Understanding Midgut Embryological Development

Hey guys, my name is Minas and today we're going to be talking about the embryological development of the GIT, namely the midgut. In a previous video we spoke about development of the foregut, in the next video we'll talk about the hindgut, but today's video will be on midgut. So the portion of GIT distal to where the bile duct enters the duodenum all the way until the junction of the proximal two-thirds and the last third of the transverse colon. And although watching the previous video on hindgut would be a good idea before you watch this one, I've made this video in a way so that you don't have to watch any other video. If you're new to embryology, you'll still understand if you want to focus on midgut embryology. So let's begin at the beginning, at the ball of cells called the blastula, which is the result of fertilization when a sperm fertilizes an egg. This ball travels down the uterine tube into the uterine canal, implants into the uterine wall, and the process of gastrulation will form three joints. germ layers and these three germ layers are the ectoderm the mesoderm and the ectoderm in endoderm sorry the ectoderm will become your cns and your skin the mesoderm has three components the paraxial mesoderm which will become muscle and somites the intermediate mesoderm which are kidneys and gonads and lateral plates and there are two lateral plates there is the somatic lateral plate and there's the splanchnic lateral plate and another name for that is the parietal mesoderm and the visceral mesoderm. Finally we have the endoderm in green and that's going to be the focus because that's pretty much where the bulk of GIT comes from. And so this is a cross section, as is this, of the fetus, just in different times. This is when you were a pancake and this is when you're starting to fold and differentiate. So what happens is this folds in like this and as you can see the skin is starting to cover you and we have the neural tube pinching off the ectoderm. At the same time, the endoderm is forming the GIT. So let's quickly label this. We have the parietal mesoderm and the visceral mesoderm. The visceral will end up being the peritoneum covering. And then there is also a parietal peritoneum. That's where those come from. We have the yolk sac here, which is connected to the midgut through a vitiline duct. We'll go through that in a bit. And we have the skin covering you, but as you can see, it's not complete just yet. This is in a time between week three and week four. All right, so let's have a look at this week six embryo. The development of the GIT is continuing. And this is the biggest embryo I've drawn in my videos probably, just to outline the entire GIT. So let's quickly look at the GIT. label this one. Here's the heart and the aorta and I've drawn in the blood supply of the GIT. It's important to know the divisions of the GIT because there is a different blood supply for each one which we'll go through. We have the liver here in blue, the stomach duodenum and the loops and the cloaca at the bottom over here, yolk sac which is connected to the midgut through a vitiline duct. And so at this point let's assume that's where the bile duct enters that's the beginning of the midgut and the foregut is supplied by celiac artery The midgut is supplied by the superior mesenteric artery and the hindgut is supplied by the inferior mesenteric artery. That's important, you have to know that. The superior mesenteric artery is especially important for mid-gut development because it's its axis of rotation and what I mean by that is that if you have the superior mesenteric artery here and then the loop of bowel here it'll rotate along this superior mesenteric artery. So let's have a look right here. Here's the aorta and the superior mesenteric artery with its branches not drawn in but all the way extending throughout the midgut. The midgut is made out of primary intestinal loops and the primary intestinal loops have two ports. above the vitiline duct it's called the cephalic portion and below it's the caudal but that pretty much means is that above and below there are two limbs of the primary intestinal loops and what happens is that these these loops will rotate and the rotation it's kind of like this so we have some bowel here And as the GIT elongates, rotation occurs. And I'm pulling this out of the amniotic cavity because that's where its first 90 degrees of rotation happens. happens there is a herniation that is physiological which is in other words meaning it's normal herniation of the bowel temporarily so it herniates and it rotates counterclockwise 90 degrees while it is herniated it rotates another 180 degrees in the next month, however, this is when it's back inside. So if we're going to do the whole rotation, so we have 90 degrees and then another 100, 180 degrees of rotation of the bowel. first 90 degrees happens while it's herniated the other 180 degrees happens when it's back in tra abdominally and the reason it herniate is because there is a lack of space with the growing liver and everything else that's growing in the abdomen and it's assumed that it retracts back inside the abdomen because the liver stops growing and the abdominal cavity has grown to allow for its space to continue so what we get is the super superior mesenteric artery is the axis of which those intestinal loops rotate So what happens is that the cephalic portion is actually at the bottom now, and the caudal portion, which is your colon, will be at the top. Okay, so what do each bits become? So we've already spoken about the primary intestinal loops. The cephalic portion of the primary intestinal loop will become the distal duodenum after the bile duct, the jejunum, and the proximal ileum. Whereas the caudal, the top bit now, caudal initially, but now the higher part, is going to become the distal ilium, the cecum, the appendix, the ascending colon, and the proximal two-thirds of the transverse colon. So in other words, all this together is the midgut. So the midgut is this. All of this that's turning is your midgut. Okay, and this image pretty much outlines the physiological herniation. So we can tell, this being the abdomen and this being extra-embryonic, we can see that the intestines actually protrude out of the embryo and this is completely normal during this herniation there is a 90 degree turn that's clockwise and as it retracts there is another 180 degree rotation so in total there is about 270 degrees of rotation of the gut anti-clockwise or counterclockwise And that's at two months that the physiological herniation occurs. And in the third month, that's when the retraction occurs. And the last 200 and the final 270 degrees rotation will complete. Okay. Now let's move on to what happens after this rotation when the midgut is back in the abdomen. We have this. So think about it as step one and step two. So over here We've got retraction of the midgut, and we're getting all these bits that are differentiating. So your initial cephalic intestinal loop will become, you'll see the jejunum and the rest of the small intestine. And what happens is that there is coiling of the jejunum. But this doesn't happen with the colon. The colon remains uncoiled, although it'll go on to... have hostra, the jejunum and the small intestine will coil so it'll loop on along each other and we have here the cecum which is initially in the right upper quadrant. The cecal bud developing here and if we look at this developed GIT We will see here that, let's assume the bile duct enters here, that's where the midgut begins, and this is the proximal two-third of the transverse colon, and at this junction, with the one-third portion of the transverse colon, That's where the midgut ends and the hindgut begins. So the hindgut is everything after that. So you'll notice that the right upper quadrant, secant, will eventually descend down to be in the right iliac fossa in a normal scenario. So now let's have a look at this. This is where we have initially the cephalic. excuse me the caudal loop this already been rotated and the cecal bud is being grown and the cecal bud is pretty much a dilatation of this loop and like we mentioned the cecum is temporarily in the right upper chondron and with development you have an appendix elongating and you'll see here at week eight this is where we had this physiological herniation at around about that time and when it descends into the Right iliac fossa. I've drawn in over here this tinea coli and I was watching a surgery where, well it was a appendectomy and the surgeon asked me if we can't find the appendix how can we find it if i'm having difficulty what kind of landmarks will i be able to use so that it'll guarantee me finding this appendix and because i had done my some reading prior to that i said if you follow the tinea coli which the large bowel has three of they will eventually meet at the appendix and so that's a good uh kind of nugget to know so there are three tinea coli will eventually meet at the appendix eventually all meet at the appendix and in 50% of the time the appendix is on the back of the cecum and that's because of its growth during its descent of the cecum. And so that is essentially the end of midgut embryology. I hope that everyone has understood everything that went on. If not, leave a comment in the box below or send a message on Facebook and I'll be happy to help more. Thanks for watching.

