Lecture 6: Anatomy & Development of the Foregut PDF

🐣 Lecture 6: Anatomy & development of the foregut Part 1: Introduction to the foregut The Gut Tube Develops into Structures From Cranial to Caudal Portions of the foregut in the : head & neck, thorax, and abdomen L...

🐣 Lecture 6: Anatomy & development of the foregut Part 1: Introduction to the foregut The Gut Tube Develops into Structures From Cranial to Caudal Portions of the foregut in the : head & neck, thorax, and abdomen Lecture 6: Anatomy & development of the foregut 1  The diaphragm is the division between the thorax and abdomen Part 2: Folding and foregute anatomy Foregut anatomy Only the stomach, pancreas and liver are part of the gut tube proper during development The spleen migrate into the gut tube area later The liver sits off to the right side and superiorly the stomach is more of a mid line structure The pancreas sits posterior, behind the stomach, nestled against the posterior abdominal wall Lecture 6: Anatomy & development of the foregut 2  Lateral Folding Incorporates the Gut Tube into the body Dorsally of the embryo is the amniotic cavity and the ectoderm, ventrally the yolk sac and endoderm In the process of lateral folding: Lecture 6: Anatomy & development of the foregut 3  The amniotic cavity takes on fluid rapidly → growing so significantly that the lateral walls of that amniotic cavity grow towards the yolk SAC (the ectoderm also coming around/folding) → helps create a great little space where the gut tube is now encased The somatic mesoderm lining the amniotic cavity, which is going to be the peritoneum, is now lining both the gut tube and the body wall (inner red semi-circle thing) The part covering the body wall from lateral folding = parietal peritoneum The part covering the gut tube = visceral peritoneum The mesentery connects the parietal and visceral peritoneum Cephalic-caudal Folding (Head to Tail Folding) Lecture 6: Anatomy & development of the foregut 4  Cephalocaudal (Longitudinal) Folding is Induced by Differential Cell Growth of Ectoderm and Endoderm The brain grows rapidly - forcing the head forward and pushing the heart and the diaphragm from the neck region into the thoracic region The diaphragm is actually innervated by the phrenic nerve, phrenic nerve is innervated by C3, C4 and C5 nerves. Those are cervical nerves. They're telling you that the origin of the diaphragm is actually from the neck Folding continutes to occur, forming the three different parts of the gut tube (fore, mid, and hind) Cranial caudal folding is what creates these different regions of the gut tube. The lateral folding is what leads to the gut tube being enclosed in the body wall. Lecture 6: Anatomy & development of the foregut 5  Parietal/somatic mesoderm ⇒ gets innervated by the same nerves that innervate your body wall. So it's sensitive to pain. It can feel, temperature, pressure Splanchnic/visceral mesoderm ⇒ allow for movement of the gut tube The visceral layer lining the endoderm or gut tube (yellow) and we have the parietal layer lining the ectoderm (blue) → this is one sheath Part 3: Mesentery The Gut Tube Develops into Many Different Structures Lecture 6: Anatomy & development of the foregut 6  Foregut - have both ventral and dorsal mesentery (the only area of the gut tube that has this) Septum transversum in the foregut region is what allows the foregut to have both a ventral and a dorsal mesentery. Ventral Mesentery: The septum transversum provides a structure from which the ventral mesentery can develop. This ventral mesentery connects the foregut structures (like the stomach and liver) to the anterior body wall. The ventral mesentery is essential for the development of structures such as the lesser omentum (which connects the stomach to the liver) Unlike the foregut, the midgut and hindgut do not have a ventral mesentery. This is because the septum transversum does not extend into these regions. The diaphragm, derived from the septum transversum, does not reach down to the midgut and hindgut, so these parts of the gut tube only retain a dorsal mesentery. Lecture 6: Anatomy & development of the foregut 7 Lecture 6: Anatomy & development of the foregut 8  ^^ Liver grows into the septum transversum and ventral mesentery Liver Growth into the Septum Transversum and Ventral Mesentery 1. Liver Development: The liver begins to develop as a bud (hepatic diverticulum) from the ventral part of the foregut. As it grows, it extends into the ventral mesentery, which is the double layer of peritoneum that connects the developing liver to the anterior body wall and stomach. 