Body Folding PDF

Body Folding List the steps from primitive streak formation to neural plate formation. Primitive streak forms along caudal midline of bilaminar embryonic disk; Its cranial end is expanded as primitive node. Position of future oropharyngeal membrane is indicated at cranial end o...

Body Folding List the steps from primitive streak formation to neural plate formation. Primitive streak forms along caudal midline of bilaminar embryonic disk; Its cranial end is expanded as primitive node. Position of future oropharyngeal membrane is indicated at cranial end of embryonic disk During gastrulation, epiblast cells ingress along primitive streak. First cells to ingress displace hypoblast and form definitive endoderm. Subsequent ingression cells migrate between epiblast and endoderm to form mesoderm. Mesoderm extends cranially from primitive node to form notochordal process, while mesoderm lateral to midline becomes paraxial, intermediate, and lateral plate mesoderm. Lateral plate mesoderm begins to split into two ways In future head paraxial mesoderm forms head mesoderm, while in the future trunk, paraxial mesoderm forms somites. Two layers of lateral plate mesoderm have now formed, somatic mesoderm and splanchnic mesoderm, and the two layers are separated by intraembryonic coelom Oropharyngeal and cloacal membrane are present and neural plate forms; expanded cranial part represents future brain, whereas narrow caudal part represents future spinal cord What is the process of folding and neurulation? What are the three primary germ layers? Ectoderm, mesoderm, endoderm What are the characteristics of ectoderm? After body folding, surface ectoderm surrounds the embryo and becomes the epidermis  Stomodeum and proctodeum are the primitive oral cavity and lower anal canal (both derived from ectoderm) Oropharyngeal and cloacal membranes are points where ectoderm and endoderm meet with no mesoderm in between What are the characteristics of mesoderm? Mesoderm differentiation in the trunk o Notochord (axial mesoderm) ▪ Rigid midline mesoderm structure ▪ Location of future vertebral bodies and the nucleus pulposus of intervertebral discs ▪ Induces neurulation ▪ Experiments (to the right) reveal importance of notochord in neurulation o Paraxial mesoderm (segments into somites from the occipital to coccygeal region) ▪ Somite: condensed paraxial mesoderm ▪ Segmentation into somites progresses from rostral to caudal ▪ Somites are condensed paraxial mesoderm and differentiate into: Dermatome (dermis of skin), Myotome (muscles), Sclerotome (vertebrae & ribs) Each myotome and dermatome retain its innervation from its segment of origin ▪ Dermamyotome: dermatome and myotome migrate together to form tissues of skin and muscle ▪ Segmentation into somites progresses from rostral to caudal o Intermediate mesoderm ▪ Temporarily connects paraxial and lateral plate mesoderm ▪ Forms urogenital structures: Gonadal ridge and Mesonephric system o Lateral plate mesoderm: Cavitates and splits into parietal and visceral mesoderm layers ▪ Splits into somatic and splanchnic layers ▪ Somatic mesoderm lines the intraembryonic (coelomic) cavity along the body wall ▪ Splanchnic mesoderm surrounds the internal organs ▪ Both somatic and splanchnic mesoderm form serous membranes (mesothelium) that line the adult peritoneal, pleural, and pericardial cavities  ▪ In the abdominal cavity, the serous membranes are the visceral and parietal peritoneum What are the differences between lateral and longitudinal folding in mesoderm? Lateral: transverse section through abdominal region o Brings lateral aspect of embryo ventrally o Wraps amniotic cavity around the embryo o Creates coelomic cavity that will become the peritoneal cavity in the abdominal region o Seals off gut tube (except where it is in contact with the yolk sac) Longitudinal: sagittal section o Head fold – due to rapid brain development ▪ Brings the developing heart ventrally and caudally ▪ Creates foregut ▪ Septum transversum separates heart from foregut o Tail fold – due to convergent extension and lengthening of spinal cord ▪ Creates hindgut ▪ Places connecting stalk on ventral aspect of embryo How does cardiogenic mesoderm develop from splanchnic mesoderm? Cardiac progenitor cells migrate through the primitive streak to the splanchnic mesoderm (of the lateral plate mesoderm) Cardiogenic mesoderm migrates anterior to the oropharyngeal membrane How does