Embryology | Development of the Urinary System

All right ninjas, in this video, we're going to be talking about the development of the urinary system. Before we get started, please hit that like button, comment down in the comment section, and please subscribe. Also, down in the description box, we have links to our Facebook, Instagram, and Patreon account. Go check that out. All right ninjas, let's get into it.

All right, so when we talk about developing the urinary system, the basic foundation for developing the urinary system, if you guys remember from the mesoderm video, is the intermediate mesoderm, right? So, what we have to do is kind of really quickly go through how do we make the intermediate mesoderm, and then how does that intermediate mesoderm make the urinary system? That's kind of the basics, those steps, right?

So, first things first, let's start up here with our bilaminar disc around week two. If you guys remember up here, we have the amniotic cavity, right? And then you have these cells here, these blue cells, which is called your epiblast. Then below that, you have these green cells, which is called your hypoblast. And then below that, you have the yolk sac.

Now, what happens is, if you guys remember, you develop a little cavity or a little like space called the primitive streak. The epiblast cells move through the primitive streak and take the hypoblast and turn it into a new layer. What is that new layer here? It's called the endoderm. So, it replaces the hypoblast cells and converts that into endoderm. Then it makes a new layer, right? It comes down, replaces the hypoblast, that makes a new layer above the endoderm, which is called the mesoderm, right? Then what happens is the epiblast cells differentiate themselves and turn into ectoderm. So, what do we know? We went from a bilaminar disc to a trilaminar disc. And again, amniotic cavity and yolk sac. All right, this process for which we went from a bilaminar to a trilaminar disc is called gastrulation, right? Ba-boom!

Now, the next thing that we have to talk about is when you look at the mesoderm, there's actually three components of it, right? What are those three components? This first one, we'll label it as one, then two, then three. One is what? This is the paraxial mesoderm, right? Then two is what? That's the intermediate mesoderm. That's the one we really, really care about in this lecture. And then the last one, just for kind of consistency's sake, the third one is your lateral plate mesoderm. And the lateral plate mesoderm has two layers, right? Which is going to be this top layer going with the ectoderm, which is called the somatic layer, and this layer here going with the endoderm, which is called your splanchnic. We know that we're pretty good at that part, right?

What happens though in the next step here is that we have to undergo folding. And we're going to fold this kind of three-layer disc, which kind of looks like a pancake in this situation. We're going to fold it in a lateral folding process, and then another way, which is craniocaudal, which you can't really see in this type of two-dimensional space with the board, but it would be coming out at you. It would be this type of process. When we go through the lateral folding process, it gives us this nice little kind of like structure that looks somewhat similar to our actual internal organs, right? And what you see here is very important to kind of build your foundation here. We're going to kind of consistency go off of this number system. Paraxial mesoderm was one, two here was your intermediate mesoderm, and then this portion here, which is going outwards, right? This one here, it would be your somatic layer of lateral plate mesoderm. And then this one going here to wrap around your gut tube would be the splanchnic layer of the lateral plate mesoderm, right? We have an idea here of where that intermediate mesoderm is, right here, right? Number two.

What I want you guys to remember here is that once this intermediate mesoderm is formed, it starts to really condense and really, really condense near this splanchnic layer of the lateral plate mesoderm. It makes a little divot into it. And what does that start to look like? So, what starts happening is you start getting condensation, condensation of the intermediate mesoderm. And once it starts kind of really condensing and kind of bulging in, look here. So now you have what is this? Your paraxial mesoderm, right here. Your intermediate mesoderm, here. And then again, you have your lateral plate mesoderm, which will go out this way as the somatic layer, and then this one here, which will be your splanchnic layer. If you look here, you see how the intermediate mesoderm kind of condensed near this splanchnic layer and created like a little divot? That little divot there, which is created by that this big chunk of mesoderm, is called your urogenital ridge. And this core of mesoderm, which is a part of that urogenital ridge, is given a really special name. That is called your nephrogenic cord. It's called your nephrogenic cord.

