54 terms

Abdominal Embryology

What week of develpment does folding produce he primitive endodermal gut tube?
The 4th week. All other layers will be splanetic mesoderm.
What is the only outlet of the near the gut at the 4th week?
Yolk stalk/omphaloenteric duct (future umbilical cord)
What does the endoderm produce?
Produces lining of gut and gut derived abdominal organs
What doe the splanchnic mesoderm produce?
Produces muscle, connective, tissue, and other layes of the gut and gut derived abdominal organs
What is lined with ectoderm?
The proctodeum and stomdeum
At the beginning of the development of the foregut, what membrane separates the foregut from the site of the future mouth?
oropharyngeal membane
When does the orophayrngeal membrane rupture and what does this allow?
At 4 weeks, allows fetal swallowing of amnotic fluid
What is the stomodeum?
ectodermally lined pit, the primitive mouth
What membrane separates the hindgut from the future proctodeum?
Cloacal membrane
When does the cloacal membrane rupture?
8 weeks, at that point it is the anal membrane, this allows passage of amniotic fluid through fetal digestive system
What is the proctodeum?
Ectodermally lined pit, the primtive anal pit
What is the arterial supply for the foregut?
celiac trunk

exception=structures lying outside tof abdominal cavity-pharynx, respiratory tract, and majority of esophagus
What is the arterial supply for the midgut
superior mesenteric artery (SMA)
What is the arterial supply for the hindgut
inferior mesenteric artery (IMA)
What is the specialized deverticula for the foregut
pharyngeal pouches
What is the specialiezed diverticula for the midgut
yolk stalk/omphaloeneteric duct
What is the specialized diverticula for the hindgut
allantosi-a receptical or liquitory waste that also has respiratory function
Describe recanalization of the gut, what happens if an error occurs?
primitive gut tube gets filled by proliferation of epithelial cells and gets completely plugged up

then remade into a tube by being broken down, a process known as recanalization.

Now the endodermal gut tube is lined with an epithelial layer of cells.

An error in this process, may occur anwhere along the digestive tract producing an atresia (blockage) or stenosis (narrowing) of adult gut lumen, if you don't have break down this could happen-more often than not it happens in the deuoduem
Descirbe the peritoneum, parietal, viceral and periotneal cavity
Pariteal peritoneum-very sensitive to pain

visceral peritoneum-senstive to pressure changes, painful but not well localized

peritoneal cavity-no breaches in the cavity except for one the uterine tubes in females
double layer of peritoneum lining an organ, formed by splanchnic mesoderm

connects organs to the body wall

essentially everything running to or form the gut travels through mesentery (nerves and vessels)
dosal mesentery
provides mesentery for most abdominal organs
ventral mesentery
degrades in the midgut and hindgut, making a single periotneal cavity

remains as the lesser omeentum, mesentery of the liver and falciform ligament in the foregut
intraperitoneal, what organs?
gut susupended in mesentery (most abdominal organs)

ex stomach, tranverse colon, liver, gall bladder, jujunum and ileum, cecum, sigmoid colon, the first part and fourth part of the duoedumn
secondarily retroperitoneal, what organs?
maybe initially the organs are suspened in mesentery but due to rotations and repositiong during development, orgnas get pressed against abdominal wall, and get stuck there-think of the mesentery as sticky event hought it is not really in real life, their mesenteries fuse and they become SRP, it is no longer intraperiotenal now, it is behind and is therefore called retroperitoneal,

Ex: distal 2/3rd of duodenum, ascending colon and descending colon, pancreas
primarily retroperitenoal
has always been behind/external to peritoneum

ex: thoracic esophagus, rectum, anal canal
What does the foregut produce?
pharynx, esophagus, stomach and proximal 1/3 of duodenum
What are the foregut outgrwoths
pharyngeal pouches, lower respiratory system, liver, pancreas and gallbladder
How does the esophagus grow?
grows in length with descent near septum transversum
Describe the trancheoesophageal fold
fold betwen the pharynx and primordium of layrngotracheal tube

eventually fuses to form the tracheoesophageal septum, which the eventually forms two separate tubes, the esophagus and the laryngotracheal tube

The trachea is an outgrowth of the esophagus.
What results in imporper fusion of the tracheosophageal fold?
Tracheosesophageal fistual-most commonly assoicated with esophageal atresia (blockage of esophagus), (also failure of recanalizaiton can cause blockage), 1/2,500 birth result in imporper fusion of tracheoesophageal fold
too much amniotic fluid surrounding developing embryo or fetus, only mechanism to reduce amniotic fluid in utero is fetal drinking of amnotic fluid, after 4 weeks, the oropharyngeal membrane breaks down and the opens the cranial foregut to amniotic cavity and its contents, the embryo starts drinking, fetal gut is responsible for returuning the amniotic fluid to mother...,can cause problems like missing skull or brain
Describe the development of the stomach.
forms as dilation in distal foregut

growth: dorsal border grows faster than ventral border, this differntial growth produces greater and lesser curvature

rotation of 90 degrees clockwise around longitundinal access

ventral border lesser curvature move to the right

dorsal border greater curvature moves to the left

Right and left vagus nerves rotate with stomach

LARP (Left anterior/ Right Posteior for vagus)

