Digestive System Flashcards
The primitive gut tube is formed from the incorporation of the dorsal part of the yolk sac into the embryo as a result of what two types of folding in the embryo?
craniocaudal folding
lateral folding
Week 4: The primordial gut is closed at its cranial and caudal ends by what membranes?
cranial = oropharyngeal membrane
caudal = cloacal membrane
What tissue of the primordial gut gives rise to most of the epithelium and glands?
endoderm
What part of the adult G.I. tract is derived from endoderm and has a lamina propria and muscularis mucosae?
Mucosa
epithelial lining and glands
What 3 parts of the acult G.I tract are derived from visceral mesoderm?
submucosa
muscularis externa
adventitia or serosa
*(lamina propria + muscularis mucosa are also mesoderm)
In early development the epithelial lining of the gut proliferates and obliterates the lumen. What precess reverses this later?
recanalization
The epithelium of the cranial and caudal end of the gut is derived from what primordial tissue type?
ectoderm of the stomodeum and proctodeum
The primordial gut is divided in what three sections?
- foregut
- midgut
- hindgut
The Foregut:
What are the derivitives?
Primordial pharynx (+ derivatives)
Lower respiratory system
Esophagus and stomach
Duodenum, distal to the opening of the bile duct
Liver, biliary apparatus (hepatic ducts, gallbladder + bile duct)
Pancreas
The foregut derivatives are supplied by the what artery/branch?
celiac trunk
The foregut is divided into the esophagus dorsally + trachea ventrally by what folds?
These fuse to form what structure?
The esophagus reaches its final length by what week?
- tracheoesophageal folds
- tracheoesophageal septum
- week 7
The epithelium and glands of the esophagus are derived from what primordial tissue?
It proliferates and obliterates the esophageal lumen. By what week does recanalization occur?
endoderm
- week 8
Esophagus: superior third
- striated muscle forms muscularis externa
- derived from mesenchyme in caudal pharyngeal arches
- innervated by CN X
Esophagus: inferior two thirds
- smooth muscle
- derived from splanchnic mesenchyme
- innervated by CN X
Esophageal Atresia
incomplete separation of the esophagus from the trachea
- results from deviation of tracheoesophageal septum in a posterior direction
- associated w. tracheoesophageal fistula (TEF) - 85% of cases
- other congenital defects associated with the VACTERL syndromes - 33% of cases
- can not swallow amniotic fluid = polyhydramnios
- Inability to pass a catheter through esophagus into stomach
V.A.C.T.E.R.L. syndromes/associations
V = vertebral defects
A = anal atresia
C = cardiovascular defects
T/E = tracheoesphageal fistula
R = renal defects
L = upper limb defects
*syndrome = all defects; association = only some*
Esophageal Stenosis
narrowing of the lumen (usually in midesophagus)
- week 8 = recanalization process is incomplete, or blood vessels did not develop in this region= atrophy of segment
Esophageal duplication
usually a congenital esophageal cyst
**(usually lower esophagus) **
- cysts may lie on posterior aspect of the esophagus protruding into the posterior mediastinum
Vascular Compression of the Esophagus
abnormal origin of one of the vessels derived from the pharyngeal arteries (usually right subclavian artery)
- anomalous artery passes behind the esophagus and may cause dysphagia (difficulty swalling)
The foregut is initially a simple tubular structure. At week 4, a slight dilation becomes ____________?
primordium of the stomach
(oriented in the median plane)
Rotation of the Stomach
90 degrees clockwise
- lesser curvature (ventral) moves right; greater
curvature (dorsal) moves left
- cranial region moves left + slightly inferiorly; caudal region moves right + superiorly
- Final position = long axis almost transverse
Due to rotation, the dorsal mesentery is carried to the left and eventually forms …?
What nerves innervates the ventral and dorsal surfaces of the stomach?
** greater omentum**
ventral = left vagus nerve (CN X)
dorsal = right vagus nerve (CN X)
Regarding the stomach, which cells are derived from endoderm?
