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