GI Anatomy Week 2 PDF

GI Anatomy BMS 200 Week 2 Learning Outcomes How does embryonic folding contribute to the development of the gut? How does the septum transversum form the ventral mesentery and diaphragm? How are the respiratory tract and digestive organs related? Explain the development o...

GI Anatomy BMS 200 Week 2 Learning Outcomes How does embryonic folding contribute to the development of the gut? How does the septum transversum form the ventral mesentery and diaphragm? How are the respiratory tract and digestive organs related? Explain the development of abdominal wall, peritoneal cavity, dorsal mesentery, visceral and parietal peritoneum What are the components of the GI tract? What is their arterial supply? Relate the embryology to various clinical disorders. Case Study A 14-year-old boy presents with severe lower left quadrant (LLQ) abdominal pain, abdominal distension, nausea, vomiting, and absent bowel movements (signs of bowel obstruction). ▪ What are you thinking? Pre-Assessment Quiz Which of the following structures is NOT derived from the intra-embryonic coelom? A. Pericardial cavity B. Pleural cavity C. Peritoneal cavity D. Intrathecal cavity Pre-Assessment Quiz Which of the following structures is NOT derived from the intra-embryonic coelom? A. Pericardial cavity B. Pleural cavity C. Peritoneal cavity D. Intrathecal cavity Pre-Assessment Quiz Which structure gives rise to the wall of the gut? A. Notochordal process B. Splanchnopleure C. Somatopleure D. Extraembryonic coelom Pre-Assessment Quiz Which structure gives rise to the wall of the gut? A. Notochordal process B. Splanchnopleure C. Somatopleure D. Extraembryonic coelom From BMS 150 The primordium of the intraembryonic mesoderm appears as isolated spaces in the lateral mesoderm & cardiogenic mesoderm ▪ These spaces coalesce & form intraembryonic coelom (embryonic body cavity) Intraembryonic coelom divides lateral mesoderm into: ▪ Somatic (parietal) layer: beneath ectodermal epithelium & continuous with extraembryonic mesoderm covering the amnion ▪ Splanchnic (visceral) layer: next to the endoderm & continuous with the extraembryonic mesoderm covering the umbilical vesicle Important Structures: ▪ Somatopleure: somatic mesoderm + overlying ectoderm → forms body wall ▪ Splanchnopleure: splanchnic mesoderm + underlying intraembryonic endoderm → forms embryonic gut During the 2nd month, the body cavity forms pericardial, pleural, & peritoneal cavities Embryonic Folding How does a ‘flat’ embryonic disc become cylindrical? ▪ Embryonic folding which begins at the end of 3rd week Folding occurs in 2 planes: ▪ Median (cranial-caudal or cephalo-caudal folding): anterior & posterior ends of an embryo move ventrally → brain vesicles & somites appear ▪ Horizontal (lateral folding): lateral edges of the embryonic disc move ventrally toward the umbilical vesicle → forms body wall Transverse section Sagittal section Cranial Folding Part of the endoderm of the umbilical vesicle is incorporated into the embryo as the foregut ▪ The foregut lies between the brain & heart ▪ Oropharyngeal membrane separates the the foregut from the stomodeum (primordium of the mouth) Septum transversum lies caudal to the heart ▪ Develops into the central tendon of the diaphragm and separates the abdominal cavity from the thoracic cavity Caudal Folding As the embryo grows, the caudal eminence projects over the cloacal membrane (future site of the anus) Part of the endodermal germ layer is incorporated into the embryo as the hindgut The connecting stalk (primordium of the umbilical cord) is now attached to the ventral surface of the embryo, and the allantois is partially incorporated into the embryo Lateral Folding Caused by rapidly growing spinal cord & somites As the abdominal wall forms, part of the endoderm germ layer is incorporated into the embryo as the midgut ▪ Initially, there is a wide connection between the midgut and umbilical vesicle After lateral folding, this connection is reduced to omphaloenteric (vitelline) duct ▪ The region of attachment of the amnion to the ventral surface is also reduced to a narrow umbilical region Formation of the Body Wall Closure of the ventral body wall is complete except in the region of the connecting stalk Closure of the gut tube is complete except for a connection from the midgut region to the yolk sac which forms the vitelline duct ▪ This duct is incorporated into the umbilical cord and eventually degenerates Serous Membranes The parietal layer of the lateral plate mesoderm forms the parietal layer of the serous membranes lining outside of the peritoneal, pleural, & pericardial cavities. The visceral layer of the lateral plate mesoderm forms