Surface mucous cells lining the stomach
mucous neck cells
parietal cells
chief cells
enteroendocrine cells of the gastric glands
Regarding the stomach, which layers are derived from visceral meesoderm?
lamina propria
muscularis mucosae
submucosa
outer longitudinal layer
middle circular layer
inner oblique layers of smooth muscle of muscularis externa
serosa of the definitive stomach
Congenital Hypertrophic Pyloric Stenosis
thickening of the pylorus
- circular + longitudinal muscles in the pyloric region are hypertrophic causing a narrow pyloric lumen that obstructs food passage
- treatment = pyloromyotomy
Results in:
- distended stomach (palpated @ R. costal margin)
- projectile vomiting (no bile)
- * increased incidense in infants treated with erythromycin*
The duodenum develops from what primordial sections?
- caudal part of foregut
- cranial part of the midgut
- splanchnic mesenchyme of these regions
What vessels supply the duodenum?
When does recanalization occur?
branches of celiac + superior mesenteric arteries
- recanalization = end of embryonic period
Duodenal Stenosis
incomplete recanalization
stenosis of stomach’s contents = projectile vomiting
(USUALLY with bile)
Duodenal Atresia
Complete occlusion of duodenal lumen
- occurs at junction of bile + pancreatic ducts (hepatopancreatic ampula)
- vomiting starts a few hours after birth (ALWAYS contains bile)
What structures arise from the distal part of the foregut and form the hepatic diverticulum?
Where does the diverticulum extend to?
- liver
- gallbladder
- biliary duct system
diverticulum extends into the septum transversum
(mass of splanchnic mesoderm b/t developing heart +midgut
The tremendous growth of the liver causes what?
- liver bulges into abdominal cavity = stretches septum transversum
= ventral mesentery (falciform ligament + lesser omentum)
falciform ligament
- contains the left umbilical vein (after birth = ligamentum teres)
lesser omentum
- divided into the hepatogastric ligament + hepatoduodenal ligament
- hepatoduodenal ligament = bile duct, portal vein, and hepatic artery (i.e., portal triad)
What are important developmental features of the liver at:
* week 5-10?
* week 6?
* week 12?
week 5-10 = fast hepatic growth; liver fills lg. part of peritoneal cavity
**week 6 = hematopoiesis **begins (liver is bright reddish color)
**week 12 = **bile formation by hepatic cells start
What does the connection between the hepatic diverticulum and the foregut create?
An outgrowth from this structure gives rise to _______ and ________.
bile duct
- gallbladder rudiment + cystic duct
The cystic duct divides the bile duct into the ________ an _______?
common hepatic duct + common bile duct
*(closure & recanalization of the lumen occur)
At Week 13, what gives the meconium (intestinal contents) a dark green color?
** bile** entering the duodenum through the bile duct
Biliary Atresia
obliteration of extrahepatic and/or intrahepatic ducts
- ducts are replaced by fibrotic tissue due to acute and chronic inflammation
- Clinical symptoms = progressive neonatal jaundice, white clay-colored stool (acholia), and dark-colored urine
- average survival time w/o treatment = 12 to 19 months (100% mortality rate)
- treatment = Kasai hepatoportoenterostomy (surgical procedure) or liver transplantation
What primordial structures arise from endoderml cells in pancreatic development?
dorsal + ventral pancreatic buds
(rotation of the duodenum causes these buds to eventually fuse)
ventral pancreatic bud
= uncinate process
= head of the pancreas
dorsal pancreatic bud
= accessory pancreatic duct
= body of pancreas
= tail of pancreas
The main pancreatic duct and common bile duct form a single opening into the duodenum called what?
Where is this opening located?
hepatopancreatic ampulla of Vater
- at the tip of a major papilla (hepatopancreatic papilla)
Acinar cells, islet cells, and simple columnar or cuboidal epithelium lining the pancreatic ducts are derived from what type of tissue?
endoderm
(Surrounding connective tissue + vascular components = visceral mesoderm)
1. Insulin secretion starts at what week in the pancreas?
- Glucagon + somatostatin-containing cells develop before differentiation of insulin-secreting cells.*
2. Endodermal cells from tubules accumulate within mesoderm to form islet cells:
name 4 types.
