14 Anatomy (embryo 2) Aseel Al- khader 29/11/2015 Mohammad al Haidari 1 P a g e
بسم هللا الرحمن الرحيم *I advice you to watch this video ( from omar outom s post ) before studying this lecture for better understanding. https://drive.google.com/file/d/0b7cz_2hai1tatlvxx0jyu3cywku/vie w?pli=1 Development of the liver, Gallbladder, and Biliary Apparatus *In the 4 th week of development between the two membranes of ventral mesogastrium a specialized bud will appear, this is actually the:primodia of the hepatic sac(hapatic diverticulum )which will form the liver a biliary system later on,it will increase rapidly in size, very rapidly to form a very large gland later on. *The liver paranchyma is made of two segments : -A caranial part (upper part): which will form the main bulk of the liver (primodia of the liver) -A caudal part (lower part); whichwill form the gallbladder and the ductsconnected with it( biliary apparatus ). 2 P a g e
And origanally these two segments of the liver primodia ( caudal and cranial ) which will be later on the gallbladder and the main bulk of the liver are made from the same hepatic cells which appear in the ventral mesentry ( the same type of cells ), and both of them are connected by ducts but classicaly the are separated into cephalic (cranial) part and caudal part. As the liver increases in size it actually occupys most of the abdominal cavity so the gut tube which will develophave no space to grow there so the midgut (which will form the small intestine and part of the large intestine ) will leave the abdomin into the umblical region ( into the yolk sac)so they will had a space there and the will elongate, twist and rotate there and then they will return back when the liver attains its final size. When the liver increases in size it will actually ascends to push the diaphragm so it will make a small space below it. The cranial part of the hapatic primodia when it increases in size it actually separates the two membranes of the ventral mesentery that coveres the liver (it is like to inflate a baloon btween two layers until it separates them completely when it reaches the diaphragm ). 3 P a g e
That means: under the diaphragm( which was septum transversum) there is an area where the liver is in direct contact with the diaphragm whithout peritoneum and this is what we call the bare area. These two layers which the liver will tear will form on the top of the liver the coronary ligaments and its extention which is the right triangular, and on the left lobe the left triangular. This bare area is importanat clinically because when there is an accumulation of pus there, it is very difficult to diagnose it because the patient will not complain (no peritoneal irritation ) but instead there will be symptoms of infection and this what we call the subphrenis abcess. The primodia of the liver and the gallbladder are connected by a duct to the duodenum (common bile duct ),this duct opens in the ventral aspect of the dudenum. Development of the pancreas Another gland is formed in the area which is the pancreas, originally the pancreas will appear in two regions: ventral one and dorsal one, the ventral one will join the bile duct and it will rotate with the rotation of the duodenum until it will become in contact with posterior abdominal wall and then it will fuse posteriorly with the dorsal part of the pancreas. 4 P a g e
The pancreatic duct and the common bile duct will fuse to form theampulla of vater andthis ampulla will open in the second part ( vertical part) of the duodenumto form the major duodenal papila,this area of the duodenum marks the junction between the foregut and midgut so the foregut ends at the duodenal papila and the midgut starts there ( the midgut includes the rest part of the duodenum and the illeum, jejunum,cecum, ascending and anterior 2/3 of the transverse colon )and the midgut is supplied by the superior mesenteric artery and the hindgut which includes the ( posterior 1/3 of the transvere colon until the upper anus is supplied by the inferioir mesentric. the midgut The increasing in the size of the liver and the dilatation and the rotation of the stomach will be in transvere position. The whole midgut will have no space to grow in the abdomin so it will leave the abdomin toward the yolk sac 5 P a g e
( remember the yolk sac and its connection which the vitelline duct). This midgut is a u- shape loop by the pulling of the vitelline connection) this midgut will rotate in an anti-clockwise direction (90 degrees )around the superior mesenteric artery. Note : in some books it is mentioned that the upper limb of the u -shape midgut is going to rotate anti clockwise 90 degrees. The midgut will continue to elongate and convolute until the end of the embryonic period (10 th week) and then it will return back to the abdomin. 6 P a g e
Small intestine formed from the cranial limb returns first,passing posterior to superior mesenteric artery and occupies the central part in abdomen. Large intestine which is formed from the caudal limbreturns with another rotation (180 degree anti clockwise rotation) and Later on it occupies right side of abdomen. In total the midgut will rotate apon itself 270 degrees (90+180)around the axis of the superior mesenteric artery and, so the blood cannot approach these areas where the twisting which is huge ( the vesseles will be occluded because of this huge twisting ). logically,, when you twist a sraight vessel it will be occluded!!to overcome this you should branch this straight vessel and there should be arching in these branches to follow the twisting direction easily whithout being occluded. so during twistingthere will be more branching of the distal vesseles and they will form arches to follow the twisting directionof the area that they supply easily. That s why the vesseles that supply the inestine are forming arches.. To prevent the oclusion that may occur beacause of the twisting. This arching will be few in the beginning of the small intestine (because the twisting is less there ) and then it will start to increase when you move down in the intestine (because the twisting there will be more ) so more twisting more arching in the vesseles. In the jejunum the arching is less( 2 or 3 arches)and in the terminal illeum the arching will be more ( 7 or 8 or more arches). 7 P a g e
Again,, there is a huge rotation in the intestine to overcome this, arching and new branches will be formed and this arching system wil be more in the distal part because the rotation there will be more there.. The Cecum and appendix As we said before, the midgut will herniate in the umblical region,this herniation of the intestine in the umblical region is a physiological herniation. The midgut is in a u shape loope : upper limb and lower limb the proximal limb or( the upper limb) will rotate 90 degree as we said beforethe returning to the abdomin,the lower ( caudal limb or distal limb ) will show an enlargement,this is what we call the cecal elargment and it marks the future cecum and then it will return to the abdomin, andduring its returning it will rotate 180 degrees. 8 P a g e