the visceral layer of the serous membranes lining outside of the abdominal organs, lungs, and heart. Visceral & parietal layers are continuous with each other as the dorsal mesentery. ▪ Extends from the posterior body wall to the gut tube. Ventral mesentery exists from the caudal foregut to the upper portion of the duodenum. ▪ Extends from the anterior body wall to the gut tube ▪ Results from the thinning of the mesoderm of the septum transversum. Septum Transversum Thick mesodermal tissue between the thoracic cavity & stalk of the yolk sac. Derived from the visceral (splanchnic) mesoderm surrounding the heart and assumes its position between the primitive thoracic and abdominal cavities when the cranial end of the embryo grows and curves into the fetal position. This septum does not separate the thoracic and abdominal cavities, but leaves large openings, the pericardioperitoneal canals, on each side of the foregut. Development of the Diaphragm During further development, the opening between the (future) pleural & peritoneal cavities is closed by pleuroperitoneal folds. Eventually, the pleuroperitoneal folds & then membranes (thinner) grow medially to fuse with the mesentery of the esophagus that forms the crura of the diaphragm, around the aorta. The pleuroperitoneal membranes then grow until they fuse and cover the septum transversum to form the central tendon of the diaphragm. ▪ Also provides a connective tissue scaffold for muscle cells migrating in on the periphery. Overall, the diaphragm is derived from: ▪ 2 pleuroperitoneal membranes ▪ Muscular components from somites at C3-C5 ▪ Mesentery of the esophagus Congenital Diaphragmatic Hernias 1/2,000 Typically, occur when muscle cells fail to populate a region of the pleuroperitoneal membranes, resulting in a weakened area and subsequent herniation of abdominal organs into the thoracic cavity ▪ Muscle deficiency: Fibroblasts in the pleuroperitoneal membranes fail to provide the appropriate scaffolding and/or guidance cues for migrating myoblasts. Quiz Which embryonic layer contributes to the formation of the serous membranes that line the outside of the abdominal organs, lungs, and heart? A. Ectoderm B. Endoderm C. Mesoderm D. Epiblast Quiz Which embryonic layer contributes to the formation of the serous membranes that line the outside of the abdominal organs, lungs, and heart? A. Ectoderm B. Endoderm C. Mesoderm D. Epiblast Quiz The diaphragm develops from the fusion of which of the following structures? A. Mesonephros and metanephros B. Notochord and neural tube C. Somites, pleuroperitoneal membranes, mesentery of the esophagus D. Sclerotome and dermatome Quiz The diaphragm develops from the fusion of which of the following structures? A. Mesonephros and metanephros B. Notochord and neural tube C. Somites, pleuroperitoneal membranes, mesentery of the esophagus D. Sclerotome and dermatome GI System Overview 4 sections: ▪ Foregut (1): Pharynx extends from the oropharyngeal membrane to the respiratory diverticulum ▪ Foregut (2): Caudal to the pharyngeal tube and extends to the liver outgrowth ▪ Midgut: Caudal to the liver bud and extends to the junction of right two- thirds and left third of the transverse colon in adults ▪ Hindgut: Extends from the left third of the transverse colon to the cloacal membrane Endoderm: epithelial lining (parenchyma) Visceral mesoderm: stroma Overview & Foregut Before the 4th week, the gut tube is closed at both ends (oropharyngeal & cloacal membranes) All associated glands of the digestive tract are derived from the primitive gut Muscles, connective tissue, and other layers of the wall of the GI tract are derived from the splanchnic mesoderm Foregut (supplied by the Celiac Trunk): ▪ Primordial pharynx (oral cavity, pharynx, tongue, tonsils, salivary glands) & respiratory system ▪ Esophagus, stomach ▪ Duodenum (proximal to the opening of the bile duct) ▪ Liver, biliary apparatus (hepatic duct, gall bladder, bile duct), pancreas Development of the Esophagus Develops from the foregut, caudal to the pharynx Separated from the trachea (ventral) via the esophagotracheal (tracheoesophageal) septum Elongated growth due to the growth of the heart & lungs Upper 2/3rd: striated & innervated by vagus nerve Lower 1/3rd: smooth & innervated by celiac plexus Esophageal Atresia & Tracheoesophageal Fistula Atresia: Esophagus ends in a blind tube. Fistula: Abnormal connection between the trachea and esophagus. Most malformations are caused by the tracheoesophageal septum deviation. One of its clinical symptoms is polyhydramnios. Development of the Stomach Fusiform enlargement of the caudal part of the foregut Different rates of growth of borders: ▪ Dorsal (faster) → greater curvature ▪ Ventral (slower) → lesser curvature Stomach rotates rotates 90° (around the long axis) clockwise: ▪ Ventral (Ant.) border moves to the right (lesser curvature) ▪ Dorsal (Post.) border moves to the left (greater The left vagus nerve, initially innervating the left curvature). side of the stomach, now innervates the anterior wall; similarly, the right nerve innervates the posterior wall. Stomach & Mesenteries Attached to the dorsal body wall by the dorsal mesogastrium and to the ventral body wall by the ventral mesogastrium (part of the septum transversum). As the liver grows, the mesoderm forming the ventral mesogastrium becomes thinner and forms: ▪ The peritoneum covering the liver; ▪ The lesser omentum, connecting the stomach to the liver; Free margin becomes the portal pedicle & roof of epiploic foramen ▪ The falciform ligament, connecting the liver to the ventral body wall. Contains the umbilical vein Rotation pulls the dorsal mesogastrium to the left, creating a space behind the stomach called the omental bursa (lesser peritoneal sac). This rotation also pulls the lesser omentum to the right. Stomach & Spleen In the 5th week of development, the spleen primordium forms within the dorsal mesogastrium. ▪ The stomach's rotation causes the dorsal mesogastrium to lengthen and shift to the left. ▪ Thus, portion of the dorsal mesogastrium between the spleen and dorsal midline attaches to the posterior abdominal wall via Toldt fascia. ▪ The spleen becomes connected to: The posterior body wall near the left kidney through the lienorenal ligament. The stomach through the gastrolienal ligament. Stomach & Pancreas The lengthening and attachment of the dorsal mesogastrium also determine the final position of the pancreas. ▪ Initially, the pancreas grows into the dorsal mesoduodenum. ▪ The tail of the pancreas extends into the dorsal mesogastrium and ultimately lies against the posterior body wall due to the mesogastrium's attachment. Stomach & Mesenteries The rotation of the stomach along causes the dorsal mesogastrium to bulge downward. ▪ This forms a double-layered sac, the greater omentum, which extends from the greater curvature of the stomach to the transverse colon. Pyloric Stenosis Pylorus hypertrophies and thickens Narrowing of the pyloric lumen → passage of food is obstructed Symptoms: projectile vomiting, polyhydramnios Development of the Duodenum Develops from the caudal part of the foregut and the proximal part of the midgut. The junction of the two parts is directly distal to the origin of the bile duct (major duodenal papilla after birth). ▪ Grows rapidly and forms a C-shaped loop ▪ With the rotation of the stomach and duodenum together, it becomes a retroperitoneal organ. ▪ As it is derived from both foregut and midgut, it is supplied by the branches of both celiac and superior mesenteric arteries. Development of the Duodenum Proliferation of the epithelial cells and obliteration of the lumen of the duodenum ▪ Recanalization (cell death) and re-opening of the lumen Duodenal Stenosis & Atresia Duodenal stenosis: Partial occlusion of the duodenal lumen ▪ Usually caused by incomplete recanalization of the duodenum Duodenal atresia: Complete occlusion of the lumen of duodenum ▪ Distention of the epigastric region in infants and vomiting begins within a few hours of birth. The vomit almost always contains bile. ▪ Polyhydramnios. Development of the Liver & Biliary Apparatus Hepatic diverticulum (liver bud): A ventral outgrowth from the caudal part of the foregut. It gives rise to the liver, gallbladder, and biliary duct system. ▪ The connection between the hepatic diverticulum and the foregut (duodenum) narrows, forming the bile duct. A small ventral outgrowth is formed by the bile duct and gives rise to the gallbladder (caudal part) and cystic duct. ▪ During further development, epithelial liver cords merge with the vitelline and umbilical veins, which form hepatic sinusoids. The cranial part of the liver bud grows superiorly and meets the septum transversum. The liver bud forms the parenchyma of the liver and biliary ducts. Kupfer cells and hematopoietic tissue of the liver are derived from mesenchyme in the septum trasversum. Extrahepatic Biliary Atresia & Bifid Gall Bladder Extrahepatic biliary atresia: Part of the bile ducts are either blocked or do not fully develop ▪ Failure of recanalization Bifid gall bladder: duplication of the gall bladder Development of the Pancreas Develops from pancreatic buds that arise from the caudal part of the foregut Due to the rotation of the duodenum, the ventral bud is carried dorsally with the bile duct. ▪ Fusion of the two buds. ▪ Ventral bud forms a part of the