Development of digestive system, midgut & hindgut

1. Development of digestive system, midgut & hindgut By: Dr. Mujahid Khan

2. Midgut The derivatives of the midgut are: • The small intestine including most of the duodenum • The cecum, appendix, ascending colon and the right half to two-thirds of the transverse colon

3. Midgut • Midgut derivatives are supplied by the superior mesenteric artery • The midgut loop is suspended from the dorsal abdominal wall by an elongated mesentery • As the midgut elongates, it forms a ventral U-shaped loop of gut, the midgut loop • It projects into the remains of the extraembryonic coelom in the proximal part of the umbilical cord

5. Midgut • At this stage the intraembryonic coelom communicates with extraembryonic coelom at the umbilicus • This movement of the intestine is a physiological umbilical herniation • It occurs at the beginning of the 6th week • The midgut loop communicates with the yolk sac through the narrow yolk stalk or vitelline duct until the 10th week

7. Midgut • Umbilical herniation occurs because there is not enough room in the abdomen for the rapidly growing midgut • The shortage of space is caused mainly by the relatively massive liver and two sets of kidneys during this stage of development • The midgut loop has a cranial limb and a caudal limb • Yolk stalk is attached to the apex of the midgut loop where the two limbs join

9. Midgut • The cranial limb grows rapidly and forms small intestinal loops • The caudal limb undergoes very little change except for development of cecal diverticulum which is a primordium of the cecum and appendix

11. Rotation of Midgut Loop • While it is in the umbilical cord the midgut loop rotates 90º counterclockwise around the axis of superior mesenteric artery • This brings cranial limb to the right and the caudal limb to the left • During rotation the midgut elongates and forms intestinal loops e.g. Jejunum and ileum

13. Return of Midgut Loop to Abdomen • During the 10th week the intestines return to the abdomen • What causes the return of the intestine is not known • The decrease in the size of the liver and kidneys and the enlargement of the abdominal cavity are important factors • This process is called reduction of the physiological midgut hernia

15. Return of Midgut Loop to Abdomen • The small intestine formed from cranial limb returns first • It passes posterior to the superior mesenteric artery and occupies the central part of the abdomen • As the large intestine returns, it undergoes further 180º counterclockwise rotation • Later it comes to occupy the right side of the abdomen

16. Cecum and Appendix • The primordium of cecum and appendix, the cecal diverticulum appears in the 6th week as a swelling on the antimesenteric border of the caudal limb of the midgut • The apex of the cecal diverticulum does not grow as rapidly as the rest of it • The appendix is initially a small diverticulum of cecum

18. Cecum and Appendix • The appendix increases rapidly in length so that at birth it is a relatively long tube arising from the distal end of the cecum • After birth the wall of the cecum grows unequally with the result that appendix comes to enter the medial side • Appendix is considerably variant in position, retrocecal, retrocolic and pelvic appendix

19. Hindgut The derivatives of the hindgut are: • The left one-half of the transverse colon • Descending and sigmoid colons • Rectum and the superior part of the anal canal • The epithelium of the urinary bladder and most of the urethra

20. Hindgut • All hindgut derivatives are supplied by the inferior mesenteric artery • The junction between the segment of transverse colon derived from the midgut and that originated from the hindgut is indicated by the change in blood supply • Superior mesenteric artery is the midgut artery • Inferior mesenteric artery is the hindgut artery

21. Hindgut • The descending colon becomes retroperitoneal as its mesentery fuses with the peritoneum on the left posterior abdominal wall and then disappears • The mesentery of the sigmoid colon is retained but it is shorter than in the embryo

22. Cloaca • This terminal part of the hindgut is an endoderm-lined chamber that is in contact with the surface ectoderm at the cloacal membrane • This membrane is composed of endoderm of the cloaca and ectoderm of the proctodeum or anal pit • The cloaca, the expanded terminal part of the hindgut receives the allantois

24. Partitioning of Cloaca • The cloaca is divided into dorsal and ventral parts by a wedge of mesenchyme, the urorectal septum • It develops in the angle between the allantois and hindgut • As the septum grows toward the cloacal membrane , it develops forklike extensions that produce infoldings of the lateral walls of the cloaca

25. Partitioning of Cloaca • These folds grow toward each other and fuse to form a partition that divides the cloaca into two parts • The rectum and cranial part of the anal canal dorsally • The urogenital sinus ventrally

27. Partitioning of Cloaca • By the seventh week, the urorectal septum has fused with the cloacal membrane • Dividing it into a dorsal anal membrane and a larger ventral urogenital membrane • The area of fusion of the urorectal septum with the cloacal membrane is represented in the adult by the perineal body

