Development Of The GI system

Question 1

What is the midgut?

Answer 1

Begins inferior to the major duodenal papilla & ends at the junction between the 2/3 & distal 1/3 of the transverse colon.

• Duodenum (inferior to the major duodenal papilla),
• Jejunum,
• Ileum,
• Cecum,
• Appendix,
• Ascending colon, and
• Right two-thirds of the transverse colon.

Question 2

What are the supplies and drainages of the midgut?

Answer 2

Arterial supply : Superior mesenteric artery (SMA)

Venous drainage: Superior mesenteric vein

Lymphatic drainage : Superior mesenteric nodes

Question 3

Describe the medgut development

Answer 3

Midgut elongates to form a U shaped loop which is suspended by the dorsal mesentery. It communicates with the umbilical vesicle (yolk sac) via the narrow omphaloenteric (vitelline) duct . Ventrally the loop projects into the umbilical cord [6th week→physiological umbilical hernia – because peritoneal cavity is smaller]

Question 4

Describe the midgut loop function

Answer 4

Midgut loop has cranial and caudal limbs. The cranial limb grows rapidly and gives rise to the distal part of the duodenum, jejunum, and part of the ileum. The caudal limb gives rise to the rest of the ileum, cecum, appendix, ascending colon and proximal 2/3 of the transverse colon.

Question 5

Describe the first rotation of the midgut

Answer 5

Within the umbilical cord midgut loop rotates 90 degrees anticlockwise around the axis of superior mesenteric artery

Rotation brings cranial limb to the right & caudal limb to the left

Question 6

Describe the second midgut rotation

Answer 6

10Th week midgut returns to the abdominal cavity

Further 90 degrees anti-clockwise rotation occurs as the midgut
returns to the abdomen. Cranial limb returns first, occupies the
central and left part of abdomen

Question 7

Describe the third midgut loop rotation

Answer 7

Another rotation of the viscera for 90 degrees for a total of 270 degrees.

Large intestine returns to abdomen and occupies right side.

Cecum and appendix descend to right lower quadrant of the abdomen

Question 8

What is the impact of non-rotation of the midgut?

Answer 8

Non-rotation – caudal limb returns first →small intestine lies to the right, generally asymptomatic

Question 9

What is the impact of reversed midgut rotation?

Answer 9

Reversed rotation – midgut loop rotates in a clockwise direction. Duodenum lies anterior to transverse colon posterior.

Question 10

What is a Subhepatic Cecum and Appendix?

Answer 10

– Cecum gets adhered to the liver / doesn’t descend to the iliac fossa. May cause difficulty in diagnosis of appendicitis

Question 11

What is a Mixed Rotation and Volvulus?

Answer 11

– cecum lies inferior to the pylorus and is fixed to the posterior abdominal wall by peritoneal bands.

– may cause duodenal obstruction.[due to failure of midgut loop to complete the final 900 rotation]

Question 12

What is cecal diverticulum?

Answer 12

Cecal diverticulum – 6th week on the caudal limb of the midgut. Appendix remains narrow but increases in length. Cecum undergoes differential growth, appendix enters its medial side

Question 13

What is Omphalocele?

Answer 13

Embryological defect: Persistence of the abdominal herniation.

Size of the hernia depends upon its contents

•In contrast to umbilical hernia it is covered by fetal membranes.

Question 14

What is an umbilical hernia?

Answer 14

Embryological defect:
Incomplete closure of the umbilical ring

Clinical presentation:
•It appears as a soft swelling covered by skin.
•It protrudes during crying ,coughing and straining.
• Can be easily reduced through the fibrous ring at the umbilicus
•The hernia can contain omentum and small portions of the small intestines.

Question 15

What is Gastroschisis?

Answer 15

Due to defect near the median plane of the abdominal wall.

Viscera protrudes into the amniotic cavity. Anomaly is due to incomplete closure of lateral folds during 4th week of development [1 in 10,000]

Question 16

What Meckels Diverticulum? What is the Rule of 2’s?

Answer 16

A Meckel’s diverticulum is a fingerlike projection from the ileum,
representing retention of the embryonic Omphaloenteric duct (intestinal stalk/vitelline duct ).

“Rule of 2s”:
Approximately 2 feet proximal to the ileocecal junction. It occurs in 2% of the population.
It occurs 2x as often in males as in females.
It is most commonly about 2 inches long.
It can contain 2 kinds of secretory tissue: gastric and pancreatic (it still remains unknown why).
It can mimic the signs of 2 problems: appendicitis and peptic ulcer.
It can produce periumbilical pain and internal bleeding.

Question 17

What is the significance of Meckeks Diverticulum in the population?

Answer 17

In 2 - 4% of the population a small portion of the omphaloenteric [vitelline duct] may persist giving rise to an Ileal (Meckel’s) diverticulum.

It is located 40 - 60cm from the ileocecal valve on the anti-mesenteric border of the ileum. May be asymptomatic, may contain gastric or pancreatic tissue. May become infected→symptoms resemble appendicitis.

Question 18

Describe the umbilical fistula

Answer 18

Omphaloenteric duct remains patent forming a communication between the umbilicus and intestinal tract .

Clinical signs:
• Fecal discharge at the umbilicus
• Infection at the umbilical stump

Question 19

What is the hindgut?