2. Interaction with the Septum Transversum: As the liver expands, it grows into the septum transversum, a thick plate of mesodermal tissue that is the precursor to the diaphragm. This connection anchors the liver to the developing diaphragm. 3. Peritoneal Coverage of the Liver: Most of the liver is covered by peritoneum, a serous membrane that lines the abdominal cavity. The peritoneum associated with the ventral mesentery covers the liver's surface, except where the liver directly contacts the diaphragm. 4. Bare Area of the Liver: The bare area refers to the portion of the liver that is in direct contact with the diaphragm and is not covered by peritoneum. This area forms because, as the liver grows into the septum transversum, it becomes fused with the diaphragm. The bare area is located on the posterior (upper) surface of the liver, where it attaches directly to the diaphragm. This region is significant because it lacks the peritoneal covering that is present on the rest of the liver’s surface. 5. Anatomical Implications: The bare area of the liver is bordered by reflections of the peritoneum, known as the coronary ligaments, which help define the extent of the bare area. Lecture 6: Anatomy & development of the foregut 9  This area is important in surgical and clinical contexts because it represents a region where the liver is fixed to the diaphragm, influencing how the liver moves with respiration and its relationship to surrounding structures. Side stitch example Side Stitch Pain when Running: Description: When running, some people experience a sharp pain on the right side of their abdomen, often referred to as a "side stitch." In severe cases, this pain can radiate to the shoulder. Cause: The pain occurs because the liver, which is connected to the diaphragm by peritoneal reflections (ligaments), pulls on the diaphragm during the repetitive motion of running. If the abdominal muscles are not strong enough to stabilize the liver, this pulling action causes the diaphragm to spasm. Diaphragm Spasms and Phrenic Nerve: Phrenic Nerve: The diaphragm is innervated by the phrenic nerve, which originates from cervical spinal nerves C3, C4, and C5. Referred Pain: The phrenic nerve also has connections to the supraclavicular nerves, which supply sensation to the shoulder area. Because both the diaphragm and shoulder share innervation from the same cervical nerves, the brain can misinterpret the source of pain, leading to referred pain in the shoulder. Practical Advice: Breathing Technique: To alleviate a side stitch, it’s recommended to take a deep breath and exhale strongly. This helps relax the diaphragm and can reduce the spasm. Strengthening Abdominal Muscles: Strengthening the abdominal muscles can help stabilize the liver and reduce its "bouncing" effect on the diaphragm during activities like running. This can prevent the Lecture 6: Anatomy & development of the foregut 10  diaphragm from spasming and reduce the likelihood of experiencing a side stitch. Part 4: Mesentery and foregut rotation Each gut region is supplied by a main artery Blood Supply to the Gut Tube: The gut tube is supplied by three major arteries, each responsible for different regions: Celiac Trunk: Supplies the foregut (e.g., stomach, liver, spleen, pancreas). Superior Mesenteric Artery (SMA): Supplies the midgut (e.g., small intestine, caecum, ascending colon). Inferior Mesenteric Artery (IMA): Supplies the hindgut (e.g., descending colon, sigmoid colon, rectum). Anatomical Branching: Lecture 6: Anatomy & development of the foregut 11  These arteries branch from the abdominal aorta, the main artery that runs down the abdomen. The arteries branch off at specific vertebral levels: Celiac Trunk: At the level of T12. Superior Mesenteric Artery (SMA): At the level of L1. Inferior Mesenteric Artery (IMA): At the level of L3. Intraperitoneal Travel: As these arteries branch off the abdominal aorta, they travel between the layers of peritoneum (the membrane lining the abdominal cavity), becoming intraperitoneal. This means they are within the peritoneal cavity and supply the organs of the gut tube. The Stomach is the Division of Foregut Mesenteries from Ventral to Dorsal! The ventral mesentery originates from the septum transversum (which later contributes to the diaphragm) and connects the front (ventral) of the Lecture 6: Anatomy & development of the foregut 12  stomach to the anterior body wall. The dorsal mesentery connects the back (dorsal) of the stomach to the posterior body wall. It runs along the entire length of the gut tube. In the described cross-sectional view, the stomach is centrally located, with the liver positioned ventrally (in front of the stomach). The stomach is connected to the posterior body wall by