Digestive System Embryology PDF

Dr ZN Tshabalala  The Developing Human (11th Edition) - Pages 193-217 Osmosis links - Development of body cavities - Development of the gastrointestinal system Complete Anatomy  Describe and discuss the development of the oesophagus Describe and discuss the development of the stoma...

Dr ZN Tshabalala  The Developing Human (11th Edition) - Pages 193-217 Osmosis links - Development of body cavities - Development of the gastrointestinal system Complete Anatomy  Describe and discuss the development of the oesophagus Describe and discuss the development of the stomach and duodenum up to the Ligament of Treitz Describe and discuss the development of the omental bursae and the lesser and greater omentum and the peritoneal cavity. Describe and discuss the development of the: o Liver and gallbladder o Pancreas o Spleen  Describe and discuss the development of the jejunum and ileum with regards to: o Herniation of the midgut loop o Rotation of the midgut loop o Retraction of intestinal loops Describe and discuss the development of the caecum and appendix Describe the rotation of the large bowel into its anatomical position Describe and discuss the development of the anal canal and the problem on an imperforate anus  Introduction Alimentary tract runs from the mouth to the anus Primordial gut forms during 4th week - Initially closed by the oropharyngeal membrane at cranial end - Initially closed by the cloacal membrane at caudal end Endoderm and splanchnic mesoderm - Form most of the gut, epithelium and glands Epithelium of the cranial and caudal ends - Derived from ectoderm of the stomodeum and proctodeum  Foregut Has the following derivatives - Primordial pharynx and its derivatives - Lower respiratory system - Esophagus and stomach - Duodenum, distal to the opening of the bile duct - Liver, biliary apparatus (hepatic ducts, gallbladder, and bile duct), and pancreas Most of the foregut is supplied by the coeliac trunk from the abdominal aorta  Foregut Oesophagus development Develops caudal to the pharynx Partitioned from the trachea by the tracheoesophageal septum Oesophagus initially short and rapidly elongates - Result of growth and relocation of heart and lungs Reaches its final length by the 7th week Epithelium and glands are derived from endoderm that proliferates - Partly or completely obliterates lumen  Foregut Oesophagus development Striated muscle forming muscularis externa (Superior 3rd) - Derived from mesenchyme in the 4th and 6th pharyngeal arches Smooth muscle develops from surrounding splanchnic mesenchyme (Inferior 3rd) Both muscles are innervated by the vagus nerve  Foregut Stomach development Distal part of the foregut is initially tubular Slight dilation occurs during the 4th week - At the site of the primordial stomach Dilation first appears as fusiform enlargement of the caudal part - Initially oriented in the median plane Left wall soon enlarges and broadens ventrodorsally Dorsal border of the right wall grows faster during the next 2 weeks - Demarcates developing greater curvature of the stomach  Foregut Stomach development (Rotation) Rotation contributed by the enlargement of mesentery and adjacent organs As stomach enlarges (final shape), it rotates 90⁰ clockwise - Along its longitudinal axis (Viewed from cranial end)  Foregut Stomach development (Rotation) Effects of the rotation - Ventral border (lesser curvature) moves to the right - Dorsal border (greater curvature) moves to the left - Original left side becomes the ventral surface - Original right side becomes the dorsal surface - Cranial region moves to the left and slightly inferiorly - Caudal region moves to the right and superiorly - Long axis almost transverse to the long axis of the body  Foregut Stomach mesentery Stomach suspended from dorsal wall by dorsal mesentery - Primordial dorsal mesogastrium Dorsal mesentery originally lies in the median plane - Carried left during stomach rotation and omental bursa formation Dorsal mesentery contains spleen and coeliac artery Primordial ventral mesogastrium attaches: - To the stomach - Duodenum to liver and ventral abdominal wall  Foregut Omental bursa Isolated clefts develop in mesenchyme - Forms thick dorsal mesogastrium Clefts combine to form a single cavity (Omental bursa) Stomach rotation pulls dorsal mesogastrium to the