development of the GI tract

Question 1

what are the 3 parts of the gut ?

Answer 1

foregut:
* Pharynx
* Oesophagus
* Stomach
* Cranial half of duodenum
* Ampulla of Vater
(joining of common bile duct and pancreatic duct)

midgut:
* Caudal duodenum
* Jejunum
* Ileum
* Cecum
* Appendix
* Ascending colon
* Proximal 2/3 of transverse colon

hindgut:
* Distal 1/3 of transverse colon
* Descending colon
* Rectum

Question 2

what do the 3 germ layers give rise to in terms of the gut ?

Answer 2

ectoderm - epithelium of the gut tube
mesoderm - surrounding muscle , connective tissue and mesenteries
endoderm - innervation of the gut

Question 3

describe initial gut folding and tube formation

Answer 3

• The gut tube is formed by folding of sheets of cells in two directions
• Folding towards the midline along the cranial-caudal axis
• Folding towards the yolk sac at the cranial and caudal ends

Question 4

describe the steps involved in the formation of the gut tube

Answer 4

1. Gastrulation: The embryo forms three layers: ectoderm, mesoderm, and endoderm. The gut tube will develop from the endoderm layer.
2. Folding: As the embryo grows, the endoderm folds inwards to form a tube-like structure called the primitive gut.
3. Subdivision: The primitive gut is divided into three sections:
   * Foregut (will form the mouth, esophagus, stomach, and part of the intestine),
   * Midgut (forms most of the small intestine and some of the large intestine),
   * Hindgut (forms the rest of the large intestine, rectum, and anus).
   * Support and Blood Supply: The gut tube gets a blood supply and is supported by structures called mesenteries, which help hold it in place.
4. Differentiation: The inner lining of the gut forms from the endoderm, while the surrounding muscles and connective tissue develop from the mesoderm. This shapes the digestive organs.

Question 5

how are HOX genes involved in GI tract development ?

Answer 5

• HOX genes play a key role in GI tract development by determining the body’s basic layout along the head-to-tail axis.
• HOX genes are activated in specific regions of the developing embryo, providing instructions for where different parts of the GI tract will form (e.g., where the stomach, small intestine, and colon should be).
• These genes help create distinct regions in the gut, like the foregut, midgut, and hindgut.
• HOX genes guide the growth and positioning of different organs in the digestive system. Without proper HOX gene function, the organs might form in the wrong places or be absent.
• HOX genes act like a blueprint for organizing the GI tract, making sure each part forms in the right place and at the right time.

Question 6

describe the arterial blood supply to the GI tract

Answer 6

• Gut surrounded by network of blood vessels, joining vessels to aorta.
• network resolves to form the arteries that supply the GI tract from the aorta
• These define the boundaries of the gut
• celiac artery to the foregut
• superior mesenteric artery to the midgut
• inferior mesenteric artery to the hindgut

Question 7

describe the innervation of the gut

Answer 7

• Sympathetic ganglia develop next to major branches of the aorta
• Post-ganglionic, sympathetic axons innervate the same tissues that the arteries supply with blood
• Celiac ganglion – foregut
• Superior mesenteric ganglion – midgut
• Inferior mesenteric ganglion – hindgut

Question 8

describe development of the foregut

Answer 8

1. Formation of the Foregut
   * develops from the endoderm, which forms the inner lining of the gut, while the surrounding mesoderm contributes to the muscle, blood vessels, and connective tissue.
   * It is initially a simple tube that extends from the pharynx (near the head) down to the upper part of the duodenum.
2. Subdivision and Organ Formation
   * The foregut gives rise to several important structures:
   * Pharynx → Develops into parts of the throat and respiratory system.
   * Esophagus → Elongates as the embryo grows, with its upper part forming skeletal muscle and the lower part forming smooth muscle.
   * Stomach → A bulge appears in the foregut, which later rotates and expands to form the stomach.
   * Duodenum (proximal part) → The beginning of the small intestine, forming from the lower end of the foregut.

• Additionally, important foregut-derived organs form as outgrowths:
• Liver → Forms from the hepatic diverticulum and later develops into the liver, bile ducts, and gallbladder.
• Pancreas → Develops from two buds (dorsal and ventral) that eventually fuse to form the pancreas.
• Lungs & Trachea → The respiratory diverticulum buds off from the foregut to form the trachea, bronchi, and lungs.

