Lecture 7: Midgut and Hindgut Development PDF

🐣 Lecture 7: Midgut and Hindgut development Part 1: Terminology Ventral Mesentery (origin is septum transversum) = Falciform Ligament + Lesser Omentum Lesser Omentum = Hepatogastric ligame...

🐣 Lecture 7: Midgut and Hindgut development Part 1: Terminology Ventral Mesentery (origin is septum transversum) = Falciform Ligament + Lesser Omentum Lesser Omentum = Hepatogastric ligament + Hepatoduodenal ligament Dorsal Mesentery = Greater Omentum + gastrosplenic ligament + splenoral ligament All Mesentery = aka Peritoneum started as Mesoderm sometimes referred to as Ligament Lecture 7: Midgut and Hindgut development 1  Part 2: Midgut Rotation The superior mesenteric artery (intraperitoneal structure) supplying the intestine is also the axis of rotation for the intestine During development, it is normal to have the intestines “outside” the abdominal cavity! Herniation of the gut tube is necessary to accommodate the mid-gut loop size The midgut loop, which is attached to the umbilical cord via the vitelline duct, forms around the SMA, which acts as the axis of rotation during development. The loop is divided into a cranial limb (towards the head) and a caudal limb (towards the tail). Lecture 7: Midgut and Hindgut development 2  Physiological Herniation: During early development, it's normal for the midgut loop to extend into the umbilical cord, a process known as physiological herniation. This occurs because the midgut grows rapidly and temporarily lacks space in the abdominal cavity. Eventually, the midgut loop returns to the abdomen as it continues to develop. Congenital Anomalies: If the midgut loop fails to return to the abdominal cavity, it results in congenital anomalies where portions of the intestines remain outside the abdomen after birth. While the presence of the intestines outside the abdomen is normal during a specific stage of development, failure to return is abnormal. Could be due to a variety of reasons: Differential growth isn't appropriate Anterior abdominal wall doesn't grow enough to allow the intestines back in, or The signal is telling the intestines to come back in are not appropriately delivered Mid-Gut Rotation (rotation vid ) Lecture 7: Midgut and Hindgut development 3  The Cranial Limb Outgrows the Caudal Limb → Rotation of the gut 90° in counter-clockwise direction Herniation and Differential Growth: As the midgut loop develops, it undergoes physiological umbilical herniation because the fetal abdominal cavity is too small to accommodate all the developing structures. This herniation involves both the cranial and caudal limbs of the midgut loop extending into the umbilicus. The cranial limb grows faster and more extensively than the caudal limb. This differential growth causes the cranial limb to rotate and move towards the right side of the embryo. As a result, the cranial limb flops down and becomes positioned inferior to the caudal limb. Rotation Mechanics: The rotation of the midgut loop around the SMA is a 90° counterclockwise rotation when viewed from the front of the fetus. If you were the fetus, this rotation would move towards the right side. Lecture 7: Midgut and Hindgut development 4  Initially, the cranial limb is positioned cranially (towards the head) and the caudal limb caudally (towards the tail). However, as rotation occurs, the cranial limb moves inferiorly, and the caudal limb moves superiorly, effectively reversing their positions. Final Positioning: Despite their reversed positions, the limbs are still referred to as the cranial and caudal limbs. The cranial limb, now positioned more inferiorly, remains connected to the rest of the small intestine and the stomach. The caudal limb, after rotation, moves to a more superior position and will give rise to parts of the colon. The transverse colon forms as the caudal limb stretches across the abdominal cavity. The cecal bud, which is the beginning of the large intestine, becomes important later in development as it helps define the final positioning of the intestines. “Retention bands” help guide folding Midgut Rotation Limits: The midgut loop's rotation is controlled by peritoneal structures to prevent excessive twisting, which could lead to complications or "knots" in the abdomen. Retention Bands: Two important structures known as retention bands help guide and limit the rotation: Ligament of Treitz: This is the superior retention band. It is a thickening of the peritoneum that connects the diaphragm to the duodenum (part of the small intestine). The Ligament of Treitz plays a crucial role in anchoring the intestines and controlling the rotation of the cranial limb of the midgut loop. Lecture 7: Midgut and Hindgut development 5  Phrenicocolic Ligament: This is the inferior retention band. It extends from the diaphragm to the colon. This ligament also helps guide the proper rotation of the caudal limb of the midgut loop by providing structural limitations. Role in Foregut and Hindgut Development: These retention bands are essential not only for the midgut rotation but also for ensuring that the rotation does not interfere with the ongoing development of the