anatomy exam 4

GI embryology

GI tract is 1 tube that folds in on itself

day 30: embryo stops AP growth and starts folding laterally → yolk sac thins out to become vitelline duct into umbilical cord (gut tube formed by week 4 but all cells are pluripotent and organ differentiation is caused by a blast of retinoic acid)

dorsal mesentery: provides nutrents and attaches to gut wall (will become peritoneum)

foregut embryology

esophagus and trachea: respiratory diverticulum forms at 4 weeks and then the respiratory and esophageal separates (if they don’t separate properly it can lead to atresia)

• superior 2/3 of esophagus = striated muscle (vagus nerve), inferior 1/3 smooth muscle (sphlancnic plexus)

stomach: at week 4 is just dilation of gut tube then undergoes 90 degrees clockwise rotation (left wall becomes anterior) → greater and lesser curvature, vagus nerve get dragged along

spleen: around week 5 it begins to form in dorsal mesogastrium (not connected to gut tube bc it’s mesodermal)

liver: begins at week 4 in ventral mesogastrium to intertwine with vitelline veins to anastamose → hepatic portal vein, bile duct created by liver’s connection to gut tube thinning, bud from bile duct will form gallbladder

pancreas: dorsal pancreatic bud in dorsal mesogastrium and ventral pancreatic bud and ventral mesogastrium, eventually they are moved next to each other and fuse to form pancreas (failure to fuse → pancreas divisum)

duodenum: in line with gut tube initially but gets pushed right and posterior by week 6, 1/3 is intraperitoneal

midgut embryology

begins as rapid elongation of gut tube following vitelline duct and superior mesenteric artery → primary loop

physiological herniation: necessary for development at 6 weeks midgut grows out of abdominal wall and into umbilicus following vitelline duct, cephalic limb grows the most, then 90 degree rotation around superior mesenteric artery, at 10 weeks abdominal cavity is bigger, so SI and LI stop growing and retreat into abdominal cavity, rotating total 270 degrees (including the first rotation)

• meckel’s diverticulum- small part of vitelline duct persists → constricts intestine or extraperitoneal tissue attached to it (these are rare, most cases are asymptomatic)

• intestinal malrotation (largely asymptomatic)

hindgut embryology

does not herniate or rotate

blood supply: inferior mesenteric artery

cloaca: opening for urogenital and anorectal system (fetal butthole), urorectal septum separates them to make poop hole and pee hole

watershed areas: areas that receive dual blood supply (duodenum, splenic flexure, anorectal canal) blood supply overlap at terminal vessels, high stress events → reduce blood flow

peritoneum

2 layers: parietal = lines abdominopelvic cavity, visceral peritoneum = surrounds viscera

in embryo = peritoneal cavity (in adults it’s a potential space)

peritoneal ligaments: peritoneum folds in on itself → omenta (connect organs) and mesenteries (connect organ to body wall)

• greater omentum- between greater curve of stomach and transverse colon → gastrophrenic, gastrosplenic, gastrocolic ligaments

• lesser omentum- between lesser curve of stomach and liver → gastrohepatic, hepatoduodenal ligament

compartments- omental bursa, supracolic (anterior to stomach, superior to transverse colon), infracolic (inferior to transverse colon)

omental foramen- how omental bursa communicates with greater sac

• press on omental foramen to stop bleeding from cystic artery

blood supply to abdomen

3 arteries branch from abdominal aorta

foregut: esophagus → middle of duodenum, accessory digestive organs (supplied by celiac trunk)

midgut: middle of duodenum → 2/3 of transverse colon (supplied by branches of superior mesenteric artery)

hindgut: distal 1/3 trnasverse colon → anus (supplied by brances of inferior mesenteric artery)

foregut

esophagus: passes through diaphragm at T10, and joins stomach at gastroesophageal junction

stomach: fundus, body, pyloric canal (empties into duodenum)

• pyloric sphincter controls stomach emptying

duodenum: c shaped loop of small intestine that curves around pancreas (L1-L3), superior (connects to pyloric sphincter), descending, horizontal, ascending, joins with jejunum at duodenal-jejunal junction

• only proximal part is part of foregut

• distal is part of midgut

pancreas: accessory digestive organ, in omental bursa, head, uncinate process, neck, body, tail, main pancreatic duct starts in tail and forms ampulla

spleen: not part of digestive system, derived from embryonic foregut, very delicate

midgut

jejunum- intraperitoneal, red, thick, highly vascular (more vasa recta)

ileum- intraperitoneal, pale pink, thin, more fat

• in both jejunum and ileum mesenteries are organized into straight vessels (vasa recta) and arcades (loops)

hindgut

large intestine- tenia coli = thick bands of smooth muscle, haustra (pouches), omental appendices (fat hanging off)

• poop: ileocecal valve, cecum, ascending colon, right colic flexure, transverse colon, left colic flexure, descending colon, sigmoid colon, rectum, anus

appendix- near ileocecal junction, appendicitis causes visceral referred pain around T10, then towards Mcburney point (somatic pain), psoas sign (flexion of thigh against resistance)

arteries of the abdomen

hepatic portal system

inferior and superior mesenteric veins and splenic vein → hepatic portal vein (to liver)

liver drained by hepatic veins which empty into IVC

anastomoses are important for collateral circulation but high BP can cause a lot of backup

portal hypertension: too high BP in portal system → blood backs up in portal veins and where there are anastomoses, there is also back up into systemic veins (esophageal varicose, caput medusae, anorectal varices)

liver

attached to diaphragm by coronary ligament

bare area of liver: has no peritoneum

anatomical lobes: left and right (caudate and quadrate) divided by falciform ligament