3. Rotation and Positioning
   * The stomach rotates 90 degrees to its final position.
   * The duodenum shifts to the right, becoming a C-shaped loop.
   * The liver and pancreas grow and take their positions in the abdomen.

Question 9

what is pyloric stenosis ?

Answer 9

• condition where the pylorus (the muscle at the exit of the stomach) becomes too thick, making it hard for food to pass into the small intestine.
• Happens in babies (usually around 2–8 weeks old).
• Causes forceful vomiting after feeding.
• Leads to dehydration and weight loss.
• Can be treated with minor surgery to loosen the muscle.

Question 10

describe mesentery development

Answer 10

• Mesenteries are thin sheets of tissue that support and hold the gut in place while also carrying blood vessels, nerves, and lymphatics. They develop from the mesoderm during embryonic development.

1. Formation of the Mesenteries
   * In early development, the gut tube is suspended inside the embryo by a tissue layer called the dorsal mesentery (on the back side).
   * The ventral mesentery (on the front side) is only present in the foregut region and later disappears in the midgut and hindgut.
2. Changes in the Mesenteries
   * The foregut (stomach, liver, pancreas) keeps both dorsal and ventral mesenteries.
   * The ventral mesentery forms parts of the liver’s ligaments (like the falciform ligament) and the lesser omentum (a connection between the stomach and liver).
   * The dorsal mesentery forms the greater omentum, which hangs from the stomach.
   * The midgut and hindgut only keep the dorsal mesentery, which forms the mesentery of the small intestine and colon.
3. Rotation and Fixation
   * As the intestines grow and rotate, parts of the mesentery fuse with the back wall of the abdomen, making some organs (like the pancreas and parts of the colon) retroperitoneal (fixed in place).
   * Other parts (like the small intestine) remain intraperitoneal, meaning they stay freely suspended by mesentery.

Question 11

describe the liver budding from the foregut

Answer 11

1. Heart Signals the Gut to Form the Liver
   * The developing heart sends signals to the ventral foregut endoderm, telling it to start forming the liver.
2. Hepatic Diverticulum Forms
   * A small liver bud (called the hepatic diverticulum) grows out from the foregut into the surrounding tissue called the septum transversum (which later helps form the diaphragm).
3. Liver Cells and Blood Vessels Develop
   * The liver bud contains endoderm cells, which multiply and form liver cords (early liver tissue).
   * These liver cords connect with developing blood vessels and bile ducts, creating a network of sinusoids (small blood channels).
4. Liver Grows and Expands
   * The liver grows bigger and outgrows the original space in the septum transversum.
   * It pushes into the ventral mesentery, a thin sheet of tissue that connects the gut to the body wall.
5. Formation of Supporting Structures
   * The remaining ventral mesentery forms:
   * The falciform ligament (connecting the liver to the front body wall).
   * The lesser omentum (connecting the liver to the stomach).

Question 12

describe the pancreas budding from the foregut

Answer 12

1. Two Pancreatic Buds Form
   * The dorsal pancreatic bud grows from the duodenal endoderm (helped by signals from the notochord).
   * The ventral pancreatic bud grows from the hepatic diverticulum (helped by signals from the hepatic mesoderm).
2. Duodenum Rotates, Buds Fuse
   * As the duodenum rotates, the ventral bud moves around and joins the dorsal bud.
   * Together, they form one pancreas with ducts that drain digestive enzymes into the duodenum.

Question 13

describe the development of the midgut - intestines

Answer 13

1. Midgut Stays Attached to the Dorsal Mesentery
   * The midgut is held in place by the dorsal mesentery, but there’s no ventral mesentery.
2. Uneven Growth Causes Folding
   * The midgut and its mesentery grow at different rates, which makes the intestines fold and twist into their normal shape.
3. Blood Supply from the Superior Mesenteric Artery (SMA)
   * The SMA (a major artery) grows from the aorta to supply blood to the midgut.
4. Intestines Grow Too Fast & Herniate
   * The midgut grows very quickly, but the abdomen is too small to hold it.
   * So, at 6–7 weeks, the growing intestines move out into the umbilical stalk (a normal temporary herniation).
5. Intestines Rotate & Return
   * While outside, the intestines rotate around the SMA to position themselves correctly.
   * By 10 weeks, the abdomen gets bigger, and the intestines return inside to their final positions.