28. Partitioning of Cloaca • The perineal body is a fibromuscular node and a landmark of perineum where several muscles converge and attach • The urorectal septum also divides the cloacal sphincter into anterior and posterior parts • The posterior part becomes the external anal sphincter • The anterior part develops into the superficial transverse perineal, bulbospongiosus and ischiocavernosus muscles

29. Partitioning of Cloaca • This developmental fact explains why one nerve, the pudendal nerve supplies all these muscles • The anal membrane usually ruptures at the end of the eighth week • This brings the distal part of the digestive tract into communication with the amniotic cavity

30. Anal Canal • The superior two-thirds of the adult anal canal is derived from the hindgut • The inferior one-third develops from the proctodeum • The junction between the epithelia of the two parts is indicated by the irregular pectinate line • This line is located at the inferior limit of the anal valves • It indicates the former site of anal membrane

32. Anal Canal • About 2cm superior to the anus is an anocutaneous line or white line • This demarcates where the anal epithelium changes from columnar to stratified squamous • At the anus, the epithelium is keratinized and continuous with the skin around the anus • Because of its hindgut origin the superior two-thirds of the anal canal are mainly supplied by the superior rectal artery

33. Anal Canal • Because of its origin from the proctodeum, the inferior one-third of the anal canal is supplied mainly by the inferior rectal arteries • The differences in blood supply, nerve supply, venous and lymphatic drainage of anal canal are important clinically, when considering the metastasis of cancer cells

34. Anal Canal • The characteristics of carcinomas in the two parts are different • Tumors in the superior part are painless and arise from columnar epithelium • Tumors in the inferior part are painful and arise from stratified squamous epithelium

35. Congenital Omphalocele • This anomaly is a persistence of the herniation of abdominal contents into the proximal part of the umbilical cord • Herniation of intestines into the cord occurs in about 1 in 5000 births • Herniation of liver and intestines in 1 of 10,000 births • Size of the hernia depends on its contents

36. Congenital Omphalocele • The abdominal cavity is proportionately small when there is an omphalocele • Immediate surgical repair is required • Omphalocele results from failure of the intestines to return to the abdominal cavity • The covering of the hernial sac is the epithelium of the umbilical cord which is a derivative of the amnion

37. Umbilical Hernia • When the intestines return back to the abdominal cavity during the 10th week and then herniate through an imperfectly closed umbilicus, an umbilical hernia forms • This is different from the omphalocele • In umbilical hernia the protruding mass is covered by subcutaneous tissue and skin • Hernia reaches its maximum size at the end of the first month after birth

38. Umbilical Hernia • It usually ranges from 1 to 5 cm • The defect through which the hernia occurs is the linea alba • Hernia protrudes during crying, straining, or coughing • It can easily be reduced through the fibrous ring at the umbilicus • Surgery is not usually performed until it persists to the age of 3 to 5 years

39. Meckel Diverticulum • This outpouching is one of the most common anomalies of the digestive tract • This congenital ileal diverticulum occurs in 2 to 4% of people • 3 to 5 times more prevalent in males than females • It sometimes becomes inflamed and causes symptoms that mimic appendicitis

41. Meckel Diverticulum • The wall of the diverticulum contains all layers of the ileum and may contain small patches of gastric and pancreatic tissues • The gastric mucosa often secretes acid, producing ulceration and bleeding • It is the remnant of the proximal part of the yolk stalk • It typically appears as a fingerlike pouch about 3 to 6 cm long

42. Meckel Diverticulum • It arises from the antimesenteric border of the ileum 40 to 50 cm from the ileocecal junction • It may be connected to the umbilicus by a fibrous cord or an omphaloenteric fistula

43. Congenital Megacolon • A part of the colon is dilated because of the absence of autonomic ganglion cells in the myenteric plexus distal to the dilated segment of colon • The enlarged colon has the normal number of ganglion cells • The dilation results from failure of peristalsis in the aganglionic segment • In most cases only rectum and sigmoid colon are involved

44. Congenital Megacolon • It is the most common cause of neonatal obstruction of the colon • Accounts for 33% of all neonatal obstruction • Males are affected more often than females • It results from failure of neural crest cells to migrate into the wall of the colon during the 5th and 6th weeks • This results in failure of parasympathetic ganglion cells to develop in Auerbach plexuses

45. Imperforate Anus • It occurs about once in every 5000 newborn infants • More common in males • Most anorectal anomalies result from abnormal development of the urorectal septum • Results due to incomplete separation of the cloaca into urogenital and anorectal portions

46. Imperforate Anus • There is normally a temporary communication between rectum and anal canal dorsally, from the bladder and urethra ventrally • It closes when the urorectal septum fuses with the cloacal membrane • Lesions are classified as low or high depending on whether the rectum ends superior or inferior to the puborectalis muscle

47. Imperforate Anus Following are the low anomalies : • Anal agenesis with or without a fistula • Anal stenosis • Membranous atresia of anus • Anorectal agenesis with or without fistula