By: Dr. Mujahid Khan

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Following are the low anomalies :

Anal agenesis with or without a fistula

Anal stenosis

Membranous atresia of anus

Anorectal agenesis with or without fistula