6. Development of Digestive System

Question 1

Summary of Digestive System Development

Answer 1

Head fold, lateral folding, tail fold of embryo creates a gut tube with foregut, mudgut and hindgut regions

ENDODERM: Epithelium of the gut tube (mesenteries)- liver, gall bladder and pancrease all develop as outgrowths of the endoderm from distal foregut, with CT elements from mesoderm

MESODERM: Smooth muscle, CT, peritoneum

ECTODERM: Enteric Nervous system from neural crest from ectoderm?

Question 2

PHARYNX- explain the development of the pharyngeal pouches

Answer 2

Pharyngeal Pouches- lateral outpockets of the pharynx endoderm.

First: Middle ear cavity and the eustachian (auditary tube)

Second: Palatine Tonsil area

Third: Inferior parathyroid and thymus

Fourth: Superior parathyroid, thyroid C cells (make calcitonin)

Question 3

PHARYNX- explain the development of the pharyngeal arches

Answer 3

Pharyngeal (branchial) arches appear during the 4th & 5th weeks on the ventral side of the pharyngeal gut.

They are the mesenchymal tissue surrounding the pharyngeal pouches (overlying CT of the pouches)- mesenchyme = mesoderm derivative

1st arch= cartilage model of the mandible, two auditory ossicles

2nd arch = one auditary ossicle, styloid process of temporal bone, part of hyoid bone

3rd arch = most of hyoid bone

4th & 6th arches = cartilages of the larynx.

Question 4

THYROID

Ventral Outgrowths of the pharyngeal ENDODERM -> Thyroid

Answer 4

Thyroid- develops from endoderm of the tongue, connected by the thyroglossal duct to the tongue during development.
- Between the 1st and secondpouches you get the outgrowth which is the thyroid gland, at the junction between anterior 2/3 and posterior 1/3 of the tongue.

Migrates a significant distance down the front.

Question 5

THYROID
What is the name of the remanent of the thyroglossal duct?

What are some errors which can occur in the development of the thyroid?

Answer 5

Foramen cecum = remnant of site of origin of the thyroglossal duct

Errors
thyroglossal cyst- patent thyroglossal duct, may have some cysts growing off it

Fistula- may open into the neck

Question 6

What are some otther ventral outgrowths of the PHARYNX?

Answer 6

RESPIRATORY DIVERTICULUM

Question 7

How does the Respiratory Diverticulum form?

Answer 7

Oesophageal ridges fuse to form tracheoesophageal septum, which separates trachea from oesophagus. Larynx remains in communication with laryngopharynx at the laryngeal orifice. Branching begins => lung buds.

Respiratory diverticululm gives rise to the:
Larynx
Trachea
Lungs (branching morphogenesis)

Question 8

Errors in respiratory diverticulum development

Answer 8

Proximal Atresia with distal tracheosophageal fistula- most common error, the upper portion of the oesophagus ends in a blind pouch, and the lower segment forms a fistula with the trachea

Question 9

Peritoneal Cavity Development

Answer 9

Intraembryonic Coelom = ventral body cavity

Begin as cavities in the lateral plate mesoderm, initially continuous with the extraembryonic coelom

Lateral folding- cavities merge on midline forming the ventral body cavity

Question 10

Peritoneum- visceral and parietal

Answer 10

There are two layers of the lateral plate mesoderm which contribute to the peritoneum

• Splanchnic layer: covers organs -> visceral peritoneum
• Somatic layer: covers body cavity wall -> parietal peritoneum

Question 11

Describe the development of the mesenteries of the GUT

Answer 11

The primitive gut is enveloped by a mesentery that has a dorsal and ventral aspect.

The DORSAL MESENTARY contains nerves, arteries, veins and lymphatics: it is a double layer of peritoneum from the abdominal wall to the gut . The dorsal mesentery is not lost but in some parts it comes to lie against the abdominal wall.

The VENTRAL MESENTERY of the midgut & hindgut breaks down- but the foreguts is maintained.- the liver grows into the ventral mesentary of the stomach.

Question 12

Intraperitoneal vs. Retroperitoneal Organs

Answer 12

Organs that keep their dorsal mesentary are INTRAPERITONEAL
- attached to the posterior abdominal wall.

Organs that lose it by contacting posterior abdominal wall= secondarily RETROPERITONEAL
= duodenum (except for the proximal segment), pancreas, ascending and descending colon

Question 13

Explain the neurovascular supply of the FOREGUT

Answer 13

Foregut derivatives in the abdomen: Oesophagus, stomach, first part of the duodenum (superior, descending), liver, gallbladder, spleen, superior portion of the pancreas

Coelic Artery
Various veins which drain directly or indirectly into the hepatic portal vein

Coeliac ganglion and plexus- autonomic innervation
Coeliac lymph nodes - lymphatic drainage (liver is the exception)

Question 14

Describe the rotation of the stomach and its mesenteries

Answer 14

Spleen develops in the DORSAL mesentary,
Liver in the VENTRAL mesentary of the stomach

90 degreee clockwise rotation of the stomach (viewed from anterior and superior aspect) accounts for location of spleen to left and liver to the right

• and left vagus nerve mainly forming the ANTERIOR VAGAL TRUNK (rotates anteriorly)
• and right vagus nerve mainly forming hte posterior vagal trunk (rotates posteriorly)