head and dorsal bud forms the rest of the pancreas. Annular Pancreas Abnormal rotation → the ventral bud forms a ring of pancreatic tissue around the duodenum, rather than fusing completely. ▪ The ring of pancreatic tissue can compress the duodenum, leading to partial or complete obstruction. Midgut The midgut is suspended from the dorsal abdominal wall by an elongated mesentery and is supplied by the Superior Mesenteric Artery (SMA). Formation of the midgut loop (a ventral U-shaped loop of the gut) with a cranial and a caudal limb due to the elongation of the midgut. The midgut is projected into the proximal part of the umbilical cord. ▪ Happens because there is not enough room in the abdomen for the rapidly growing midgut. Structures: ▪ Small intestine, including most of the duodenum. ▪ Cecum, vermiform appendix, ascending colon, and the right two-third of the transverse colon. Midgut Loop The Midgut loop has two counterclockwise rotations during its development: ▪ A 90-degree rotation during protrusion and a 180-degree rotation during returning to the abdomen. ▪ Rotation occurs around the axis of the SMA. ▪ During the 10th week of development, the intestines return to the abdomen (reduction of the physiological umbilical or midgut hernia). ▪ The small intestine is the first part that comes back to the abdomen and the large intestine comes after. Fixation of the Intestines Rotation of the stomach and duodenum brings the duodenum and pancreas to the right side of the abdominal cavity. ▪ They are pressed to the posterior abdominal wall by the colon. The adjacent layers of the peritoneum fuse and subsequently disappear. Most of the duodenum and pancreas (except its tail) become retroperitoneal. With this mechanism (pressing some parts of the midgut and hindgut to the posterior abdominal wall) the ascending colon and descending colon become retroperitoneal. Jejunum and ileum as well as the transverse colon and sigmoid colon retain their mesentery and take their intra-abdominal final position. Development of the Cecum & Appendix The primordium of the cecum and appendix – the cecal diverticulum – appears as a swelling on the midgut. During the descending of this diverticulum, the ascending colon, cecum, and appendix form. Growth of the cecum and appendix continues after birth and the appendix takes its final position. Congenital Omphalocele Persistence of physiological herniation of the intestinal loops into the proximal part of the umbilical cord Gastroschisis Due to the malformation of the anterior abdominal wall (incomplete closure of the lateral folds during the 4th week) the abdominal viscera protrude into the amniotic cavity. Umbilical Hernia After coming back to the abdominal cavity, the intestines herniate again through an imperfectly closed umbilicus. Meckel (Ileal) Diverticulum The proximal portion of the yolk stalk remains and forms this diverticulum. It can become inflamed and cause symptoms that mimic appendicitis. ▪ Even the wall of the diverticulum may contain small patches of gastric and pancreatic tissues. Gastric mucosa often secretes acid, producing ulceration, and bleeding. Umbilical Fistula & Vitelline Cyst Cyst: Both ends of the vitelline duct transform into fibrous cords, and the middle portion forms a large cyst Fistula: The vitelline duct remains patent over its entire length, forming a direct communication between the umbilicus and the intestinal tract. Quiz During the embryonic development of the stomach, which of the following statements is correct regarding its rotation? A. The stomach rotates 180° clockwise around its long axis. B. The stomach rotates 90° clockwise around its long axis. C. The stomach rotation involves the ventral (anterior) border moving to the left. D. The stomach rotation involves the dorsal (posterior) border moving to the right. Quiz During the embryonic development of the stomach, which of the following statements is correct regarding its rotation? A. The stomach rotates 180° clockwise around its long axis. B. The stomach rotates 90° clockwise around its long axis. C. The stomach rotation involves the ventral (anterior) border moving to the left. D. The stomach rotation involves the dorsal (posterior) border moving to the right. Quiz Which statement accurately describes innervation of the esophagus? A. Vagus nerve in the upper 2/3rd; Celiac plexus in the lower 1/3rd B. Hypoglossal nerve in the upper 2/3rd; Pelvic splanchnic nerves in the lower 1/3rd C. Facial nerve in the upper 2/3rd; Sympathetic trunk in the lower 1/3rd D. Glossopharyngeal nerve in the upper 2/3rd; Lumbar splanchnic nerves in the lower 1/3rd Quiz Which statement accurately describes innervation of the