So let's begin at the beginning, at the ball of cells called the blastula, which is the result of fertilization when a sperm fertilizes an egg. This ball travels down the uterine tube into the uterine canal, implants into the uterine wall, and the process of gastrulation will form three joints. germ layers and these three germ layers are the ectoderm the mesoderm and the ectoderm in endoderm sorry the ectoderm will become your cns and your skin the mesoderm has three components the paraxial mesoderm which will become muscle and somites the intermediate mesoderm which are kidneys and gonads and lateral plates and there are two lateral plates there is the somatic lateral plate and there's the splanchnic lateral plate and another name for that is the parietal mesoderm and the visceral mesoderm. Finally we have the endoderm in green and that's going to be the focus because that's pretty much where the bulk of GIT comes from.

And so this is a cross section, as is this, of the fetus, just in different times. This is when you were a pancake and this is when you're starting to fold and differentiate. So what happens is this folds in like this and as you can see the skin is starting to cover you and we have the neural tube pinching off the ectoderm.

At the same time, the endoderm is forming the GIT. So let's quickly label this. We have the parietal mesoderm and the visceral mesoderm.

The visceral will end up being the peritoneum covering. And then there is also a parietal peritoneum. That's where those come from. We have the yolk sac here, which is connected to the midgut through a vitiline duct.

We'll go through that in a bit. And we have the skin covering you, but as you can see, it's not complete just yet. This is in a time between week three and week four. All right, so let's have a look at this week six embryo.

The development of the GIT is continuing. And this is the biggest embryo I've drawn in my videos probably, just to outline the entire GIT. So let's quickly look at the GIT.

label this one. Here's the heart and the aorta and I've drawn in the blood supply of the GIT. It's important to know the divisions of the GIT because there is a different blood supply for each one which we'll go through. We have the liver here in blue, the stomach duodenum and the loops and the cloaca at the bottom over here, yolk sac which is connected to the midgut through a vitiline duct.

And so at this point let's assume that's where the bile duct enters that's the beginning of the midgut and the foregut is supplied by celiac artery The midgut is supplied by the superior mesenteric artery and the hindgut is supplied by the inferior mesenteric artery. That's important, you have to know that. The superior mesenteric artery is especially important for mid-gut development because it's its axis of rotation and what I mean by that is that if you have the superior mesenteric artery here and then the loop of bowel here it'll rotate along this superior mesenteric artery.