foregut and hindgut. They help maintain a balance, ensuring that the midgut rotates within a controlled range without affecting the development of other gastrointestinal structures. Peritoneum's Role: Both the Ligament of Treitz and the Phrenicocolic Ligament are thickenings of the peritoneum or mesentery. The peritoneum, a membrane lining the abdominal cavity, provides the necessary structural support to guide and limit the rotation of the intestines. Lecture 7: Midgut and Hindgut development 6  Anatomical Locations of “Retention Bands” are Close Suspensory Ligament (Muscle) of duodenum (Ligament of Treitz) This ligament is a combination of skeletal muscle fibers from the diaphragm and smooth muscle fibers from the duodenum. It serves to anchor the duodenum to the diaphragm, suspending the duodenum and helping maintain its position in the abdominal cavity. In adults, the Ligament of Treitz is covered by parietal peritoneum, which is a membrane that lines the abdominal cavity and covers the Lecture 7: Midgut and Hindgut development 7  internal organs. The ligament is therefore attached to structures that are closely associated with the body wall. Phrenicocolic Ligament: This ligament is part of the dorsal mesentery (a fold of peritoneum that attaches the intestines to the posterior abdominal wall) and connects the diaphragm to the colon. It is an extension of the peritoneum from the diaphragm down to the colon. The Phrenicocolic Ligament also plays a role in stabilizing the position of the colon within the abdominal cavity. Lecture 7: Midgut and Hindgut development 8  The caudal limb's rotation is more limited due to the stabilizing effect of the phrenicocolic ligament (a retention band). After rotation, the cranial limb ends up on the right side of the abdomen, while the caudal limb ends up on the left side. The caudal limb eventually forms part of the left third of the transverse colon and the descending colon. Lecture 7: Midgut and Hindgut development 9  Variation in Anatomical Structures Can Occur through Developmental Anomalies Final Stages of Midgut Development: After the midgut loop returns to the abdomen, it follows a specific sequence where the cranial loop (which forms the small intestine) returns first, and the caudal loop (which forms the large intestine) returns second. This process leads to the formation of various peritoneal and retroperitoneal structures: The duodenum and descending colon become secondarily retroperitoneal (meaning they initially develop intraperitoneally but later become retroperitoneal as they adhere to the posterior abdominal wall). The small intestine remains intraperitoneal. Separation from the Umbilicus: The final step in this process involves separating the midgut from the umbilicus. If this separation does not occur properly, it can result in anomalies, one of which is Meckel's diverticulum. Meckel's Diverticulum: Meckel's diverticulum is an anomaly that results from the failure of the vitelline (omphalomesenteric) duct to completely obliterate during development. The vitelline duct originally connects the midgut to the yolk sac and is supposed to disappear as the fetus develops. Location: Meckel's diverticulum typically forms at a specific location on the ileum, which is the midpoint between the cranial and caudal limbs of the midgut loop. Types of Meckel's Diverticulum and Associated Risks: Tethering: In some cases, a small remnant of the vitelline duct remains as a fibrous band tethered to the umbilicus. This tethering can cause Lecture 7: Midgut and Hindgut development 10  volvulus, a condition where the intestine twists around itself, leading to severe pain, obstruction, and potentially life-threatening complications due to the cutoff of blood supply. Infection: The remnant can also become a site for infection if food or bacteria from the intestine enter this structure. This can lead to inflammation, pain, and other complications. Complete Volvulus: In more severe cases, the presence of Meckel's diverticulum can lead to complete volvulus, where the intestines twist entirely around the vitelline duct remnant, causing significant obstruction and risk of necrosis. Part 3: Ascending Colon Growth Lecture 7: Midgut and Hindgut development 11  During development, the ascending colon grows downward (inferiorly) Transverse and Descending Colon: The transverse colon, which is horizontal, and the descending colon, which moves downward on the left side of the body, are related to the "caudal limb" during development. Vitalian Intestinal Duct: This is also known as the vitelline duct, a structure connecting the embryonic gut to the yolk sac. It typically disappears during development. If it doesn't, it could lead to complications like twisting of the intestines. Lecture 7: Midgut and Hindgut development 12  Appendix Position: The position of the appendix is highly variable in adults, influenced by how much the ascending colon grows downward. The appendix is attached to the cecum, the first part of the ascending colon. Peritoneum and Retroperitoneal Structures: As the ascending colon grows, it becomes covered by peritoneum, a membrane lining the abdominal cavity. Over