embryology

Question 1

how is the primitive gut tube formed?

Answer 1

lateral and craniocaudal folding

Question 2

what are the foregut organs supplied by?

Answer 2

coeliac trunk

Question 3

what are the midgut organs supplied by?

Answer 3

superior mesenteric artery

Question 4

what are the hindgut organs supplied by?

Answer 4

the inferior mesenteric artery

Question 5

how is the vitelline duct formed?

Answer 5

• the midgut is open into the yolk sac
• as folding continues, it narrows
• it becomes incorporated into the umbilical cord

Question 6

how is the oesophagus formed?

Answer 6

• resp diverticulum appears end of week 3
• weeks 4-7 = oesophagus lengthens rapidly due deacent of heart and lungs
• failure of oesophagus to grow in proportion to neck and thorax can lead to a short oesophagus
• this results in the stomach being placed cranially in the thorax forming a congenital hiatal hernia

Question 7

how is the stomach formed?

Answer 7

• starts as a fusiform dilation
• dorsal wall undergoes raipid growth to form the greater curvature
• ventral wall growth is slower resulting in the lesser curvature

Question 8

what does anteroposterior axis rotation of the stomach do?

Answer 8

• brings pyloris upwards and fundus downwards

Question 9

what are mesenteries?

Answer 9

a continuos later of serous membrane

they attach the gut tube to the anterior and posterior walls of the abdomen

Question 10

how is the omental bursa formed?

Answer 10

• omental bursa is the space posterior to the stomach
• rapid growth of dorsal mesentery and stomach rotation around the longitudinal axis
• the rest of the space in the peritoneal cavity is known as the greater sac
• epiploric foramen connects the two

Question 11

how is the greater omentum formed?

Answer 11

• dorsal mesentery continues o develop as a double layered sac over the small intestine and transverse colon
• these layers fuse to extend from the greater curvature of stomach
• forming the greater omentum (aprin like structure)

Question 12

how is the duodenum formed?

Answer 12

• both foregut and midgut structure
• as stomach rotates the duodenum oves from the midline to the right side of the abdominal acavity
• dorsal mesentery fuses with peritoneum covering the posterior abdomina wall
• duodenum (parts 2+3) and pancreas become retroperiotoneal

Question 13

how is the midgut developed?

Answer 13

• in week 5, the gut rapidly expands forming the primary intestinal loop
• cranial limb - distal duodenum, jejunum, upper illeuum
• caudial limb . - lower illeum, caecum, ascending colon and proximal 2/3rds of transvers colon

Question 14

what is physiological herniation of the midgut?

Answer 14

In week 6, rapid growth and expansion of the liver causes physiological herniation of midgut

The cranial (cephalic) limb continues to lengthen

Intestinal loops move through umbilical cord to lie outside the developing embryo.

Question 15

what is rotation of the midgut?

Answer 15

As lengthening continues the midgut rotates 90 degree clockwise (patients perspective) and herniates through umbilical cord.
Small intestine and large intestine growth continues to form coils (large intestine does coil).
During week 10, intestines retract back into abdomen with a further 180 degree clockwise turn. (270 degrees total)
Jejunum first then cecal bud.
Cecal bud then descends from right lobe of liver to right iliac fossa.

Question 16

what is fixation of intestines?

Answer 16

The dorsal mesentery of the gut is still present and connects to the posterior abdominal wall.

Connected around the axis of the superior mesenteric artery.

The ascending and descending colon mesentery fuse with peritoneum to place them in retroperitoneal space.

Question 17

what is an omphalocele?

Answer 17

failre of intestinal loop to return to abdomen

• presents as a shiny sac at the base of the umbilical cord
• high mortality rate

Question 18

what is gastroschisis?

Answer 18

protrusion of abdominal content through wall lateral to umbilical cord
- due to abnormal closure

Question 19

what is the development of the hindgut?

Answer 19

Terminal end of hindgut is an endodermal-lined pouch called the cloaca.
In contact with surface ectoderm of proctodeum to form the cloacal membrane
Urorectal septum partitions the cloaca into: upper rectum/anal canal and urogenital sinus
Urorectal septum fuses with the cloacal membrane at the future site of the perineal body

Question 20

how is the liver and biliary system formed?

Answer 20

A ventral outgrowth of foregut end week 3 into ventral mesentery.

Outgrowth termed hepatic diverticulum (liver bud) that penetrate septum transversum
Cranial portion – Liver
Caudal portion – Bile duct

Question 21

how is the liver formed?

Answer 21

Hepatic cords form, which intermingle with the umbilical and vitelline veins to give hepatic sinusoids.

Vitelline veins carry poorly oxygenated by nutrient rich blood to the developing embryo

Endoderm – Liver cells (parenchyma) and lining of biliary tree

Septum transversum – Hematopoietic cells, Kupffer cells and connective tissue.

Question 22

how is the spleen formed?

Answer 22

Mesoderm derivative (not gut tube)

Develops in 5th week

Initially haematopoetic

Develops into lymphatic organ during weeks 15-18

Lobular appearance but lobes normally regress before birth → notches in adult