Development of GI system Flashcards
What are the germ layers?
Ectoderm
- nervous system, face, skin, teeth, eyes and inner ear
Mesoderm
- skeletal, muscular, vascular, connective tissue, epithelia of genital and urinary system
Endoderm - gut, epithelium and digestive organs (liver, pancreas), respiratory tract epithelium
How does the gut tube form?
Trilaminar disc undergoes folding in a longitudinal (cephalocaudal) and lateral direction
By the end of WEEK 8 (embryonic period) the main organ systems have been established (organogenesis)
How does stomach develop from foregut?
Start with gut tube
Expansion 4th-5th week, 7th week, characteristic shape of stomach visible
Combined rotation –> correct positioning
Rotates 90 degrees on longitudinal axis so dorsal mesogastrium forms greater omentum (dorsal swings left, ventral swings right)
Also shunts down a bit (pylorus moves up, fundus moves down)
Peritoneum during stomach development?
Gut tube suspended in abdominal cavity by double layers of peritoneum called ‘mesenteries’
- mesenteries is term that remain used for adult
In the region of the stomach these are called mesogastrium
In the region of the stomach only (foregut only), there is an additional ventral mesogastrium to accompany the dorsal mesogastrium
Nerve supply of peritoneum?
Nerve supply to visceral peritoneum same as one supplying organ (autonomic)
Nerve supply to parietal peritoneum same as the one supplying the body wall (usually somatic)
Secondary retroperitoneal?
Retroperitoneal = organ covered with peritoneum but doesn’t leave posterior abdominal wall - no mesentery
Secondary peritoneal = used to have a mesentery but lost during development
Duodenum development?
Loses mesentery quite early in development - comes to lie against posterior abdominal wall and mesentery is absorbed
Quite easy to get to stomach, quite difficult to get to duodenum (front ok but not the back of it)
What are peritoneal pathways?
As the peritoneum cannot be pierced, vessels and nerves etc. must find suitable pathways to reach viscera
Aorta → branch into mesenteries → peritoneum
Liver development?
The HEPATIC DIVERTICULUM or LIVER BUD, buds from the gut tube and develops BETWEEN the layers of the VENTRAL MESOGASTRIUM (WEEK 3)
Liver cells proliferate as CORDS
Diverticulum NARROWS to form BILE DUCT, outgrowth giving rise to gall bladder and cystic duct
Spleen development?
Aggregation of mesenchymal cells BETWEEN the layers of the DORSAL mesogastrium in WEEK 5
Several SPLENULES which usually merge to form one organ (clinical)
Spleen:
Lymphatic tissue
mops up blood, gets rid of dodgy red blood cells
- sickle cell anaemia RBCs - spleen removes them so can remove their spleen (and accessory spleens otherwise they can grow)
Pancreas development?
WEEKS 5-8
The pancreas develops from the VENTRAL and DORSAL pancreatic BUDS which lie in the dorsal and ventral mesogastria
- The VENTRAL bud is closely associated with the developing gall bladder and bile duct
Differential GROWTH of the wall of the DUODENUM and ROTATION of the duodenum brings the BUDS INTO CONTACT and moves the BILE DUCT POSTERIOR to the 1st part of the duodenum
This is clinically significant - relationship between gall bladder and pancreatic disease
- tumour in head of pancreas (where C of duodenum is) - may present with jaundice due to obstruction of bile duct
- gall stones may also cause pancreatitis
Ligaments formed by peritoneum?
Ventral mesogastrium forms:
- FALCIFORM ligament - LESSER omentum
Dorsal mesogastrium forms:
- LIENORENAL ligament - GASTROSPLENIC ligament - GREATER omomentum
Summary - foregut?
Up to 2nd part duodenum Celiac artery Veins follow but drain to HPV Lymph ollow arteries to nodes around celiac Para = vagus Symp = T8 (celiac plexus)
Summary - midgut?
Up to 2/3rds along transverse colon Superior mesenteric artery Veins follow but drain to HPV \:lymph follow arteries to nodes around SM Para = vagus Symp = T10
Summary - hindgut?