time, parts of the ascending colon may adhere to the posterior abdominal wall, becoming "secondarily retroperitoneal," meaning they were initially within the peritoneal cavity but later moved behind it. The lower part of the ascending colon might remain intraperitoneal (within the peritoneal cavity), while the upper part becomes retroperitoneal. The appendix is typically located in the inferior right quadrant / right lower quadrant following re-entry Re-entry refers to the process during fetal development when the intestines temporarily protrude out of the abdominal cavity into the umbilical cord (a process called physiological herniation) and then re-enter the abdominal cavity as the embryo grows. This is a normal part of midgut development. Variable Positioning of the Appendix: The appendix can have a highly variable position in adults. It might be located anywhere from the right upper quadrant (RUQ) to the right lower quadrant (RLQ) of the abdomen. This variability is due to the way the appendix and the ascending colon develop and migrate during embryonic growth. ^^ Because of this variability, appendicular pain (such as in appendicitis) can be felt in different areas, not just the RLQ, where the appendix is traditionally thought to be located. Eg) Pain could also manifest in the RUQ or anywhere in between. Referred pain Lecture 7: Midgut and Hindgut development 13  Even if the appendix ends up in the RLQ (its typical adult position), pain from appendicitis can still be referred back to the RUQ, where the appendix originally developed. This is due to the embryological pathway the appendix follows during development. So, pain might start in the RLQ but could also be felt in the RUQ as the condition progresses. Greater omentum Lecture 7: Midgut and Hindgut development 14  A large fold of the peritoneum that hangs down from the stomach and covers the intestines Development and Rotation: The greater omentum develops as part of the foregut, which includes the stomach. During development, the stomach rotates, and as it does, the dorsal mesogastrium (a double layer of mesentery attached to the stomach) grows significantly. This growth causes the greater omentum to expand and extend into the area of the midgut. The term "midgut loop" refers to a segment of the embryonic gut that temporarily herniates out of the abdomen and then re-enters. After this re-entry, the expanded greater omentum can extend down to and attach to the transverse colon, which is part of the midgut. Structure and Layers: The greater omentum is composed of four layers of mesentery (two double layers). Lecture 7: Midgut and Hindgut development 15  Two layers connected to the stomach and two additional layers that have grown from the mesentery. The greater omentum also contains blood vessels and nerves, which are important for its function and can cause pain if the omentum twists or turns. Function: One of the most fascinating functions of the greater omentum is its ability to migrate toward areas of infection within the abdomen. If there is an infection, the omentum can move to that area and help wall off the infection, preventing it from spreading to the rest of the abdomen. This "apron" can essentially limit the spread of bacteria and other harmful agents. However, because the omentum is rich in blood vessels and nerves, this migration can sometimes cause pain, especially if the omentum twists and disrupts its blood supply or innervation. In severe cases, this twisting can lead to a medical emergency. Diagram Greater momentum is draping off the greater curvature of the stomach because it's just a continuation of that dorsal mesentery Lecture 7: Midgut and Hindgut development 16  When you raise up the greater momentum → you see intraperitoneal structures like the transverse colons Part 4: Hindgut Development Cloaca Division: The cloaca is initially a single pouch that needs to divide into two parts: Lecture 7: Midgut and Hindgut development 17  Anteriorly: Forms the bladder. Posteriorly: Forms the rectum and anus. Importance of Proper Division: Proper division ensures that urine and feces are separated, preventing them from mixing. Failure in division can lead to a condition where both substances mix, similar to how birds' fecal matter shows a combination of liquid and solid waste due to their undivided cloaca. Embryological Process: The division of the cloaca involves epithelial migration and growth from the anterior body wall: Epithelium from the body wall migrates inward to divide the cloaca into distinct parts—bladder anteriorly and rectum/anus posteriorly. The distal end of this epithelial wave forms what is known as the perineal body, an embryological remnant of the division. Outcome of Successful Division: Successful division results in the formation of: The urogenital septum, which separates the urogenital triangle (bladder and genital structures) from the anal triangle (rectum and anus). Proper positioning of the bladder anteriorly and the rectum/anus posteriorly ensures normal waste elimination without mixing. Potential Complications: If the cloaca fails to divide properly: There can be a persistence of mixing between