lateral folding merge the two sides of the cardia crescent? Two sides of cardiac crescent merge to from the endocardial tube Myocardial cells arise from splanchnic mesoderm Coelomic cavity becomes the pericardial cavity around the heart What are Somitomeres and Somites? Somitomeres in cephalic region Somites (42-44 pairs): 4 occipital (disappear), 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal (last 5-7 disappear) What happens after body folding? Medial mesoderm stays dorsal Lateral mesoderm moves ventrally Amniotic cavity surrounds the embryo  Coelomic cavity is within the embryo What are the characteristics and derivatives of an endoderm? Forms the inner lining of the gut tube within the folded embryo Derivatives: gut tube epithelium, trachea & respiratory epithelium, tonsils , thymus, thyroid, parathyroid, auditory canal, tympanic membrane, liver epithelium, gallbladder, pancreas, urinary bladder and urethra How is a gut tube formed? In the 4th week the ventral aspect of the trilaminar disc (endoderm) gets internalized to form the epithelial lining of the gut tube Simultaneous Cephalocaudal and lateral folding form the gut tube The splanchnic mesoderm gives rise to the wall of the gut tube The tube begins as a closed structure: Oropharyngeal membrane and Cloacal membrane Open at the level of the midgut to the yolk sac What is the primary germ layer contributions to the gut tube: Endoderm and splanchnic membrane? Stomodeum – ectoderm-lined invagination cranial to the oropharyngeal membrane Proctodeum – ectoderm-lined invagination caudal to the cloacal membrane Oropharyngeal membrane ruptures in week 4 Cloacal membrane ruptures in week 7 What is cephalocaudal folding of gut tube? view is a sagittal section Head fold o Places developing heart ventrally o Creates foregut o Brings septum transversum ventrally Tail fold o Creates hindgut o Places connecting stalk on ventral aspect of embryo What is septum transversum? Separates developing heart from midgut Mesenchyme contributes to diaphragm, stroma of liver, ventral mesentery What is lateral folding of gut tube?  Endoderm lines the lumen of the GI tract (epithelium) Splanchnic mesoderm against gut tube Somatic mesoderm against body wall Celomic cavity lies between splanchnic and somatic mesoderm Dorsal mesentery is derived from splanchnic lateral plate mesoderm What is a mesentery and how does it develop? It is a sheet of CT tissue lined by serous membrane on both sides – a double layer of peritoneum It develops from a broad sheet of mesenchyme What are the two different types of mesentery? Dorsal mesentery is a continuous sheet that maintains connection between gut tube and posterior abdominal wall o Keeps gut tube and derivatives in proper anatomical location o Provides a route for blood vessels, nerves, lymphatics Ventral mesentery – derived from mesenchyme of septum transversum What are Intraperitoneal, Retroperitoneal, & Secondarily Retroperitoneal? Intraperitoneal – organs are suspended from a mesentery Retroperitoneal – organs are outside of the peritoneal cavity Secondarily retroperitoneal – organs began as intraperitoneal, but dorsal mesentery fuses with dorsal body wall o Duodenum, pancreas, ascending and descending colon o Mechanism: What are the derivatives of the gut tube?  FOREGUT: Runs from oropharyngeal membrane to liver outgrowth (major duodenal papilla) o Pharyngeal gut – oropharyngeal membrane to lung bud – not reviewed in this lecture o Foregut proper – esophagus, stomach, proximal duodenum ▪ Esophagus: dorsal to respiratory primordium. Remains almost entirely within the thoracic cavity (thoracic vs abdominal esophagus). Gives rise to respiratory diverticulum Esophagus and RD: Striated muscle derived from somites o Smooth muscle derived from splanchnic mesoderm o Muscle innervated by CN X o Trachea and lung buds form from a diverticulum off the foregut Tracheoesophageal septum: mesoderm that separates the trachea from the esophagus ▪ Stomach – begins in the thoracic cavity and moves caudally as the esophagus lengthens. Rotates along a longitudinal and anteroposterior axes. Failure of the stomach to descend into the abdominal cavity results in a congenital diaphragmatic (hiatal) hernia Differential growth of the greater and lesser curvatures Rotates 90 degrees along a longitudinal axis (image C) Rotates along dorsoventral axis (image D) After Rotation: o Left Vagus innervates anterior wall, right vagus