From 2/3rds transverse colon Inferior mesenteric artery Veins follow but drain to HPV Lymph follow arteries to nodes around IM Para = vagus Symp = L1
Primary intestinal loop?
Gut grows very fast and with pressure from liver and kidneys is pushed out of cavity through umbilicus: physiological umbilical hernia (from about 6-8 weeks, returns to cavity 12-13 weeks)
Elongated midgut is the primary intestinal loop
Axis of the loop is SMA
Described as having proximal and distal limbs
Proximal limb - jejunum, ileum
Distal limb - caecum, appendix, colon
Midgut development - 5th week?
Gut tube elongates and bulges ventrally
Midgut development - 6th week?
Loop grows so long it enters umbilical coelom
How does midgut rotate?
- Anticlockwise rotation along axis of SMA
- Proximal limb continues to elongate and becomes pleated
- Caecum forms as a diverticulum of caudal limb
- Vitelline duct regresses
- Abdominal cavity becomes larger
- Further rotation of 180 degrees
- Week 10
- Proximal limb re-enters first
- Distal limb re-enters later and caecum lies in subhepatic position
- Caecum descends to right iliac fossa
- Appendix develops as outgrowth on caecum
Peritoneum of large intestine?
Transverse and sigmoid colon retain their mesentery (mesocolon) which attaches them to the posterior abdominal wall
The ascending/descending colon lose their mesentery and become (secondary) retroperitoneal
- Transverse mesocolon derived from dorsal mesentery
- Greater omentum derived from dorsal mesogastrium
- These become adherent so that the transverse colon appears stuck to the posterior aspect of the greater momentum
Peritoneum of rectum?
Progressively loses peritoneum as it descends through the pelvis
Upper third of rectum: anterior and lateral peritoneal covering
Middle third: anterior covering only
Lower third: no peritoneum
Lower third of rectum is ‘sub peritoneal’
Development of cloaca?
Blind ending - terminal portion of the hindgut (ENDODERM) - primitive cloaca (sewer) with ECTODERM forming CLOACAL MEMBRANE
URORECTAL SEPTUM (mesenchyme) separates cloaca into UROGENITAL SINUS anteriorly and RECTUM posteriorly
CLOACAL MEMBRANE divided into urogenital and anal membranes
Allantois degenerates into fibrous cord - the URACHUS (median umbilical ligament)
Development of anal canal?
Upper part of anal canal derived from hindgut endoderm
Lower part of anal canal derived from ectoderm (proctadaeum)
Proliferation of endoderm OCCLUDES the anal canal (week 7)
Week 8 - ECTODERM proliferates so that ANAL PIT forms
Weel 9 - Recanalises, breakdown of membrane
Division between endoderm and ectoderm (PECTINATE line at base of anal columns)
Clinically significant because of nerve supply, blood supply and lymphatic drainage
Vascular supply of rectum?
Arterial supply from superior, middle & inferior rectal arteries
Plexuses around the rectum/anal canal drain to superior, middle & inferior rectal veins
Superior rectal vein drains to inferior mesenteric vein and then to the liver (hepatic portal system)
Middle and inferior rectal veins drain into caval system
Abnormalities of GI development?
Persistent urachus, cysts or fistulae - closure of allantois
Fistulae - hindgut and urethra or vagina - misconnection between parts of the cloaca
Malroatation, herniation - omphalocele (hernia where abdominal organs protrude into umbilical cord), remnants of vitello-intestinal duct - Meckel’s diverticulum, (slight bulge in small intestine with remnants of vitelline duct) ligaments and fistulae
Imperforate anus - anal membrane thickens/fails to rupture
Anal agenesis - anal pit fails to form or incomplete separation of the cloaca by the urorectal septum
Rectal atresia - abnormal recanalisation/damaged blood supply - thick layer of CT between the anal canal and rectum
Epispadias (urethra ends up on dorsum of penus) exstrophy of the bladder/rectum - associated abdominal wall defects
Hirschprung’s disease - neural crest may not develop - aganglionic segment
Types of fistulae?
Rectourethral bulbar fistula (low)
Rectobladderneck fistula (high)
High rectovaginal fistula
Vestibular fistula