Now, what I want you to remember is that the urogenital ridge will become the urinary system, but it'll also become your reproductive system. So, what we're going to do is we're going to focus on a portion of that urogenital ridge, which is going to be the nephrogenic cord. Now, the nephrogenic cord in a cross-sectional view isn't the best way to view it. The best way to really see the nephrogenic cord is from a sagittal section. So, this was kind of our cross-section, and this is a sagittal section here. So, we're going to look at this in a sagittal view, and it's already undergone that craniocaudal folding process. So, here you're going to see this long nephrogenic cord. You see this nice long nephrogenic cord, and it's actually going to be found behind what? Your gut tube. So, this right here would be your gut tube, and we already know that you would have your foregut, your midgut, your hindgut, all that good stuff.

What we really need to focus on throughout the rest of this lecture is that we started off with our intermediate mesoderm. It condensed. It made this really important structure called the nephrogenic cord. It spans really far down the length of the embryo. What I want us to do now is to take this nephrogenic cord and make our urinary system, and we're going to look at that now. So, let's go ahead and start with the nephrogenic cord and start building a bunch of stuff.

So, we're taking a look here at our nice little embryo, our cute little embryo, right? Now, again, what is this structure here called? This is called your nephrogenic cord. Now, here's what I want you to remember. Remember I told you that the gut tube was kind of located anteriorly to this nephrogenic cord? There's a real important portion of the gut tube that we actually do care about. It's right here at the bottom. You know what this bottom portion here is called? It's called the cloaca. And the cloaca is basically what will become your drainer and your stainer, right? So, it's going to become the pooper and the peeper. It's going to become your anal canal and the bladder and urethra. So, that's important because what do we say the nephrogenic cord is really becoming? Your urinary system. And guess what? That nephrogenic cord has to kind of interact, and we'll talk about it a little bit later, interact with the cloaca.

So, let's start off taking the nephrogenic cord and looking at how it develops from the beginning of week four. So, we're going to start around week four within the nephrogenic cord around the cervical region. So, around the cervical region of the actual baby, right? Around the cervical region of the nephrogenic cord, you start to have some development of a very specialized structure called the pronephros. When I say special, it's not really special because it actually regresses a little bit later. But the pronephros is going to be developing around week four, and this is one of the parts that are, you know, developed, derived from the nephrogenic cord. The pronephros is made up of two parts. It's made up of a duct and little tubules that form in front of it called a nephrotome. Okay? So, little tubules that form in front of it called the nephrotome.

What I want you to remember here is, big thing: pronephros develops at week four, starts where? Cervical region. Made up of a ductal system. So, let's draw here a nice, long tube here, cervical region, the duct, and then in front of it, what do we have? Your nephrotomes. Here's the big thing though. The pronephros, it actually starts kind of regressing and degenerating by the end of week four. It's pretty much gone. So, it's pretty weird, right? So, generally, the pronephros starts kind of degenerating by the end of week four. So, what's its purpose? We don't really know, to be honest with you. One of the things that we may say that it actually could be important with is that it helps with the kind of providing a structure that the next part from the nephrogenic cord derives.

So, we start off with the first thing, which is your pronephros, which is your duct and your nephrotome in front of it. By the end of week four, it starts to degenerate. But something starts to build off of that, and that's what's important. And I'm going to start kind of showing it. We're not going to develop it yet because we're going to go into the next stage. But another structure starts to start slowly forming by the end of week four that develops off of the pronephros. What is that called? It's called the mesonephros. So, that's what we have so far. Week four developed the pronephros, which is made up of the duct and the nephrotomes in front of it, starts degenerating by the end of week four, and the cervical region. So, now let's show this pronephros starting to kind of break down, and what starts actually forming coming off of it.

