L10 Embryology II

Question 1

What is the primary intestinal loop? When does it form?

Answer 1

In week 5 the primary loop, gut tube, elongates rapidly. At the same time, other organs particularly the liver, grows rapidly as the intestinal loops. This forms a primary intestinal loop. There is a cranial limb and a caudal limb of the primary loop.

Question 2

What makes up the cranial and caudal limb of the primary intestinal loop?

Answer 2

The cranial limb will form: distal duodenum, jejunum and proximal ileum.
The caudal limb will form: distal ileum, caecum, appendix, ascending colon and proximal 2/3 transverse colon.

Question 3

In which direction does the midgut rotate?

Answer 3

As it elongates, the primary intestinal loop herniates into the umbilical cord as there is not enough space. As it protrudes out, it rotates 90 degrees anticlockwise. The jejunoileal loops form.

As they go back in, they rotate a further 180 degrees anti-clockwise. By the time the intestinal contents are back in the abdominal cavity, we have rotates 270 degrees anti-clockwise. This is opposite to the 90 degree clockwise rotation of the stomach in the foregut.

Question 4

What is the vermiform appendix?

Answer 4

As we go back into the abdominal cavity, we have developed a dilation which will go onto form the cecum. The caecum develops an appendage called the vermiform appendix.

Question 5

When does the vitelline duct disappear?

Answer 5

Generally, the duct fully obliterates (narrows and disappears) during the 5–6th week of fertilization age (9th week of gestational age), but a failure of the duct to close is termed a vitelline fistula.

Question 6

what is the effect of the descent of the cecum?

Answer 6

When the cecum moves into the right iliac fossa, it pulls the colon into place Rotational movement of the gut tube causes the ascending and descending colon to be pushed against the posterior wall, causing them to become secondarily retroperitoneal.

Question 7

What are the different positions of the appendix?

Answer 7

On the caecum there is the developing appendix. As the caecum descends, the appendix moves back up behind the appendix most of the time. The position of the appendix in most people is retrocecal. It is variable in people. The appendix has its own mesentery mesoappendix and so the position is variable.

Question 8

What is Meckel’s Diverticulum?

Answer 8

Remanence of the vitelline duct. This creates an outpouching of the ileal wall. Usually asymptomatic. However can cause problems is there is ectopic tissue in it e.g., from the stomach or pancreas and the cells start secreting acid. This can lead to inflammation, ulceration and bleeding. It is rare.

Question 9

What is an omphalocele?

Answer 9

Omphalocele, also called exomphalos, is a rare abdominal wall defect in which the intestines, liver and occasionally other organs remain outside of the abdomen in a sac because of failure of the intestines and other contents to return back to the abdominal cavity during around the sixth week of intrauterine development.

Question 10

What is the difference between gastroschesis and omphalocele?

Answer 10

In gastroschisis, fail to develop body wall. In this case we still have the abdominal wall formed correctly. In omphalocele there is coverings to intestinal contents, the umbilical cord and peritoneum. In gastroschisis, this does not occur.

Question 11

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

Answer 11

If rotations do not occur, we get abnormal placement of viscera. This is usually asymptomatic.

Question 12

What is reversed rotation of the midgut?

Answer 12

n the way out it rotates 90 degrees anticlockwise, when it goes back in, it rotates the extra 180 degrees the opposite way, leading to a total rotation of 90 degrees clockwise. The duodenum is now anterior to the large intestine. It doesn’t get pushed against the posterior wall. This means it doesn’t lose its mesentery and becomes mobile.

Question 13

What is volvulus?

Answer 13

A volvulus is when a loop of intestine twists around itself and the mesentery that supports it, resulting in a bowel obstruction. Symptoms include abdominal pain, abdominal bloating, vomiting, constipation, and bloody stool.

Question 14

What are the symptoms of volvulus?

Answer 14

If it twists greatly, it can cut off its blood supply and becomes narcotic. It can lead to bilious vomiting.

Question 15

What is the anorectal canal?

Answer 15

The distal end of the hindgut enters the dorsal part of the cloaca

Question 16

How does the anal canal form?

Answer 16

During weeks 4-6 a layer of mesoderm extends caudally to separate the urogenital sinus and anorectal canal – urorectal septum

Question 17

What is a imperforate anus?

Answer 17

Imperforate anus is a defect in which the opening to the anus is missing or blocked.

Question 18

How is the GI tract innervated?

Answer 18

The GIT is innervated by the enteric nervous system (division of autonomic nervous system)

There are 2 enteric plexi:
Myenteric (Auerbach’s) plexus between the circular and longitudinal muscle layers co-ordinates muscle contraction.

Submucosal (Meissner’s) plexus between the circular muscle and mucosa and regulates secretion.
The enteric nervous system is derived from neural crest cells (ectoderm origin) that migrate from neural tube to GIT

Question 19

What is Hirschsprung’s disease?

Answer 19

Hirschsprung disease is a birth defect in which nerve cells are missing at the end of a child’s bowel. Peristalsis is paralysed. The muscles cannot contact. The food that is stagnated leads to bacteria respiring, leading to gas production. The mesoderm is Aganglionic as it doesn’t contain any cell bodies. Treatment is only to move the paralysed piece of bowel.

Question 20

Where can a volvulvus occur in an adult?

Answer 20

Sigmoid colon

Question 21

What does the urogenital sinus go onto form?

Answer 21

The ventral part of the cloaca (urogenital sinus) will form the bladder, pelvic urethra, penile urethra (males) and caudal part of vagina (females).

Question 22

What is the proctodeum?

Answer 22

primitive anus

Question 23

What is the pectinate line?

Answer 23

The pectinate line delineates the change between two embryological origins.
• The upper 2/3 is derived from hindgut – endoderm
• The lower 1/3 is derived from proctodeum (anal pit) - ectoderm
Become continuous when cloacal (anal) membrane degenerates

Question 24

How does the hindgut form?

Answer 24

The cloaca is where the hindgut empties and the developing bladder is.

Mesoderm develops and extends to separate the cloaca. The mesoderm extends all the way to the surface and separates a ventral part (forming the urogenital sinus) and a dorsal part (forming the procodeum). It therefore forms the urorectal septum to separate the urogenital sinus to the anorectal canal.

Question 25

What is the pectinate line?

Answer 25

The pectinate line delineates the change between two embryological origins.
• The upper 2/3 is derived from hindgut – endoderm
• The lower 1/3 is derived from proctodeum (anal pit) - ectoderm
Become continuous when cloacal (anal) membrane degenerates

Question 26

What is congenital rectourethral and rectrovaginal fistulas?

Answer 26

Opening of hindgut is shifted ventrally to the urethra in males and the vagina in females. Caused by abnormal cloaca e.g. too small or failure of urorectal septum to extend caudally.

Question 27

What is the origin of the perineal body?

Answer 27

The septum that separates the gut tube forms the perineal body. This is something we can attach muscles of the perineum to. External muscles of these openings attach here.
Something we can attach the muscles of the perineal.

Question 28

What is the embryological origin of the cloaca?

Answer 28

Ectoderm invaginate at the anal pit. It comes up the cloacal membrane. The cloacal membrane therefore has endoderm and ectoderm.