So let's have a look right here. Here's the aorta and the superior mesenteric artery with its branches not drawn in but all the way extending throughout the midgut. The midgut is made out of primary intestinal loops and the primary intestinal loops have two ports.

above the vitiline duct it's called the cephalic portion and below it's the caudal but that pretty much means is that above and below there are two limbs of the primary intestinal loops and what happens is that these these loops will rotate and the rotation it's kind of like this so we have some bowel here And as the GIT elongates, rotation occurs. And I'm pulling this out of the amniotic cavity because that's where its first 90 degrees of rotation happens. happens there is a herniation that is physiological which is in other words meaning it's normal herniation of the bowel temporarily so it herniates and it rotates counterclockwise 90 degrees while it is herniated it rotates another 180 degrees in the next month, however, this is when it's back inside. So if we're going to do the whole rotation, so we have 90 degrees and then another 100, 180 degrees of rotation of the bowel.

first 90 degrees happens while it's herniated the other 180 degrees happens when it's back in tra abdominally and the reason it herniate is because there is a lack of space with the growing liver and everything else that's growing in the abdomen and it's assumed that it retracts back inside the abdomen because the liver stops growing and the abdominal cavity has grown to allow for its space to continue so what we get is the super superior mesenteric artery is the axis of which those intestinal loops rotate So what happens is that the cephalic portion is actually at the bottom now, and the caudal portion, which is your colon, will be at the top. Okay, so what do each bits become? So we've already spoken about the primary intestinal loops. The cephalic portion of the primary intestinal loop will become the distal duodenum after the bile duct, the jejunum, and the proximal ileum. Whereas the caudal, the top bit now, caudal initially, but now the higher part, is going to become the distal ilium, the cecum, the appendix, the ascending colon, and the proximal two-thirds of the transverse colon.

So in other words, all this together is the midgut. So the midgut is this. All of this that's turning is your midgut. Okay, and this image pretty much outlines the physiological herniation.

So we can tell, this being the abdomen and this being extra-embryonic, we can see that the intestines actually protrude out of the embryo and this is completely normal during this herniation there is a 90 degree turn that's clockwise and as it retracts there is another 180 degree rotation so in total there is about 270 degrees of rotation of the gut anti-clockwise or counterclockwise And that's at two months that the physiological herniation occurs. And in the third month, that's when the retraction occurs. And the last 200 and the final 270 degrees rotation will complete. Okay. Now let's move on to what happens after this rotation when the midgut is back in the abdomen.

We have this. So think about it as step one and step two. So over here We've got retraction of the midgut, and we're getting all these bits that are differentiating.

So your initial cephalic intestinal loop will become, you'll see the jejunum and the rest of the small intestine. And what happens is that there is coiling of the jejunum. But this doesn't happen with the colon.

The colon remains uncoiled, although it'll go on to... have hostra, the jejunum and the small intestine will coil so it'll loop on along each other and we have here the cecum which is initially in the right upper quadrant. The cecal bud developing here and if we look at this developed GIT We will see here that, let's assume the bile duct enters here, that's where the midgut begins, and this is the proximal two-third of the transverse colon, and at this junction, with the one-third portion of the transverse colon, That's where the midgut ends and the hindgut begins.

So the hindgut is everything after that. So you'll notice that the right upper quadrant, secant, will eventually descend down to be in the right iliac fossa in a normal scenario. So now let's have a look at this. This is where we have initially the cephalic.

excuse me the caudal loop this already been rotated and the cecal bud is being grown and the cecal bud is pretty much a dilatation of this loop and like we mentioned the cecum is temporarily in the right upper chondron and with development you have an appendix elongating and you'll see here at week eight this is where we had this physiological herniation at around about that time and when it descends into the Right iliac fossa. I've drawn in over here this tinea coli and I was watching a surgery where, well it was a appendectomy and the surgeon asked me if we can't find the appendix how can we find it if i'm having difficulty what kind of landmarks will i be able to use so that it'll guarantee me finding this appendix and because i had done my some reading prior to that i said if you follow the tinea coli which the large bowel has three of they will eventually meet at the appendix and so that's a good uh kind of nugget to know so there are three tinea coli will eventually meet at the appendix eventually all meet at the appendix and in 50% of the time the appendix is on the back of the cecum and that's because of its growth during its descent of the cecum. And so that is essentially the end of midgut embryology. I hope that everyone has understood everything that went on.

If not, leave a comment in the box below or send a message on Facebook and I'll be happy to help more. Thanks for watching.

Transcript for:Understanding Midgut Embryological Development

Transcript for:
Understanding Midgut Embryological Development