The next structure here, which extends all the way from the thoracic region, because remember, the pronephros develops in the cervical region, the mesonephros develops within the thoracic region and extends down to like the lumbar region and connects with the cloaca. So, around week five, let's say by the beginning of around week five, that pronephros degenerates, and what starts to developing off of it? So, the mesonephros starts developing. Developing, right? And what is the mesonephros made up of? The same thing like the pronephros. It's made up of a mesonephric duct and little tubules in front of it called mesonephric tubules. So, let's show that now.

So, the mesonephric duct and the mesonephric tubules kind of come off of the degenerating pronephros, and that's going to be forming within what region? From the thoracic region. So, it develops where? In the thoracolumbar region. Here's what I want you to remember: the mesonephros is going to become pretty much our primitive urinary system up until week 10. I'll explain how because we've only just seen the formation of the duct and the tubules. But what I want you to remember is that this will become the primitive urinary system. So, what have we talked about so far? Mesonephros develops off the pronephros with a duct and tubules, develops in the thoracolumbar region, and once starts degenerating the pronephros. Next thing I wanted you to remember is that this mesonephros will become the primitive urinary system, and you'll see how in just a second.

All right, beautiful. So, just one thing to remember before we move on to the next step is that that mesonephric duct is connected to the cloaca. And again, what does the cloaca become with respect to the urinary system? The bladder and the urethra. So, this will become part of your kidneys and ureter connecting to the bladder and the urethra, which will drain that, right? That's why it's important.

All right, so now let's move on here. Let's, what do we have degenerated? What's gone up here? We'll draw this in orange just so you know that this was kind of degenerate. What was degenerated at this point here? Your pronephros. What was built off of the pronephros from the thoracolumbar region here? Your mesonephric duct. I removed the cloaca here for a second because we're going to focus on this next structure here. But again, this was your mesonephric duct. What was coming off of that mesonephric duct? Mesonephric tubules.

Now, here's what's kind of really cool. These mesonephric tubules, when you start to form them, they continue to keep growing from the mesonephric duct. And there's a structure kind of located uh in between around the area of where the gut tube is and the nephrogenic cord. So, we just removed that gut tube, particularly the cloaca, and we're seeing this vascular structure developing along the length of the embryo. You know what the vascular structure here is called? It's called your aorta. It's called the aorta. And guess what? The aorta and the mesonephric tubules start meeting one another. They start kind of forming some structures. You know what this is called when you have a vessel that you've already formed and it starts making new vessels off of it? What is this called? Angiogenesis. So, off of the aorta, it starts forming these kind of little vessels called glomeruli, if you will, little capillary structures that come off of it via the process of angiogenesis. And these mesonephric tubules will grow from the duct and form like little capsules around that little primitive glomerulus. You know what this is actually called? Our primitive urinary system.

So, if we were to actually look at this and kind of zoom in on it, you would have here a duct. Here's your mesonephric duct. Here's your mesonephric tubule. This mesonephric tubule will continue to grow and kind of form a capsule here, like a Bowman's capsule. So, again, what is this here called? This is your tubule. The tubule extends and forms a Bowman's capsule. Then coming off over here, deriving from our actual mesoderm, you had the aorta, and the aorta is doing what? Having an arterial kind of butt off here via the angiogenesis process, and then making a capillary that will filter into this Bowman's capsule. So, then guess what happens? Blood will run through the aorta. As it runs through the aorta, it has to run through this little arterial and then through the glomerular capillaries. And then via the filtration process, some of the plasma and solutes and stuff like that will get filtered into this Bowman's capsule, then move down the mesonephric tubule into the mesonephric duct. What does the mesonephric duct connect to? The cloaca. What does the cloaca drain? Urine. That's our primitive urinary system. So, we've already started to see that process here.