left (Enlarges bursa) - Bursa expands transversely and cranially - Lies between stomach and posterior abdominal wall -Facilitates movement of the stomach  Foregut Omental bursa Superior part forms closed space (Infracardiac bursa) - Space persists medial to the base of the right lung - Inferior region of superior part of bursa persists as superior recess of omental bursa  Foregut Omental bursa Stomach enlargement expands omental bursa - Acquires inferior recess of the omental bursa - Between layers of elongated dorsal mesogastrium (Greater omentum) - Overhangs the developing intestines - Inferior recess disappears as greater omentum layers fuse Omental bursa communicates with peritoneal cavity through omental foramen  Foregut Duodenum development Early in 4th week develops from - Caudal part of foregut - Cranial part of the midgut - Splanchnic mesenchyme associated with primordial gut - Junction of the parts found distal to bile duct origin Duodenum grows rapidly, forming c-shaped loop - Projects ventrally  Foregut Duodenum development Duodenal loop rotates right as the stomach rotates - Pressed against posterior abdominal wall (Retroperitoneal) Duodenum supplied by coeliac trunk and superior mesenteric artery Lumen progressively shrinks during 5th and 6th weeks - Also becomes temporarily obliterated - Results of proliferation of its epithelial cells - Recanalisation occurs from vaculolation  Foregut Liver and biliary apparatus development Liver, gallbladder and biliary duct system - Arise as ventral endodermal outgrowth (Hepatic diverticulum) - From distal part of foregut early in 4th week Diverticulum invades into septum transversum - Septum forms the ventral mesogastrium Hepatic diverticulum enlarges rapidly and divides into 2 layers of ventral mesogastrium  Foregut Liver and biliary apparatus development Larger part of the hepatic diverticulum - Primordial liver Smaller caudal part becomes primordium of gallbladder Endodermal cells give rise to epithelial lining of intrahepatic part of biliary apparatus Hepatic cords anastomose around endothelium-line spaces - Primordia of the hepatic sinusoids  Foregut Liver and biliary apparatus development Liver, gallbladder and biliary duct system - Arise as ventral endodermal outgrowth (Hepatic diverticulum) - From distal part of foregut early in 4th week Diverticulum invades into septum transversum - Septum forms the ventral mesogastrium Hepatic diverticulum enlarges rapidly and divides into 2 layers of ventral mesogastrium  Foregut Liver and biliary apparatus development Liver grows rapidly from 5th to 10th weeks - Fills large part of upper abdominal cavity Initially, left and right lobes are the same size - Right lobe becomes bigger Haematopoiesis begins in liver after 6th week - Bile formation begins at the 12th week  Foregut Liver and biliary apparatus development Small caudal part of diverticulum becomes gallbladder Stalk of diverticulum becomes the cystic duct - Initially duct attaches to ventral duodenum Stalk of diverticulum hepatic and cystic ducts becomes common bile duct Bile entering duodenum through bile duct after 13th week - Gives meconium a dark green colour  Foregut Ventral mesentery Thin double-layered membrane Lesser omentum runs from liver to lesser curvature of stomach - Also from liver to duodenum Falciform ligament extends from liver to ventral abdominal wall - Umbilical vein passes through ligament from the umbilical cord to liver Ventral mesentery forms the visceral peritoneum of the liver - Only part not covered is the bare area in contact with diaphragm  Foregut Pancreas development Pancreas develops between layers of mesentery - From dorsal and ventral pancreatic buds of endodermal cells - Arise from caudal part of the foregut Most of pancreas is derived from dorsal pancreatic bud (Appears first) - Develops a slight distance cranial to the ventral bud Smaller ventral pancreatic bud develops near entry of common bile duct into duodenum - Grows between layers of the ventral mesentery  Foregut Pancreas development As duodenum rotates to the right, bud is carried dorsally It soon lies posterior to the dorsal pancreatic bud and later fuses Ventral pancreatic duct forms uncinate process and head of pancreas As stomach, duodenum and pancreas rotate - Pancreas comes to lie along dorsal abdominal wall (Retroperitoneal) As