the dorsal mesentery. The liver growing out ventrally from the stomach (near the duodenum) divides the ventral mesentery, establishing a connection between the liver and the ventral body wall. Thus, the stomach is connected both ventrally and dorsally by these mesenteries, with the liver playing a crucial role in the ventral connection. Foregut rotation - Liver & Stomach Liver Growth and Foregut Rotation: The liver grows out from the ventral (front) side of the developing foregut, in front of the stomach. As the liver enlarges, it exerts a force that causes the stomach and the surrounding gut tube to rotate to the right. This rotation is crucial for the proper positioning of the stomach and other foregut structures in the abdominal cavity. Uneven Growth of the Stomach: During rotation, the dorsal side of the stomach (the back part) grows faster than the ventral side (the front part). This uneven growth causes the stomach to tilt and rotate further, so it is no longer in a simple vertical orientation. This tilting results in the lesser curvature (the inner curve) of the stomach facing more to the right, while the greater curvature (the outer curve) shifts downward. Lecture 6: Anatomy & development of the foregut 13  Mesentery and Omentum Formation: The mesentery, a double layer of peritoneum that initially surrounds the gut tube, is pulled along with the rotating stomach. This movement and the resulting rearrangement of the mesentery contribute to the formation of the omentum, specifically the lesser omentum (connecting the stomach to the liver) and the greater omentum (a large fold that hangs down from the greater curvature of the stomach). Final Orientation of the Stomach: After rotation, the left side of the stomach, which was initially facing left, now faces anteriorly (to the front), and the ventral surface of the stomach (which faced forward) is now oriented more to the right. The lesser curvature is now positioned on the right side of the body, and the greater curvature faces inferiorly (downwards), leading to the stomach's final anatomical position in the abdominal cavity. Lecture 6: Anatomy & development of the foregut 14  ^^ 1) Liver 2) Stomach 3) Gallbladder 4) Ventral pancreatic bud 6) Dorsal pancreatic bud Foregut rotation - Liver & Stomach Lecture 6: Anatomy & development of the foregut 15 Lecture 6: Anatomy & development of the foregut 16  Initial Development: Ventral Pancreatic Bud: Initially, the ventral pancreatic bud forms as an outgrowth from the liver bud, which originates from the foregut. It is positioned ventrally (toward the front) in relation to the developing gut tube. Dorsal Pancreatic Bud: The dorsal pancreatic bud develops independently on the opposite side of the gut tube, positioned dorsally (toward the back). Rotation Process: 1. Foregut Rotation: As the foregut rotates to the right during development, the ventral pancreatic bud, which is connected to the liver, begins to rotate along with it. The liver, growing into the ventral mesentery, pulls the ventral bud around. Lecture 6: Anatomy & development of the foregut 17  2. Meeting of the Buds: The ventral pancreatic bud rotates around to the right and continues until it meets the dorsal pancreatic bud. This rotation effectively moves the ventral bud from its original anterior position to a more posterior location, where it comes into contact with the dorsal bud. 3. Fusion of the Buds: Once the ventral and dorsal buds meet, they fuse together to form a single pancreas. This fusion allows the ducts from each bud to connect and create the main pancreatic duct. Anatomical Consequences: Retroperitoneal Positioning: The pancreas starts as an intraperitoneal structure (surrounded by peritoneum). However, as the rotation and fusion occur, the pancreas becomes retroperitoneal (positioned behind the peritoneum) because the peritoneal lining on the posterior side of the pancreas disappears as it adheres to the posterior body wall. This is why the pancreas is described as secondarily retroperitoneal. Pancreatic Tail and Spleen: The tail of the pancreas, which is part of the dorsal bud, remains intraperitoneal as it is closely associated with the spleen, another intraperitoneal organ. There's this little piece at the bottom that's dorsal to what's growing out → unicate process Lecture 6: Anatomy & development of the foregut 18  Key Identifying Features in Adult Anatomy: Uncinate Process: The uncinate process, a small part of the pancreas that lies behind the superior mesenteric artery and vein, is derived from the ventral pancreatic bud. Its position posterior to these vessels indicates its origin from the ventral bud. Dorsal Bud Remnants: The