So, now let's actually see it drain into the cloaca and then where that actual cloacal stuff goes to. Okay? So, we go to this next part here. We have our mesonephros here, well, particularly the mesonephric duct going to where? Connecting to the cloaca. Then we have coming off of this, the mesonephric tubules forming a Bowman's capsule. Then we have coming off of the aorta, the angiogenic process making the little glomeruli. From this, you're filtering the plasma from the blood moving through the Bowman's capsule, through the mesonephric tubules, down the mesonephric ducts, emptying into the cloaca. You know what the cloaca actually does? If we were to kind of look at it like this, the cloaca has two kind of points where two things come into it. Let's say here's one part, and here is where it kind of drains. There's a structure here called the allantois. But coming from the back end, you're going to have this mesonephric duct. So, this mesonephric duct is emptying in this kind of primitive urine, if you will, right? So, it's kind of dumping in urine here. But coming here is your hindgut. What's the hindgut making? Poopoo. And that poopoo is coming down and emptying into the cloaca. So, now you have feces and urine coming into this cloaca, and then exiting the cloaca is going to be a very special structure here called the allantois. The allantois.

Okay, so that's the big thing that I want you to remember is that the cloaca is actually draining two structures: both the hindgut and the mesonephric duct. So, it's emptying out both urine and feces into the allantois. So, now as you can see here, we finally formed our primitive kidney kind of structure here, which is what? What is the primitive kidney? This primitive urinary system, urinary system is actually going to continue to make urine. It makes urine from about week five till about week 10. And we'll see what the next step kind of happens here. But again, to remember, what it's actually pretty much made up of is your mesonephros. Okay? Pretty stinkin' cool.

But guess what? A bad man about to come in. The metanephros. So, we started with the pronephros. It was like, "I'm done. I can't do this." Then the mesonephros came in. It's like, "I got this." But then the bad man came in, the metanephros, and it says, "Don't worry, I got this." So, here's what I want you to think about in the pelvic region. All right? So, we had cervical, thoracolumbar. So, cervical is the pronephros. Thoracolumbar was the mesonephros. And the pelvic region, some intermediate mesoderms kind of kind of starts condensing, kind of in front of the mesonephric duct or the mesonephros in general. So, what is this intermediate mesoderm core here? You start to develop what's called the metanephric mesoderm. But they also like to give it another name. You know, scientists love to do this. They actually also call it the metanephric blastema. And what you need to remember is that this develops where? In the pelvic region. Okay? And again, this is intermediate mesoderm, just like the mesonephros is intermediate mesoderm. So, it starts to develop here.

What we need to do now is talk about how the mesonephros starts kind of losing its job of being the primitive urinary system and how the metanephros will then become our adult urinary system. So, now let's do that. Let's zoom in on this area and talk about that next step. We have this little metanephric blastema that's starting to kind of form out here from that intermediate mesoderm, right? So, again, what is this here called? Metanephric blastema. Again, metanephric blastema. You know, it's really cool. The metanephric blastema knows that it's about to be the big dog, so it starts releasing growth factors. And these growth factors that it starts releasing starts coming over here and stimulating this mesonephric duct. Coming off of the mesonephric duct, then will be this next little kind of bud. So, the metanephric blastema will release growth factors that'll stimulate the mesonephric duct to make a little kind of bud here. You know what this bud is called? This bud that's made here is called the ureteric bud. You have an idea now what this is going to become? The ureteric bud is going to become the ureter, and then eventually become the collecting system. The metanephroblastoma will become your nephron. So, I'm kind of prefacing all of this. So, metanephric blastema releases growth factor, stimulates the mesonephric duct to make the metanephric ureteric bud.

Now, let's go on to the next step here. We have our ureteric bud. So, what is this portion here called? That's called the ureteric bud. And this portion here is called the metanephric blastema. Now, the metanephric blastema was secreting those growth factors, right? That was causing the ureteric bud to grow. At the same time, the ureteric bud starts releasing more growth factors, and these growth factors start stimulating the metanephric blastema to begin growing and getting bigger and bigger and bigger. That's an important thing to understand here, signaling-wise. What is it called when you have a two-way interaction between two structures? In that case, metanephric blastema is releasing growth factors, stimulating the ureteric bud to form. Then the ureteric bud was released in growth factors, stimulating the metanephric blastema to form. There's a particular name for that type of process, and it's called reciprocal induction. Pretty straightforward, right? So, again, what is that process called? It's called reciprocal induction. Very important for the growth of the kidney.