At the beginning the cecal enlargement with the small projection of it (this small projection will be later a larger projection which we will call the appendix ) will occupy the lowe right side where it will be finally located in the right illiac fossa. All these rotations happen at the axis of the main artery of the midgut which is the superior mesenteric artery and this artery should give large number of branches and arches to avoid the obstruction during rotation. In adults there are 30-50 branches from the suoerior mesenteric to approach the gut tube, and all of these branches pass through a membrane which will hold the whole tube into a limited position which is the root of mesentery what is the root of mesentery?? and where is it? explanation :This root of mesentery is 6 inches oblique opening in the posterior abdominal wall and it runs from the left transverse process of the L1 to the right sacroiliac articulation. This root allows the large number of branches of superior mesenteric artery to reach all this long gut tube. 9 P a g e
so if we open the abdomin we will see this root of mesentery as a skirt, the waist of the skirt ( التنوره (خصر is the root of the mesentery and the rest of the skirt is the part of the mesentry that covers the gut tube. This skirt is very large, it will reach approxiamtely 8 meters to occupy both small and large intestines(the small intestine is 6 meters in length in adults and the large intestine is 2 meters in length). * when you examine a patient with an abdominal tumor or a peritoneal mass you can move this mass easily beacuase it will be connected only to the root of mesentery and this will allow it to move freely,so this root of mesentery is clinicaly important. chloaca stomodeum which is the orifice into the oral cavity passes into the gastrointesetinal and the digestive tube from the pharynx down to the caudal part which we call the chloaca. 10 P a g e
the chloaca is an area which actually receives the endings of thetwo systems the digestive and the urogenital, and it is closed at its ending by what we call the chloacal membrane ( this membrane is composed of the endoderm of thechloaca and the ectoderm proctodeum) The proctodeumis exactly like the stomodeum, the stomodeum is the enterance and the proctodem is the exit of these systems. In the adult there is a mucocataneousjunction ( it is a junction between the skin and the mucous membrane) in the region of the lip and at the in the anus. This chloaca is going to show an invagination by a septum which we call it urorectal septum similar to what we see in the esophagus and the trachea. so the opening of the urogenital sinus ( ventrally) and the opening of the rectum ( dorsally ) is going to be separated by a septum which is the urorectal septum. 11 P a g e
The perineum: is a diamond shape regionlocated in the most caudal part of the trunck, this diamond shape- area is marked by four bony projections that we can feel them : - The lateral ones are the isichial tuberosities. - The anterior one is the pubic symphysis. - The posterior one is the coccyx. - if you divide this diamond area by a line that links between the two isichial tuberosities you will end by two triangles : - the urogenital triangle ( anterior triangle):which is different between male and female. * in the females there are two separated opening for urethra and vagina but in the male there is a combined opening -the posterior triangle : which contains the anal opening and it is the same in both sexes. 12 P a g e
* the septum that separates these opening is going to continue downward untill it reaches the cloacal membrane and the proctodeum ( which is part of the chloacal membane) and fuses with them to make a thickining there that is called the perineal body which is the center of the levator ani ( muscle in this region ). the perineal body is a thickining of certain tissue into which muscle fibers are inserted and it can control the contraction and relaxation in that region so this perineal body is a thickening in the tip of the urorectal septum. The proctodeum which is the lower part of the anal canal reaches systemic branches from the internal illiac artery which approaches this region, here also there is a connection which we mentioned in addition to the umblical region and the lower part of the esophagus( I think the doctor here is talking about the porto-systemic anastomosis which is in the : umblical region, lower part of the esophaus, rectum ) 13 P a g e
Abnormalities: 1-Omphalocele: is the failure of the intestines to return to the abdominal cavity, the infant will be born with outside bulging of the intestine. 2-esophageal atresia/ fistula is also a common disorder. 3-umblical hernia :which is different from omphalocele the difference between umblical hernia and omphalocele : That the omphalocele is the persistence of the intestine in the umblical region and this means it is covered by the yolk sac layers whilethe umblical hernia occurs when the intestine returns to its normal place in the abdomin and covred by connective tissue but there is a weakness in this connective tissue so it will bulge again from the umblical region. In the omphalocele there is a reminant of the yolk sac while in the hernia no reminat of the sac ( the yolk sac ends as an umblical cord and then structures from abdomen come). 4-meckle s diverticulum : This outpouching is one of the most common anomalies of the 14 P a g e
digestive tract, It may be connected to the umbilicus by a fibrous cord or an omphaloenteric fistula. 5-megacolon: isan enlargment in part of the colonbecause of the absence of autonomic ganglion cells in the myenteric plexus. 5-Imperforated anus: is a common problem in which the proctoduem will not open completely and this can be corrected easily by a simple syrgery. 15 P a g e questions : -A question by one of the students about the difference between chloaca and procotodeum The answer was: the chloaca is endoderm and the proctodeum is ectoderm. -Another question about malrotaions The answer was:malrotation is possible in any part of the body like in the heart there is something called decxtrocardia where the heart will be in the right side, here also there is what we call the situs inversus in which all these rotations will be in the opposite direction ( it could happen but this is vey rare ). THE END =D