Week 10
- alpha cells (glucagon)
- beta cells (insulin)
- delta cells (somatostatin)
- PP cells (pancreatic polypeptide)
What causes an Annular Pancreas?
What is raiological sign of this?
ventral pancreatic bud fuses with dorsal bud both dorsally + ventrally
(= ring of pancreatic tissue around duodenum; severe duodenal obstruction)
Radioraphy shows:
duodenal obstruction + “double bubble” sign (dilation of stomach + distal duodenum)
This pathology occurs when the distal two thirds of the dorsal pancreatic duct + the entire ventral pancreatic duct fail to anastomose (dorsal pancreatic duct persists; two separate duct systems)?
These patients are prone to…?
Pancreas Division
- prone to pancreatitis
Hyperplasia of Pancreatic Islets
- fetal islets exposed to high blood glucose levels in infants of diabetic mothers
- glucose stimulates fetal islet hyperplasia + insulin secretion = increased fat + glycogen deposition in fetal tissues
- results in increased birth weight at term (macrosomia) + episodes of hypoglycemia in the postnatal period.
The spleen is derived from what tissue type?
a mass of mesenchymal cells (b/t layers of dorsal mesogastrium)
- capsule, connective tissue + parenchyma of spleen differentiate from mesenchyme
What does the splenorenal ligament form from?
fusion of mesogastrium + peritoneum over the left kidney
The spleen functions as a hematopoietic organ during what part of fetal life?
approx. week 3 -> late fetal life (month 5)
Midgut
Its derivatives are:
- small intestine
- distal duodenum
- cecum
- appendix
- ascending colon
- part of the transverse colon
The midgut is supplied by what artery?
superior mesenteric artery
physiologic umbilical hernia
As the intestine grows it forms a midgut loop that projects into the umbilical cord; important for the shape the intestine will form
- communicates with umbilical vesicle through omphaloenteric duct (yolk sac) until week 10
- midgut rotates counter clockwise arouns SM artery = forms intestinal loops
What are the 2 limbs of the midgut loop of intestine?
cranial limb and caudal limb
(suspended by an elongated mesentery)
cranial limb:
- grows fast
- forms small intestinal loops
caudal limb:
- goes through little change
- forms the cecal sweelings (primordium of cecum), + appendix
During rotation of the midgut, which limb elongates and forms intestinal loops (primordia of the jejunum and ileum)?
the cranial limb
When & how do the intestines return to the midgut?
Week 10
- 1st: small intestine returns to abdomen
- 2nd: as large intestine returns, it rotates 180 degrees counterclockwise. (comes to occupy right side of abdomen)
- 3rd: ascending colon becomes recognizable as the posterior abdominal wall elongates
Fixation of the Intestines
(occurs as intestines enlarge, lengthen, and assume their final positions)
- their mesenteries are pressed against the posterior abdominal wall
- ascending colon becomes retroperitoneal (looses its peritoneum)
- fan shaped mesentery of small intestines attaches to the dorsal abdominal wall by a new line of fixation that passes from the duodenojejunal junction
At what week does the cecal swelling appear?
What does it become?
week 6
- primordium of cecum and appendix
- appears as an elevation of the border of the caudal limb
- after birth, wall of cecum grows unequally; 64% of people the appendix is located retrocecally
Congenital Omphalocele
herniation of abdominal contents through umbilical ring
- covered by a translucent peritoneal membrane sac protruding from the base of the umbilical cord
- abdominal cavity is small; surgical repair is required, often these infants present pulmonary + thoracic hypoplasia
*** Associated with: congenital anomalies such as **trisomy 18 (Edward’s Syndrome) + 21 (Down Syndrome)*
Umbilical Hernia
intestines return to the abdomen, but then herniate through a faulty umbilicus
- protruding mass is covered by subcutaneous tissue + skin
- hernia protrudes during crying, or coughing
- can be easily reduced through the fibrous ring at the umbilicus (slowly, over time)
Gastroschisis
Congenital abdominal wall defect; Extrusion of abdominal viscera without involving the umbilical cord.