Question 15

Mesenteries of the Stomach, Liver and Spleen

Answer 15

Ventral Mesentery - MESOGASTRIUM - forms the lesser omentum, falciform ligament

Dorsal Mesentery- greater omentum, gastrosplenic ligament, splenorenal ligament

Question 16

Lesser Sac, Greater Omentum

Answer 16

As the Liver rotates to the right, the lesser sac (omental bursa) is created posterior to stomach and lesser omentum

The greater omentum initially has an extension of omental bursa between its layers, but this usually fuses (also fuses to the transverse colon)

• The greater omentum is a large fold in visceral peritoneum (2 layers) that descends down from the greater curvature of the stomach, passing in front of the small intestines & reflects on itself (4 layers of serous membrane) to ascend to the transverse colon before reaching the posterior abdominal wall.
• It actually fuses (before the transverse colon) so you cannot tell there are four layers - usually it fuses to the transverse colon as well.
• Hence what was the lesser sac becomes much smaller -> now it is just posterior to the stomach as opposed to extending down.

Question 17

Liver and Gallbladder

Answer 17

Develop as outgrowths of the foregut ENDODERM

Within the VENTRAL MESENTERY of the stomach
- tube grows into the ventral mesentery of the stomach (liver) and get outgrowth (gall bladder)

Question 18

Development of the Pancreas

Answer 18

Ventral and Dorsal outgrowths of foregut endoderm into ventral and dorsal mesenteries of the stomach
- outgrowth near foregut/midgut junction

2 pancreas’ develop: ventral pancreas (small), growing into the ventral mesentary and a dorsal pancreas (larger) growing into a dorsal mesentery.

They then swing around and end up merging together

Ventral pancreatic duct & the bile duct: have a shared opening into the duodenum at the major duodenal papillae

Msot of the dorsal pancreatic duct joins with the ventral pancreatic duct to form the main pancreatic duct; a small part remains as the accessory pancreatic duct

Question 19

What are some abnormalities in pancreas development?

Answer 19

Annular pancreas- probably from bi-lobed ventral pancreas, with lobes rotating in opposite directions to surround the duodenum.

May compress the duodenum- duodenal stenosis

Question 20

What is the neurovascular supply of the MIDGUT

- what are the MIDGUT derivatives??

Answer 20

Midgut derivatives: MOST of SI (except for first part of duodenum), caecum => proximal 2/3 transverse colon

Superior mesenteric artery
Superior mesenteric vein to hepatic portal vein
Superior mesenteric ganglion and plexus
Superior mesenteric lymph nodes

Question 21

Development of the midgut

Answer 21

Midgut grows rapidly- can’t fit in abdominal cavity so it normally herniates into the umbilical cord, returning to the abdominal cavity several weeks later

Midgut is connected to the yolk sac via a yolk stalk

Question 22

What can go wrong developmentally with the midgut

Answer 22

Failure to return = omphalocele
- loops of bowel within the umbilical cord

Failure of resorption of viteline duct: Ileal (Meckel’s) diverticulum or fistula
- Sometimes the mucosa is more like stomach and can produce HCl-, which can lead to stomach ulcers.

Question 23

Explain the developmental rotation of the MIDGUT

Answer 23

270 degrees counterclockwise around the axis of the superior mesenteric artery: loop up and around to the right; hence SI ends up on the left, the proximal half of the LI ends up on the right

Accounts for the orientation of the large intestine

Question 24

Name some abnormalities of Midgut rotation

Answer 24

Non-rotation or failure to complete rotation -> left sides colon with LI on the Left, SI on the right: compatible with life

Clockwise instead of counter clockwise -> duodenum anterior to the colon- Reverse rotation can be an issue, resulting in the duodenum being intraperitoneal which means that the stomach, duodenum and small intestine are all hypermobile & are thus prone to twisting/volvulus.

Partially incomplete or excessive rotation -> abnormal position of the caecum, appendix etc.
- relevant for surgery and appendicitis

Question 25

What are the HINDGUT derivatives?

Answer 25

Hindgut derivatives: Distal 1/3 of transverse colon, descending colon, sigmoid colon, rectum

Inferior mesenteric artery
Inferior mesenteric vein -> splenic vein -> hepatic portal vein
Inferior mesenteric ganglion & plexus
Inferior mesenteric lymph nodes

Question 26

What are some Neural Crest problems which can occur with the hindgut?

Answer 26

Neural crest cell migration is quite far to the gut
Lead to enteric nervous system problems, most commonly in the hindgut

Hirschsprung Disease = congenital aganglionic megacolon: LI lacks ganglia (no enteric NS), nothing to regulate smooth muscle contraction in that area -> massive dilation of some parts and some very small parts -> doesnt function properly.

Question 27

Describe the development of the Rectum and the Anal Canal

Answer 27

Cloaca- common distal end of hindgut and allantois

• early in development there is a common opening between the hindgut and allantois
• Allantois eventually gives rise to the urinary bladder

Connective tissue invades to form the URORECTAL SEPTUM which divides the cloaca into urogenital sinus ventrally, and rectum & anal canal dorsally

Question 28

Probelsm with develoopment of the RECTUM

Answer 28

IMPERFORATE ANUS

• Opening of anus is missing or blocked
• Rectum may end in a blind pouch; may have openings to urethra, bladder
• May be stenosis of the anus or no anus

FIstula between hindgut & urogenital structures

• Rectovesical (opens into bladder)
• Rectourethral (into urethra) boys
• Rectovaginal (into the vagina)