rest of the pancreas (including the body and tail) remains ventral or anterior to these vessels, signifying its origin from the dorsal bud. What happens if things go wrong? Ventral pancreatic bud fails to rotate properly or if pancreatic tissue is left behind during the rotation process ⇒ Annular Pancreas - an abnormal ring or collar of pancreatic tissue that encircles the duodenum (the part of the small intestine that connects to stomach) Consequences of Annular Pancreas: Lecture 6: Anatomy & development of the foregut 19  Encircling of the Duodenum: The abnormal rotation or tissue development can cause the pancreas to form a ring around the duodenum (the first part of the small intestine). This ring of pancreatic tissue, called an annular pancreas, can constrict the duodenum. Obstruction Symptoms: The constriction of the duodenum leads to a blockage that prevents food from passing from the stomach into the small intestine. This obstruction causes symptoms like vomiting, particularly in infants and children, as they are unable to keep food down. Surgical Intervention: Because the condition involves a physical blockage, the only effective treatment is surgery. The surgical procedure typically involves bypassing the obstructed part of the duodenum to restore the normal flow of food through the digestive tract. All foregut structures start as intraperitoneal but because of rotation end up with different relationships with peritoneum! Lecture 6: Anatomy & development of the foregut 20  The portal vein is a major vessel that drains blood from the abdominal organs, including the stomach, intestines, spleen, and pancreas, and delivers it to the liver Part 5: Rotation and shunts Lecture 6: Anatomy & development of the foregut 21  Lesser sac = omental bursa The omental bursa or lesser sac is a hollow space that is formed by the greater and lesser omentum and its adjacent organs Lecture 6: Anatomy & development of the foregut 22  Foregut Rotation: As the foregut structures develop, including the stomach, liver, and spleen, they undergo significant rotation. This rotation creates two distinct spaces: the lesser sac (or omental bursa) and the greater sac. Division of Mesentery: The ventral mesentery is divided into two ligaments: Falciform Ligament: Connects the liver to the anterior abdominal wall. Hepatogastric Ligament: Connects the liver to the stomach. Collectively, these structures are part of what is known as the lesser omentum. Dorsal Mesentery: The spleen and its associated mesenteries divide the dorsal mesentery into: Gastrosplenic Ligament: Connects the spleen to the stomach. Splenorenal Ligament (or lienorenal ligament): Connects the spleen to the kidney. Formation of the Lesser and Greater Sacs: As the liver grows and pushes the stomach, the rotation shifts the structures: Lesser Sac (Omental Bursa): It is bounded by the liver anteriorly and the dorsal wall of the stomach posteriorly. A smaller cavity behind the stomach, bounded by the liver (ventrally) and the posterior body wall (dorsally) The liver's growth pushes the stomach, shifting the left side of the stomach anteriorly and the right side posteriorly. This displacement creates the lesser sac. Greater Sac: The remaining portion of the peritoneal cavity that becomes larger as the lesser sac forms. Lecture 6: Anatomy & development of the foregut 23  Shunts Fetal Liver and Liver Bypass: During fetal development, the liver and lungs are not yet functional in the way they will be after birth. The liver doesn’t perform its usual functions of metabolizing and detoxifying blood, and the lungs are not used for oxygen exchange. To adapt to these conditions, fetal blood circulation includes bypasses to avoid passing through the non-functional liver and lungs. Key Fetal Shunts: Ductus Venosus: This is the primary focus here. It is a critical shunt that allows oxygen-rich blood from the umbilical vein to bypass the liver and flow directly into the inferior vena cava. This shunt ensures that highly oxygenated blood from the placenta reaches the developing fetus’s vital organs, including the brain and heart, quickly and efficiently. Ductus Arteriosus and Foramen Ovale: These are the other key shunts associated with bypassing the lungs and directing blood flow from the right atrium to the left atrium in the fetal heart. Adult Structures: Ligamentum Venosum: This is the remnant of the ductus venosus. After birth, it becomes a fibrous band that is visible on the posterior side of the liver. It serves as a marker of the structure that once facilitated the bypass of the liver. Lecture 6: Anatomy & development of the foregut 24 Lecture 6: Anatomy & development of the foregut 25