esophagus? A. Vagus nerve in the upper 2/3rd; Celiac plexus in the lower 1/3rd B. Hypoglossal nerve in the upper 2/3rd; Pelvic splanchnic nerves in the lower 1/3rd C. Facial nerve in the upper 2/3rd; Sympathetic trunk in the lower 1/3rd D. Glossopharyngeal nerve in the upper 2/3rd; Lumbar splanchnic nerves in the lower 1/3rd Hindgut Structures: ▪ The left 1/3 of the transverse colon, descending colon, and sigmoid colon; the rectum and superior part of the anal canal. ▪ The epithelium of the urinary bladder and most of the urethra. All the hindgut derivatives are supplied by the Inferior Mesenteric Artery (except the urinary bladder and urethra). Due to the midgut rotation and returning the intestinal loop to the abdomen, the descending colon becomes retroperitoneal, and the sigmoid colon retains its mesentery. Development of the Cloaca The terminal dilated portion of the hindgut is called the cloaca. ▪ It is in contact with the surface ectoderm at the cloacal membrane. The cloacal membrane is composed of the endoderm of the cloaca and the ectoderm of the proctodeum (anal pit). The urorectal septum is a wedge-shaped mass of mesenchyme which divides the cloaca into two sections: ▪ The ventral part becomes the urogenital sinus, which will form structures like the bladder and urethra. ▪ The dorsal part becomes the rectum and the upper part of the anal canal. Development of the Cloaca As the urorectal septum grows and moves towards the cloacal membrane, it splits the membrane into: The dorsal anal membrane will cover the developing rectum and anal canal. The ventral urogenital membrane will cover the urogenital structures. The proctodeum (anal pit) is a depression in the ectoderm that forms on the anal membrane and helps in the development of the lower part of the anal canal. When the anal membrane ruptures, it creates an opening for the lower part of the digestive tract to connect with the amniotic cavity.  Development of the Anal Canal The superior 2/3 of the adult anal canal is derived from hindgut. The inferior 1/3 develops from the proctodeum (ectoderm). The pectinate line demarcates the border between these two origins Parts of anal Nerve Artery Venous Drainage Lymphatic canal Drainage Upper 2/3 of Autonomic Superior Superior rectal Inferior anal canal N.S. rectal A. vein (to the portal mesenteric lymph system) nodes Lower 1/3 of Pudendal Inferior rectal Inferior rectal vein Inguinal lymph anal canal nerve A. (to IVC) nodes (somatic N. S2/3/4) Internal & External Hemorrhoids Pectinate Line: Separates the upper two-thirds of the anal canal (where internal hemorrhoids occur) from the lower one-third (where external hemorrhoids occur). Internal Hemorrhoids: ▪ Are covered by a mucous membrane that lacks pain receptors. This is why internal hemorrhoids typically don't cause significant pain. External Hemorrhoids: ▪ Are covered by skin that contains pain receptors. This is why they are often associated with pain and discomfort. Congenital Megacolon (Hirschsprung Disease) Absence of autonomic ganglion cells in the myenteric plexus, in a segment of the colon. ▪ Ganglion cells are responsible for coordinating the rhythmic contractions that move feces through the intestines. ▪ Ganglion cells usually migrate from the neural crest cells to the entire length of the colon. In Hirschsprung's disease, there is a failure of this migration in a segment of the colon. This segment of the colon becomes unable to relax (stays dilated) and propel stool forward. Amniotic Fluid Volume & Function of Gut Tube 1. Protects the fetus in the event the maternal abdomen is the object of trauma. 2. Protects the umbilical cord by providing a cushion between the fetus and the umbilical cord. 3. Helps protect the fetus from infectious agents due to its inherent antibacterial properties. 4. Serves as a reservoir of fluid and nutrients for the fetus containing: proteins, electrolytes, immunoglobulins, and vitamins from the mother. 5. Provides the necessary fluid, space, and growth factors to allow normal development and growth of fetal organs such as the musculoskeletal system, gastrointestinal system, and pulmonary system. 6. Clinicians can use amniotic fluid as a tool to monitor the progression of pregnancy and predict fetal outcomes. (Amniocentesis). Problems: Polyhydramnios (too much) & Oligohydramnios (too little) Case Study A 14-year-old boy presents with severe lower left quadrant (LLQ) abdominal pain, abdominal distension, nausea, vomiting, and absent bowel movements (signs of bowel obstruction). ▪ What are you thinking? Symptomatic Meckel’s Diverticulum References Langman’s Medical Embryology: Chapter 15 – Digestive System Chapter 7 – The Gut Tube and Body Cavities