregut is suspended by mesenteries both ventrally (ventral mesogastrium) and dorsally (dorsal mesogastrium). The ventral mesogastrium divides into a ventral part of ventral mesogastrium forms falciform Iigament, right and left triangular ligaments, and superior and inferior layers of coronary ligament. Dorsal part of ventral mesogastrium forms lesser omentum. The Dorsal mesogastrium has large caudal part and small cranial part. Caudal part forms greater omentum. Cranial part divides into ventral part (forms gastrosplenic Iigament) and dorsal part (forms linorenal ligament). The cranial most part forms gastrophrenic ligament.
The midgut and hindgut have only a dorsal mesentery, which forms mesentery of jejunum and ileum (mesentery proper), transverse mesocolon and sigmoid colon. The mesenteries of duodenum, ascending colon, descending colon and rectum are lost during development.
Rectum and anal canal
- The rectum has two developmental parts. The upper part develops from hindgut and lies above third transvers fold (Houston's valve). Lower part (below Houstonâ€™s valve) develops from dorsal part of endodermal cloaca (cloaca is the part of hindgut) caudal to the attachment of allantoic diverticulum).
- Part of anal canal above pectinate line develops from dorsal part of endodermal cloaca (primitive rectum) and part of anal canal below pectinate line develops from ectodermal proctodeum.
Before the formation of the tail fold of the embryo, a small endodermal diverticulum called the allantoic diverticulum arises from yolk sac near its caudal end. Yolk sac proper (proximal to allantoic diverticulum) forms the gut, i.e. foregut, midgut and hindgut (see above). The part of hindgut caudal to attachment of allantoic diverticulum is called cloaca. Urorectal septum divides cloaca into urogenital sinus (ventral part) and primitive rectum (dorsal part). Urogenital sinus gives rise to some part urogenital system and primitive rectum forms lower part of rectum and upper part of anal canal.
Remember Primitive midgut communicates ventrally with yolk sac (extraembryonic part) by vitellointestinal duct.
Meckel's diverticulum refers to persistent proximal part of vitello intestinal duct.
Septum transversumSeptum transversum is formed by mesoderm (mesenchyme) lying cranial to intraembryonic coelom. The diaphragm and liver develop in relation to septum transversum. It is penetrated by hepatic diverticulum (liver bud). Structures derived from septum transversum are-
- Ventral mesogastrium; it gives rise to all ligaments of liver (except ligamentum teres): falciform ligament, right and left triangular ligaments, superior and inferior layers of coronary ligaments, and lesser omentum.
- Hematopoietic cells, kupffer cells, sinusoids of liver, capsule and fibro-areolar stroma of liver.
- Central tendon of diaphragm, epicardium, fibrous pericardium, esophageal mesentery.
Liver Gall bladder and biliary apparatus
- Liver, gall bladder and biliary apparatus develop from an endodermal diverticulum that arises from ventral aspect of terminal part of foregut. This diverticulum grows into ventral mesogastrium and reaches septum transversum and divides into two parts:-
- Pars hepatica is larger cranial part and forms liver.
- Pars cystica is smaller caudal part and forms gall bladder and cystic duct.
- Ligaments of liver are formed by septum transversum except ligamentum teres, which is formed by left umbilical Vein.
Pancreas is developed from the two pancreatic buds.
- Dorsal pancreatic bud:- It is larger and most of the pancreas is derived from it i.e. most of the head, and whole neck, body & tail.
- Ventral pancreatic bud:- It is smaller and forms lower part of the head of pancreas including uncinate process.
- During 7th week of development, the ventral and dorsal pancreatic buds fuse to form a single pancreatic mass.
- After the fusion of ventral and dorsal pancreatic buds, their ducts develop cross communications. Final duct system is formed as below -
- Main pancreatic duct (Duct of wirsung) is formed by the duct of ventral bud, distal part of duct of dorsal bud and an oblique communication between the two. The main pancreatic duct joins the bile duct to form hepatopancreatic ampulla that enters the 2nd part of duodenum at major duodenal papilla.
- Accessory pancreatic duct is formed by the proximal part of the duct of dorsal bud. It opens into 2nd part of duodenum at minor duodenal pupilla, 2 cm proximal (cranial) to major duodenal papilla.
- Anomalies of pancreatic development may be:
- Annular pancreas:- Two components of the ventral bud fail to fuse and grow in opposite direction around the duodenum and meet the dorsal pancreatic duct.
- Pancreatic divisum (divided pancreas):- Ventral and dorsal buds fail to fuse with each other. It is the most common congenital anomaly of pancreas.
- Inversion of pancreatic duct:- The main pancreatic duct is formed by the duct of dorsal bud, i.e. accessory duct is larger than the main duct and the main drainage of pancreas is through the minor duodenal papilla.
- Accessory pancreatic tissue:- May be found in
- Wall of stomach, duodenum, jejunum or ileum.
- Meckel's diverticulum