Question 14

what is Meckels diverticulum ?

Answer 14

yolk Duct Should Disappear, But Sometimes It Stays
* Normally, the yolk duct (which connects the midgut to the yolk sac) disappears during development.
If it remains, it can lead to Meckel’s diverticulum.

Common Condition, Usually No Symptoms
* This is the most common intestinal abnormality (seen in 2-4% of people).
It’s usually harmless and doesn’t cause problems.

Location: Near the Ileocecal Junction
* Meckel’s diverticulum is found on the ileum, close to the junction between the small intestine and large intestine.

Possible Problems
* Sometimes, it contains stomach-like cells, which can make acid and cause ulcers.
In some cases, a ligament connects it to the umbilicus, which can lead to twisting of the intestine (volvulus), blocking digestion.

Question 15

what is an umbilical hernia ?

Answer 15

• An umbilical hernia happens when part of the intestine or abdominal tissue pushes through a weak spot in the belly button (umbilicus).
• Before birth, the baby’s intestines temporarily move into the umbilical cord as they grow.
• Normally, they return into the abdomen, and the muscle wall closes up.
• If the muscle doesn’t close completely, a small opening remains, and intestines can push through, forming a hernia.

Question 16

what is gastroschisis ?

Answer 16

• Gastroschisis is a birth defect where the baby’s intestines (and sometimes other organs) develop outside the belly because the abdominal wall doesn’t close properly.
• During early development, the abdominal wall forms to hold the intestines inside.
• in gastroschisis, a small hole forms next to the belly button, and the intestines remain outside, exposed to the amniotic fluid.
• No protective sac around the intestines (unlike omphalocele, which has a covering).
• Usually found on the right side of the belly button.
  Can cause intestine damage due to exposure to amniotic fluid.

Question 17

what is Hirschsprung diseases ?

Answer 17

• Hirschsprung disease is a condition where part of the large intestine (colon) lacks nerve cells, making it difficult for stool to pass through. This leads to severe constipation and blockage.
• During development, nerve cells (ganglion cells) normally grow along the intestine to help it move food.
• In Hirschsprung disease, these nerve cells don’t reach the end of the colon, so that section can’t contract properly to push stool forward.
  Key Features:
• Newborns don’t pass their first poop (meconium) within 48 hours.
• Severe constipation and bloating in infants and children.
• Can cause vomiting and poor feeding.
• More common in boys and in children with Down syndrome.

Question 18

what are neural crest cells ?

Answer 18

• Neural crest cells originate from the dorsal region of the neural tube
• Contribute to a wide variety of tissues in the embryo
• Parasympathetic innervation of the gut

Question 19

describe hindgut development - anal development and the cloaca

Answer 19

• the last part of the gut tube, and it forms the lower parts of the digestive system. Here’s how it develops:

1. Formation of the Hindgut
   * The hindgut forms from the endoderm (the innermost layer of cells in the developing embryo).
   * It extends from the midgut to the cloacal membrane, which is the region where the digestive and urinary tracts will separate.
2. Cloaca Formation
   * The cloaca is a common chamber at the end of the hindgut where the digestive, urinary, and reproductive systems meet.
   * Initially, the cloaca is undivided, but over time it separates into two parts:
   * Anterior part: Becomes the rectum and part of the anal canal.
   * Posterior part: Forms the urogenital sinus, which will give rise to parts of the urinary and reproductive organs.
3. Anal Development
   * The anal membrane is a structure that covers the end of the hindgut (the future anus) early on.
   * The anal membrane ruptures around the 8th week, opening the anal canal.
   * The anal canal forms from two parts:
   * Upper part: Develops from the hindgut.
   * Lower part: Forms from ectodermal cells from the cloacal membrane.
   * These two parts fuse during development to create the full-length anal canal.
4. Separation of the Rectum and Urogenital Sinus
   * The urorectal septum (a tissue growth) divides the cloaca into the rectum (for digestion) and the urogenital sinus (for urine and reproductive organs).
   * This separation is crucial for proper formation of the rectum and anus and the urinary system.