urine and feces, analogous to bird waste. Additionally, failure to disconnect the hindgut from the anterior abdominal wall can lead to urine dribbling out of the belly button post-birth. Lecture 7: Midgut and Hindgut development 18  Final Steps of Anorectal Development: In the last stages of anorectal development, the internal structures (lined with endoderm) need to connect with the external structures (lined with ectoderm). The gut tube, which is formed from endoderm, needs to fully extend to reach the ectoderm, which forms the external surface of the body. Meeting of Endoderm and Ectoderm: After the cloaca is divided and the anorectal region is separated, the ectoderm invaginates (grows inward) to meet the endodermal gut tube. This meeting is crucial for the proper formation of the anorectal region, where the internal lining of the gut tube connects with the external skin. Pectinate (Dentate) Line: The junction where the endodermal and ectodermal layers meet is visible in the adult anus and rectum as a line known as the Pectinate Line or Lecture 7: Midgut and Hindgut development 19  Dentate Line. Pectinate (Dentate) Line Significance: The pectinate line serves as a crucial anatomical boundary separating tissues of different embryological origins: Above the Pectinate Line: The tissue is derived from endoderm (gut tube). Below the Pectinate Line: The tissue is derived from ectoderm (external body surface). Impact on Adult Anatomy: Innervation: Lecture 7: Midgut and Hindgut development 20  Above the Pectinate Line: Innervation is visceral (related to internal organs), which generally results in less intense, diffuse pain. This is because visceral innervation tends to be less sensitive to pain stimuli. Below the Pectinate Line: Innervation is somatic (related to the body wall), which is highly sensitive, leading to sharp and intense pain when stimulated. This is why conditions like hemorrhoids below the pectinate line cause significant pain. Lymphatic Drainage: Above the Pectinate Line: Lymphatic drainage is deep, directed towards visceral (internal) lymph nodes. Therefore, issues like cancer above this line would involve deeper lymph nodes, which might not be as easily palpable. Below the Pectinate Line: Lymphatic drainage is superficial, involving lymph nodes closer to the skin surface. For instance, anal cancer below the pectinate line may cause swelling of superficial lymph nodes, which can be detected upon physical examination. Vascular Supply: Above the Pectinate Line: Vascular supply is linked to the internal organs and tends to be less associated with painful conditions. Below the Pectinate Line: Vascular supply is related to the body wall, and conditions such as hemorrhoids in this region can lead to significant discomfort due to the somatic (pain-sensitive) innervation. Clinical Implications: The difference in embryological origins (endoderm vs. ectoderm) results in different adult structures, which affects how conditions are experienced and diagnosed: Hemorrhoids: Those located above the pectinate line might cause a feeling of fullness or pressure without severe pain, whereas those below the line are extremely painful due to somatic innervation. Cancer: The location of cancer relative to the pectinate line determines the pattern of lymphatic drainage and the associated Lecture 7: Midgut and Hindgut development 21  symptoms, such as swelling of lymph nodes. Normal vs. Abnormal Development: In normal development, the endoderm (lining the gut) and ectoderm (external body surface) must meet and properly form the structures of the bladder, rectum, and anus. However, when this process goes awry, various congenital abnormalities can occur, affecting the structure and function of these organs. Abnormalities Rectocele (Cloacal Fistula) - A blind-ending pouch that can result from improper division or fusion during development. Anal Stenosis - Narrowing of the anal canal, often linked to issues with ectodermal development, leading to restricted passage of faeces Lecture 7: Midgut and Hindgut development 22  Persistent Anal Membrane - This occurs when the ectoderm fails to perforate as it should, leading to a blockage that prevents normal fecal passage. Anal Pit - In this case, the ectoderm forms a pit or opening that does not align properly with the endoderm, leading to a misalignment in the anorectal region. Rectovaginal Fistula - A condition where there is an abnormal connection between the rectum and vagina, allowing feces to pass through the vaginal canal. This may go unnoticed in newborns until solid feces are produced. Lecture 7: Midgut and Hindgut development 23  Rectourethral Fistula - An abnormal connection between the rectum and urethra, leading to feces being expelled through the urinary tract. This can result in bladder infections due to the presence of bacteria in the feces. Imperforate Anus - A relatively common congenital condition where the anus is either absent or closed off because the ectoderm did not perforate or grow enough to meet the endoderm. This is one of the last steps in hindgut development and can be surgically corrected. Lecture 7: Midgut and Hindgut development 24