Answer 19

Begins at the junction between the proximal 2/3 & distal 1/3 of the transverse colon, and ends midway through the anal canal.
• The left one-third of the transverse colon,
• Descending colon,
• Sigmoid colon,
• Rectum,
• Upper part of the anal canal.
• Endodermal lining of hindgut gives rise to the urinary bladder and most of urethra

Question 20

What are the supplies and drainages of the hindgut?

Answer 20

Arterial Supply : Inferior mesenteric
Venous drainage : Inferior mesenteric vein
Lymphatic drainage: Inferior mesenteric nodes

Question 21

Explain the rekation/orientation of cloaca to the hindgut

Answer 21

The Cloaca
• Due to folding of the embryo the allantois and the hind gut opens into the cloaca .

• Therefore, the cloaca is terminal part of the hind gut before division into rectum, bladder and genital primordia.
• It is an endoderm lined cavity that is in contact with the surface ectoderm at the cloacal membrane

Cloacal membrane:
•Endodermal component = cloaca •Ectodermal component = proctodeum

Question 22

What is the embryological significance of the cloacal partitioning?

Answer 22

Urorectal septum – mesenchymal tissue :

Divides tissue into :
•Ventral primitive urogenital sinus
•Dorsal primitive rectum and Cranial part of the anal canal

Question 23

What is the significanceof the cloacal membrane ?

Answer 23

•The urorectal septum fuses with the cloacal membrane •The cloacal membrane is divided into
Urogenital (UG) membrane, &
Anal membrane
Upper anal canal – hindgut
 Lower anal canal-proctodeum

•Mesenchymal proliferations produce elevations of the surface ectoderm around the anal membrane. The membrane is soon located at the bottom of the ectodermal depression –proctodeum. The anal membrane then ruptures at the end of the 8th week.

Question 24

Describe the development of the anal canal

Answer 24

• Superior 2/3 derived from the hindgut
• Inferior 1/3 derived from proctodeum
• Pectinate line – junction of the epithelia derived from the
ectoderm of the proctodeum and endoderm from the hind gut
• Pectinate line is the former site of the anal membrane

Question 25

What is the blood supply to the anal canal?

Answer 25

Blood supply to the anal canal : Superior 2/3 supplied by the superior rectal artery – IMA Inferior 1/3 supplied by the inferior rectal artery – internal pudendal

Question 26

What is the innervation to the anal canal?

Answer 26

Above the pectinate line – autonomics | Below the pectinate line – inferior rectal nerves –pudendal n

Question 27

How are anal lesions classified?

Answer 27

Lesions are classified as low or high depending on whether the rectum ends superior or inferior to the puborectalis muscle, which maintains fecal continence and relaxes to allow defecation.

Question 28

What are the clinical features of low anorectal maalformations?

Answer 28

``` Clinical Features of low anorectal malformations: • No anal opening • Abnormal opening in the perineum • Anal dimple/stenotic opening • Bulge at the site of the anus ```

Question 29

What are the clinical features of high anirectal malformations?

Answer 29

• Generally present as fistulas that communicate with the urogenital system In the male it can present as rectourethral , rectovesical or rectoprostatic In the female it can present as rectovestibular or rectovaginal • Flat perineum • No pigmentation or dimple at the site of the anus

Question 30

What are the comsequences if anirectal malformations/lesions?

Answer 30

M3conium is excreted at an abnormal opening

Question 31

What is the significance of the high anomaly- anorectal agenesis- with fistula?

Answer 31

Although the rectum ends blindly, there is usually a fistula to the bladder (rectovesical fistula) or urethra (rectourethral fistula) in males or to the vagina (rectovaginal fistula) or the vestibule of the vagina (rectovestibular fistula) in females. Anorectal agenesis with a fistula is the result of incomplete separation of the cloaca from the urogenital sinus by the urorectal septum

Question 32

What is the signifucance of rectal atresia?

Answer 32

High anomaly The anal canal and rectum are present but are separated . Sometimes the two segments of intestine are connected by a fibrous cord, the remnant of an atretic portion of the rectum. The cause of rectal atresia may be abnormal recanalization of the colon or, more likely, defective blood supply

Question 33

Describe anal stenosis

Answer 33

Low anorectal abnormality ``` Anal Stenosis (narrowed anal canal) In anal stenosis the anus is in the normal position, but the anus and anal canal are narrow . This birth defect is probably caused by a slight dorsal deviation of the urorectal septum as it grows caudally. ```

Question 34

What is membranous atresia?

Answer 34

Low anorectal In Membranous Atresia the anus is in the normal position, but a thin layer of tissue separates the anal canal from the exterior. The remnant of the epithelial anal plug is thin enough to bulge on straining and appears blue from the presence of meconium superior to it. This defect results from failure of the epithelial plug to perforate at the end of the eighth week.

Question 35

What is imperforate anus?

Answer 35

Low anorectal anomaly The anal canal may end blindly or there may be an ectopic anus or an anoperineal fistula that opens into the perineum . The abnormal canal may, however, open into the vagina in females or the urethra in males . More than 90% of low anorectal anomalies are associated with an external fistula.

Question 36

Describe Hirschsprung Disease (Congenital Megacolon8

Answer 36

Absence of autonomic ganglia in the Myenteric Plexus of the NARROWED segment. Failure of migration of neural crest cells in weeks 5 - 7. Incidence 1:5,000 births Most common cause of neonatal intestinal obstruction Aganglionic segment no peristalsis