pancreatic bud fuses, the ducts anastomose Pancreatic duct forms from duct of ventral bud and distal dorsal bud duct  Foregut Pancreas development Proximal part of the duct of dorsal bud may persist - As an accessory pancreatic duct - Opens into the minor duodenal papilla  Foregut Spleen development Development begins during the 5th week Spleen is derived from mesenchymal cells between dorsal mesogastrium Foetal spleen is lobulated and disappear before birth As stomach rotates, left surface of dorsal mesogastrium fuses with peritoneum over kidney Fusion results in splenorenal ligament  Midgut Derivatives - Small intestines and distal part of duodenum - Caecum, appendix, ascending colon and right half of transverse colon Derivatives are supplied by the superior mesenteric artery  Midgut Herniation of midgut loop Midgut elongates to form a ventral U-shaped midgut loop - Projects into the remains of the extraembryonic coelom Loop is herniated at beginning of the 6th week Loop communicates with umbilical vesicle through omphaloenteric duct Herniation occurs because of no room in the abdominal cavity Loop has a cranial and caudal limb suspended from dorsal wall by dorsal mesogastrium  Midgut Herniation of midgut loop Omphaloenteric duct is attached to the apex of the midgut loop - Where the cranial and caudal limbs join Cranial limb grows rapidly and forms small intestinal loops Caudal limb undergoes little change except development of a caecal swelling - Primordium of caecum and appendix  Midgut Rotation of midgut loop Components rotate 90⁰ counterclockwise while loop is in the umbilical cord - Around the axis of the superior mesenteric artery Rotation brings cranial limb to the right and caudal limb to the left During rotation, cranial limb elongates and forms intestinal loop  Midgut Retraction of intestinal loop Intestines return during the 10th week - Reducing midgut hernia Small intestines return first As large intestines return - Undergo a 180⁰ counterclockwise rotation  Midgut Retraction of intestinal loop (Fixation) Rotation of the stomach and duodenum place duodenum and pancreas on the right Enlarged colon pushes duodenum and pancreas against the posterior abdominal wall - This causes most of the duodenal mesentery to be absorbed - Results in duodenum being retroperitoneal, except the first part  Midgut Retraction of intestinal loop (Fixation) As the large intestines enlarge, lengthen and assume their final position - Mesenteries press against posterior abdominal wall - Mesentery of ascending colon - Fuses with parietal peritoneum and disappears Jejenum and ileum retain their mesenteries  Midgut Caecum and appendix Caecal swelling appears in the sixth week on the caudal limb Apex of the swelling does not grow rapidly, creating the appendix The appendix is variable in position - May be retrocaecal, retrocolic or run over the pelvic brim  Hindgut Derivatives of the hindgut are - Distal third of the transverse colon and the descending and sigmoid colons - Rectum and superior part of the anal canal - Epithelium of the urinary bladder and most of the urethra All derivatives are supplied by the Inferior mesenteric artery  Hindgut Descending colon becomes retroperitoneal - As its mesentery fuses with parietal peritoneum, then disappears The mesentery of the foetal sigmoid colon is retained - Making the sigmoid colon intraperitoneal  Hindgut Cloaca Cloaca is an endoderm-lined chamber in contact with surface ectoderm - At the cloacal membrane Membrane is composed of cloacal endoderm and anal pit ectoderm Cloaca receives the allantois ventrally  Hindgut Cloaca (Partitioning) Cloaca is divided into dorsal and ventral parts by the urorectal septum Septum grows to develop extensions producing infolding of lateral cloacal walls - Folds grow toward each other and fuse, forming a partition - Divides cloaca into rectum, cranial part of anal canal and urogenital sinus Anal pit is formed by recanalisation of anorectal canal - By apoptosis of the epithelial anal plug - Plug is an epithelial plug that temporarily closes anorectal lumen  Hindgut Anal canal Superior 2/3 of anal canal is formed from the hindgut (Superior rectal artery) Inferior 1/3 formed from anal pit (Inferior rectal artery) Junction of epithelium from anal pit ectoderm and hindgut endoderm is indicated by the pectinate line