- viscera float in the amniotic fluid
- intestines are not covered by a peritoneal membrane( thickened + covered with adhesions)
Ileal Diverticulum (Meckel’s Diverticulum)
remnant of the vitelline duct (joins yolk sac to midgut)
- outpouching of the ileum = common anomaly of the digestive system
- wall of the diverticulum contains all layers of the ileum + may contain gastric and pancreatic tissues
- ectopic gastric tissue may secrete acid = ulceration + bleeding.
- appears on antimesenteric border of ileum, 40 to 50 cm from ileocecal junction
Hindgut
Its derivatives are:
- left half of transverse colon
- descending colon
- sigmoid colon
- rectum + part of the anal canal
- epithelium of urinary bladder
- most of the urethra
hindgut and its derivatives are supplied by what artery?
the inferior mesenteric artery
What is the expanded terminal part of the hindgut lined by endoderm?
Where does it contct surface ectoderm?
Cloaca
(receives the allantois ventrally, which is a fingerlike diverticulum)
- contacts surface ectoderm @ cloacal membrane
The cloaca is divided into dorsal and ventral parts by what?
What are these 2 divisions?
the urorectal septum (mesenchyme)
(develops b/t allantois + hindgut)
Dorsal: rectum + part of the anal canal
Ventral: urogenital sinus
What occurs at week 7, in regards to the urorectal septum?
fuses with cloacal membrane
- divided into anal membrane and a larger ventral urogenital membrane
- perineal body results from this fusion
urorectal septum:
divides the cloacal sphincter into anterior and posterior parts. What are these parts?
posterior = external anal sphincter
anterior = superficial transverse perineal, bulbospongiosus, and ischiocavernosus muscles.
proctodeum (anal pit):
formed by mesenchymal proliferation in the surface ectoderm around the
anal membrane
The Anal Canal:
The anal membrane ruptures by week …?
This allows communication between what 2 structures/areas?
week 7
- communication b/t digestive tract + amniotic cavity
Anal Canal
superior canal = derived from hindgut (superior rectal a.)
inferior part = develops from proctodeum (inferior rectal aa.)
(divided by pectinate line)
- approx. 2 cm superior to the anus = anocutaneous line (white line)
(epithelium changes from columnar to stratified squamous)
- anus = keratinized epithelium; continuous with external skin
Hirschsprung Disease (Congenital Megacolon)
- failure of neural crest cells to migrate into wall of colon = no development of parasympathetic ganglion in Auerbach and Meissner plexuses
- lack autonomic ganglion cells (aganglionosis) in myenteric plexus distal to dilated segment of colon
- enlarged colon or megacolon= normal number of ganglion cells
** *most common cause of neonatal obstruction of the colon***
treatment:
rescect the affected area; probably prepare an ostomy
until distal portion of colon can connect to anal canal
Imperforate Anus
Abnormal development of urorectal septum
= incomplete separation of the cloaca into urogenital +anorectal portions
- sometimes it’s obvious, sometimes there’s a “blind opening” when an entrance exists but only goes a few cm.
Anal Agenesis
(with or without fistula)
most common type of anorectal anomaly
- anal canal may end blindly; may be an ectopic anus or an anoperineal fistula opening into perineum (or into vagina or urethra)
- rectum ends superior to puborectalis muscle
- usually a fistula to the bladder (rectovesical) or to the vagina (rectovaginal)
- passage of meconium or flatus (gas) in the urine is diagnostic of rectourinary fistula.
- Higher rates of fecal incontinence.
Anal Stenosis
anus is in normal position, but the anus + anal canal are narrow
Due to a deviation of the urorectal septum
Membranous Atresia of Anus
- thin layer of tissue separates anal canal from the exterior
- anal membrane appears blue from meconium superior to it
Due to imperforation of the membrane at week 8.
Rectal Atresia
- anal canal + proximal rectum are present but separated
- sometimes two segments are connected by fibrous cord (remnant of an atretic portion of the rectum)
- cause may be abnormal recanalization of the colon or more likely defective blood supply