dual blood supply: 30% from proper hepatic artery, 70% from hepatic portal vein for filtration and nutrient processing, common bile duct travel in hepatoduodenal ligament

biliary tree: bile from liver → right and left hepatic duct → common hepatic duct → cystic duct → gallbladder (stored and concentrated) → during digestion bile leaves gb through cystic duct → common bile duct → hepatopancreatic ampulla

retroperitoneal organs

outside of parietal peritoneum, organs only have peritoneum on anterior aspects

left suprarenal vein and left gonadal vein empty into left renal vein but on right they empty directly into IVC

kidneys

retroperitoneal (right kidney is lower bc of liver)

suprarenal glands arterial supply: superior, middle, inferior suprarenal arteries

waste in kidneys emptied through ureters and filtered blood returns through renal veins

renal medulla: renal columns and pyramids (pee produced then exits through papilla into collecting system)

• minor calyces, major calyces, renal pelvis, ureter, bladder

gonadal veins always anterior to ureters

diaphrgam and posterior abdominal wall

diaphragm openings- caval foramen for IVC (T8), esophageal hiatus (T10), aortic hiatus (T12)

• medial arcuate ligament - sympathetic trunk, psoas major

• lateral arcuate ligament - quadratus lumborum

quadratus lumborum- innervated by ventral rami of spinal nerves

psoas major- lumbar plexus

iliacus- femoral nerve

intermediate mesoderm

between paraxial and lateral plate mesoderm and connected to lateral plate mesoderm

differentiates to make glomeruli and tubing, kidney is diverticulum of celom but eventually separates

mesonephric duct: excretes urine, forms from above the heart down to the bladder

pronephros: primitive and never develops into functional excretory structures

mesonephros: ducts and tubules remain in adult but glomeruli disappear in adult (forms collecting system of metanephric kidney = calyces, CD, pelvis, ureter)

• mesonephric ducts will form adult epididymis and vas deferens in men, paramesonephric ducts will form oviduct in females (diverticulum of uterus)

• 2 distillations: one in looping artery of glomerulus and one in tubules (no veins)

  • fluid accumulates outside artery and trapped outside bowman’s capsule

  • when urine enters distal convoluted tubule twists to make loop of henle and proximal convoluted tubule

metanephros: adult kidney with new glomeruli

• blastema- forms excretory portion (bowman’s capsule, PCT, loop of henle, DCT)

• uteric bud is a diverticulum of mesonephric duct (will eventually become excretory portion)

cloaca

cup at the bottom of embryo

3 tubes: anus, vagina, bladder

forms urorectal septum to separate anus and vagina

urogenital sinus is front sinus

bladder is composite organ and mesonephric trigone of bladder is formed by exstrophy (when mesonephric ducts turn inside out)

vascularization embryology

metanephric kidneys form low near the bladder and slowly rise to adult position and changes arterial supply based on where it is (stops when it hits adrenal gland), as kidney rises, ureter grows

urogenital system: male

• male: bladder above prostate and inside prostate is orifice of prostatic utricle (urine passes on either side of tube)

• female: in urethra when it empties inside vagina

embryological urogenital pathologies

VACTERL association: embryological pathologies (vertebral, anal atresia, CV abnormalities, tracheoesophageal fistula, esophageal atresia, renal anomalies, limb defects)

imperforate anus- dysfunction of urorectal septum (failure to pass meconium)

hydronephrosis- glomeruli don’t work → fluid accumulates in kidney → ureter expands

renal agenesis- kidney not formed

oligohydramnios- not enough amniotic fluid → deformed limbs, dry skin, face abnormal, pulmonary hypoplasia

polyhydrammnios- excess amniotic fluid caused by lack of swallowing from maternal diabetes

horseshoe kidney- asymptomatic but unified kidney prevented from reaching abdominal location by inferior mesenteric vein (high risk of kidney stones and UTIs)

sympathetic innervation of abdominal organs

foregut:

• preganglionic cell bodies from T5-T9 (celiac ganglia)

• postganglionic axons distribute with celiac trunk

midgut:

• preganglionic cell bodies from T9-T12 (superior mesenteric ganglia)

• postganglionic axons distribute with superior mesenteric artery

hindgut:

• preganglionic cell bodies from T12-L2 (inferior mesenteric ganglia)

• postganglionic axons distribute with inferior mesenteric artery

preganglionic pathway: start at lateral horn of T1-L2, through ventral horn, spinal nerve, ventral ramus, white ramus communicants, pass through paravertebral ganglia (NO SYNAPSE) through splanchnic nerve to subdiaphragmatic ganglia (some will descend to lower levels and leave at lower paravertebral ganglia)

greater splanchnic nerves- T5-T9, lesser splanchnic nerves- T10-T11, least splanchnic nerve- T12, lumbar splanchnic nerves- L1-L2

parasympathetics to abdominal organs

most of parasympathetic innervation still comes from vagus nerve → posterior and anterior vagal trunks → pass with arteries/sympathetics to organs they innervate

some (hindgut) will be innervated by S3-S4:

• cell bodies in lateral horn, ventral root, spinal nerve, ventral ramus, then just leave ventral ramus as pelvic splanchnic nerves → inferior hypogastric plexus

enteric nervous system

intrinsic part of autonomic nervous system (both plexuses controlled by extrinsic and enteric nervous system)

submucosal plexus: regulates secretion and degree of blood flow

myenteric plexus: controls motility

hirchsprung disease: birth defect where neural crest cells fail to migrate → lack of ganglia in distal bowel → failure to pass meconium, abdominal distention

visceral sensory from abdominal organs

usually follows path of sympathetic outflow backwards (pelvic organs are exception)