innervates posterior wall o Anterior side becomes lesser curvature o Posterior side becomes greater curvature Rotation of the Stomach, and Formation of the Greater Omentum  o Rotation of the stomach drags dorsal mesentery anteriorly and creates a sac o Greater Omentum is a double layer of mesentery that fuses and drapes from the greater curvature of the stomach in front of inferior organs ▪ Cephalic (proximal) duodenum – first part of the small intestine. Associated with abdominal foregut Parenchyma of glands is derived from the endoderm Stroma of glands is derived from splanchnic mesoderm Liver and gallbladder: o Hepatic plate – ventral endoderm thickening o Hepatic diverticulum – outgrowth from hepatic plate that gives rise to liver bud o Liver bud elongates, creating the bile duct and forms a branch, creating the gallbladder and cystic duct o Endoderm forms liver cords o The liver grows into the septum transversum: ▪ Septum transversum lines liver and becomes serosa ▪ Bare area of liver is where the septum transversum splits and liver directly contacts diaphragm ▪ Caudal septum transversum becomes the visceral peritoneum ▪ Falciform ligament forms from ventral mesentery ▪ Lesser omentum forms posterior to liver Pancreas: o The pancreas is formed by two buds ▪ The ventral pancreas buds off the bile duct (uncinate process) ▪ The dorsal pancreas buds off the duodenum (head, body, tail) ▪ Main pancreatic duct and hepatic duct merge at hepatopancreatic ampulla o The ventral pancreas rotates in the fifth week of development o Secondarily retroperitoneal – fixed to the posterior body wall o Hormone-secreting cells arise from pancreatic parenchyma and delaminate to form Islets of Langerhans  ▪ Abdominal foregut is supplied by the celiac trunk MIDGUT: Duodenum after bile duct, jejunum, ileum, cecum, ascending colon, proximal (right) 2/3 transverse colon o Runs from hepatic diverticulum (major duodenal papilla) to the junction between right two thirds and left third of the transverse colon o Distal duodenum, Jejunum and ileum, Cecum and appendix, Ascending colon, Right two thirds of transverse colon o Supplied by the superior mesenteric artery o Intestinal loop herniation: ▪ Intestinal loop: primitive midgut ▪ Umbilical Ring: Region of ventral body wall that is open to yolk sac and connecting stalk ▪ Rapid elongation of the midgut causes the intestinal loop to herniate through the umbilical ring ▪ Connecting stalk containing umbilical vessels also connects to the embryo o Pathological herniation: ▪ Umbilical hernia – small protrusion of bowel through the umbilical ring that is covered by skin ▪ Omphalocele – protrusion through umbilical ring, due to failure of intestines to return to abdominal cavity. Limited mesenchyme to close umbilical ring ▪ Gastroschisis – failure of anterior body wall to close properly. Not a protrusion through umbilical ring o Midgut rotation: ▪ During herniation, the primary intestinal loop rotates 90-degrees counterclockwise around the axis of the superior mesenteric artery ▪ Later in development, the gut rotates another 180 degrees within the abdominal cavity ▪ The cecum then descends from the liver to its adult location HINDGUT: Distal (left) 1/3 transverse colon, descending colon, sigmoid colon, rectum, upper anal canal (to pectinate line/cloacal membrane) o Runs from the left (distal) third of the transverse colon to the cloacal membrane ▪ Left 1/3 of transverse colon ▪ Descending colon ▪ Sigmoid colon ▪ Rectum and superior anal canal o The cloaca is an expanded region of the hindgut just deep to the cloacal membrane  ▪ Subdivision: Urorectal septum: mesoderm cord that divides the dorsal anorectal canal and ventral urogenital sinus o Anal Canal ▪ Superior 2/3 of anal canal derived from hindgut endoderm ▪ Inferior 1/3 of anal canal derived from ectoderm from proctodeum (anal pit) ▪ Pectinate line is the junction between endoderm and ectoderm ▪ Different nerve and vascular supply of upper and lower anal canal o Served by the inferior mesenteric artery Mention the summary of GI development Week 4 o Gut tube forms during body folding o Liver, gallbladder, pancreas bud from duodenum (distal foregut) o Oropharyngeal membrane ruptures Weeks 5 -6 o Stomach expands and rotates Week 7 o Midgut herniates through umbilical ring and begins to rotate (rotation extends through week 11) o Cloaca separates into urogenital sinus and anorectal canal o Cloacal membrane ruptures