DIGESTIVE SYSTEM DEVELOPMNENT

DIGESTIVE SYSTEM DEVELOPMNENT

1. MOLECULAR REGULATION

   • RETINORIC ACID (RA)
     • CAUSES TRANSCRIPTORS TO BE EXPRESSED IN DIFERENT REGIONS (INTERACTION BETWEEN MESODERM AND ENDODERM)
       • DETERMINE THE TYPE OF STRUCTURE THAT WILL BE FORMED
   • SONIC HEDGEHOG (SHH)
     • UPREGULATES FACTORS IN MESODERM
       • DETERMINES THE TYPE OF STRUCTURE THAT WILL BE FORMED

2. FOLDING OF EMBRYO

   • PRIMITIVE GUT
     • HINDGUT
       • 1/3 TRANSVERSE COLON
       • DESCENDING COLON
         • rotation of midgut fixes it in retroperitoneal position
       • ANAL CANAL
         • CRANIAL PART (2/3)
           • FROM ENDODERM OF HINDGUT
             • SUPPLIED BY SUPERIOR RECTAL ARTERY: CONTINUATION OF INFERIOR MESENTERIC ARTERY
           • JUNCTION DELINEATED BY PECTINATE LINE
         • CAUDAL PART (1/3)
           • FROM ECTODERM OF PROCTODEUM
             • SUPPLIED BY INFERIOR RECTAL ARTERIES: BRANCH OF INTERNAL PUDENDAL ARTERIES
           • JUNCTION DELINEATED BY PECTINATE LINE
       • RECTUM AND CLOACAL MEMBRANE
         • IN CAUDAL FOLDING, URORECTAL SEPTUM COMES CLOSE
           • END OF 7TH WEEK --> RUPTURE OF ANAL MEMBRANE
             • ANAL CANAL FORMS (HINDGUT
             • VENTRAL OPENING FORMS ( UROGENITAL SINUS)
               • ALLANTOIS
                 • FOLDING OF EMBRYO
           • FURTHER EXTENSION OF URORECTAL SEPTUM
             • UROGENITAL MEMBRANE
               • TO THEN FORM
             • ANAL MEMBRANE
               • THE ANAL CANAL --->
     • FOREGUT
       • LIVER AND GALLBLADDER
         • LIVER PRIMORDIUM = GROWTH FROM HEPATIC DIVERTICULUM
           • PROLIFERATING CELLS PENETRATING SEPTUM TRANSVERSUM
           • LIVER IN CONTACT W/ DIAPHRAGM NOT COVERED BY PERITONEUM
           • 10th WEEK liver 10% of body weight
             • Hepatopoietic function
               • large nets of proliferating cells producing RBC & WBC until 2 months before birth
         • CONNECTION BETWEEN HEPATIC DIVERTICULUM & FOREGUT NARROWS TO FORM BILE DUCT
           • OUTGROWTH VENTRALLY TO GALLBLADDER + CYSTIC DUCT
             • Week 12: bile production begins
         • CELLS IN LIVER DIFFERENTIATE INTO HEPATOCYTES & BILIARY CELLS REGULATED BY HEPATOCYTE NUCLEAR TRANSCRIPTION FACTORS (HNF3 & 4)
           • Fibroblast growth factor (FGF2) from cardiac mesoderm: blocks foregut endoderm to express liver specific cells
           • Bone morphogenetic proteins(BMP):enhance competence of liver endoderm to respond to FGF2
       • ESOPHAGUS
         • -tracheoesohageal septum divides respiratory primordial from esophagus
         • muscle coat derived from splanchnic mesenchyme
         • upper 2/3 Vagus nerve;lower 1/3 splanchnic plexus
       • STOMACH
         • week 4= dilation of foregut=>gradual changes in shape and appearance.
         • High rotation longitudinally and anteroposteriorly
         • Posterior wall grows>anterior wall + rotation = greater and lesser curvatures
         • growth in disproportion to mesenteries + rotation => alteration to mesentery
           • dorsal mesogastrium is pulled <--
           • ventral mesogastrium is pulled -->
       • DUODENUM
         • stomach rotation creates C shaped loop & rotates the duodenum
           • pushes duodenum right and later becomes retroperitoneal
         • 2nd month: lumen is obliterated and shortly recanalized
         • Since its so close to midgut : vascularised by celiac and superficial mesentery
       • PANCREAS
         • DORSAL BUD (connected to dorsal mesentery
           • Wirsungg duct
           • Santorini duct (duct not fused to the main pancreatic duct.
         • VENTRAL BUD: duodenum rotation pushed bud to lie below dorsal bud and fuse
           • Uncinate process + inferior head of pancreas
         • 3rd month: islets of langerhans scatter throughout the pancreas
         • 5th month:insulin and glucagon secretion begins
     • MIDGUT
   • EXTRAEMBRYONIC YOLK SAC (vitelline duct)
     • MIDGUT
       • COUNTERCLOCKWISE ROTATION (270*) moves organs into correct position @ week 10
         • CEPHALIC LIMB
           • DISTAL PART OF DUODENUM
           • JEJUNUM
             • first to return to abdominal cavity after final loop.
           • UPPER PART OF ILEUM
         • CAUDAL LIMB
           • LOWER PART OF ILEUM
           • CECUM
             • first located upper right quadrnt => moves Ins-Lat assuming its final position.
           • APPENDIX
             • pushed down to final location by the growth of colon
           • ASCENDING COLON
             • press posterior to body wall. mesenteries fuse with parietal peritoneum. SECONDARY RETROPERITONEAL
           • PROXIMAL 2/3 OF TRANSVERSE COLON

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