right vagus moves dorsal

left vagus movers ventral

rotation also rotates duodenal loop ot the right and presses it against the body wall (majority secondarily retroperiotneal)
Descirbe what happens to the stomach's mesentery during develpment
stomach's dorsal mesentery becomes very elaborabte-greater omentum, aprone of mesnetery draping over the abdominal caivty, attaching between greater curvature of stomach and tranverse mesocolon, omentum-2 layers of mensetery, 4 layers of perotium

lesser peritoenal sac/omental bursa-behind stomach, entery via epiploci omental foramen, inferior recess of lesser sac fuses to form greater omentum
descirbe the development of duodenum? what organs outgrow from it?
proximal 1/3rd of duodenum forms from the distal foregut, foregut ends just distal to orgin of common bile duct, ducts of the liver, pancreas and gallbladder,

form as solid ventral outgrowth of duodenum, rotate posteriorly with duodental rotation
How does the pancreas form?
forms from 2 buds (ventral and dorsal) from the duodenum

ventral bud sits in ventral mesentery, dorsal bud in dorsal mesentery,

ventral bud rotates dorsally with duodenal rotation to fuse with dorsal bud

old ventral pancreas forms uncinate process (inferior projection of head) and part of head of the pancreas
How is a pancreatic ring/annular pancreas formed?
bifid vetnral pancreas can produce this, and thus form a duodenal stenosis: proximal to the stenosis there will be dilation due to material being backed up, distal to the stenosis it will appear normal. If complete blockage (atresia) occurs, massive dilation of proximal part will appear with an underveloped duodenum
How is the spleen formed?
is NOT formed formed as an outgrowth of the gut, instead soley derived from the splanchnic mesoderm in dorsal mesentery of the stomach

accessory spleens are quite common
Describe the midgut the contents
small intestine (except proximal 1/3rd of duodenum)
cecum, appendix, acending colon, and proximal 2/3rds of the transverse colon to the left colic/splenic flexure,

arterial supply is the Superior Meseneteric Artery
How many rotations does the midgut under go during development around the SMA?
three 90 degree counterclockwise rotations
Describe the beginning of the devleopment of U-shapped loop of the midgut and the SMA as its axis and the yolk stalk at its apex
the loop has a cranial limb (forming most of the small intestine) and caudal limb (forming parts othe large intestine), the loop herniates into old extraembryonic coelom at the umbilicus
Describe the first 90 degree rotation of the midgut loop
midgut loop rotates 90 degrees coutner clockwise as viewed from the ventral side of the embryo around SMA, crainal limb elongates faster than the caudal, and starts to form loops of the small intestine
Describe Rotations 2 and 3 of the midgut loop
abd. increases in size, hernia reduces and cranial limb enter abdominal cavity first (so small intestine starts settling in center), during hernial reduction, midgut undergoes an additonal 180 degree of coutnerclockwise roations for at total of 270 degree of CCL midgut rotation

The messenteries go along for the ride
What happen to the ascending and decendign colon during the rotations of the midgut
Ascending and decending colon get pressed agains the wall, messenteries of the these organs get pressed into each other

Ascending and decending become only one layer of perotienal, now they are secondary retroperioteneal, stuck to the posterior wall of the abd.
Describe somethings that could go wrong if intestines did not get come in right.
several things could occur:

1. If rotation did not occur correctly, the acending and decending colon could be intraperiotneal (not attached to the wall) which at first is asympatmatic but could create problems if it gets twisted , and kills a segment due to lack of blood supply,

2. If the cecum did not drop down. cecum and appendix could not have decended all the way, which is not a problem clinically unless the surgon doesn't realize during a procedure that the appendix is misplaced. Desent of the cecum is the last step of development.

3. SMA compresses tranverse colon, the cecum become dilated.
Descirbe what could happen if a volvulus formed in the midgut during development
duodenal obstruction-where the secom cuts off the stomach from the duodenum, happens when during rotation instead of sliding off, a side catches on a structure
Descirbe the hindgut contents
distal 1/3rd of transverse colon form left colic flexure to descending colon, signmoid colon, rectum superior portion of anala canal:

arterial supply is the IMA
what are some of the outgrowths of the Hindgut
urinary bladder and most of the urethra
From where does the caudal hindgut form
the cloaca
Describe the urorectal septum
Septum growing down from area between the allantois and yolk stalk, divdes the cloacal membrane into: urogential membrane-anterior and the anal membrane-posterior
Describe the anal membrane-posterior
Separates caudal hindgut from proctodeum, breaks down 7 weeks, producing an anal canal from caudal hinddgut and ectodermally lined proctodeum
What is the pectinate line?
pectinate line= dividing line where blood supply, lymphatic drainage and innervation above are same as hindgut (autonomic) and below are same as the body wall (somatic),

Marks the point where the cloeca membrane was in development

Above the line the rectum was derived from the hindgut-autonomatic

Below the line the rectum was derived from the proctodeum which is ectodermally lined-somatic
If you had a laceration in the anal canal where would it be more painful below the pectinate line or above the pectinate line
it would be more painful below the pectinate line because below is somatic where it would be more localized and above is autonomic where it would be more diffuse
What are some possible malformations of the urorectal septum?
non-perforate anus-persistent anal membrane, anoperineal fistual-?

rectourethreal fistual-rectum and urethra comibne to form same tube

rectovaginal fistual-rectus and vagina are the same tube