All right, beautiful. So, now as this reciprocal induction is occurring, growth factors being released between the two structures, the ureteric bud starts to kind of form a nice little, as it continues to grow and grow and grow, it now creates a nice stalk. So, now we have to kind of really define this ureteric bud a little bit more technically. This knobby portion here that's kind of invading into the metanephric blastema because of that reciprocal induction process, this portion is technically called the ureteric bud. And then this, this little stalk-like structure that connects the ureteric bud to the mesonephric duct, this is actually called the ureteric stalk. Okay? Very important to understand.

So, to quickly recap it: metanephric blastema releasing growth factors, stimulating the mesonephric duct to make ureteric bud. The ureteric bud starts releasing more growth factors, stimulating the metanephric blastema to grow. As it continues to occur, the ureteric bud continues to grow inwards to the metanephric blastema and makes an elongated tube that connects the ureteric bud to the mesonephric duct, which is called the ureteric stalk. Beautiful.

Now, as the ureteric bud continues to grow and grow and grow, it then kind of takes on a larger structure that we really have to start defining now, and we're going to go in kind of a sequential order. So, the ureteric bud continues to grow under the influence of the reciprocal induction process, and as it grows, it makes this big kind of like tubular structure in this metanephroblastoma. This right here is called your renal pelvis. So, we started off with the ureteric bud invading the metanephric blastema, continuing to grow. The next thing that it becomes is the renal pelvis.

All right, so now what happens is the renal pelvis, right? From that ureteric bud, we had ureteric bud invade the metanephric blastema, it started growing. The next thing it grew into is the renal pelvis. Now, the renal pelvis is going to again bifurcate and form more structures. So, here we had at the center here, this is our renal pelvis. Growing off of that, you see how it comes off of that? It kind of grows these little knobbies coming off over here. These little knobbies are going to become what's called the major calyx. And over here, this would be another major calyx. Beautiful.

Now, what happens next? We're going to continue to keep kind of replicating, proliferating, and going through that growth process. So, now we had here the renal pelvis. You guys are going to get the gist here. Then here, we're going to have the major calyx. And then guess what happens to the growth off of that major calyx? It's going to butt off even more, and this is going to make your minor calyx. Pretty crazy, right? Now, the minor calyx is going to continue to keep replicating and growing and growing and growing more structures off of it. So, this is going to become consistently pretty repetitive here. But in the center is the renal pelvis. Here, coming off of this portion here is your major calyx. Coming off of that portion here is going to be your minor calyx. And then coming off here, you see like all these little blue like finger projections coming off? This right here is going to be your collecting tubules. And you're going to make millions of these collecting tubules that come off of that minor calyx.

So, here's what I want you guys to remember. We had over here coming off again, what was that ureteric bud? Started, became, became renal pelvis, major calyx, minor calyx, collecting tubules. What was the ureteric bud connected to? The mesonephric duct. What was that structure called? What is this one here called? This was called the ureteric stalk, right? But what does the ureteric stalk start to look like? The ureter, right? So, the ureteric stalk technically is going to start really becoming the ureter. What have we formed from the ureteric stalk and ureteric bud? We have pretty much formed all of the collecting system for the urinary system. We've formed the ureter, we've formed the renal pelvis, the major and minor calyces, and the collecting tubules. We're ready to collect urine now. What we've got to do is we've got to start making structures that are going to help to filter and really make urine. That's the next step. Let's do that now.

It's also really important to remember what is pretty much still functioning as our urinary system, our primitive urinary system, at the same time all of this is happening? That mesonephros. So, remember that mesonephros is still continuing to be our urinary system while this is developing.

All right, now let's go in and zoom in on this next part, which is we got to really start making our nephron, the structure that's going to help to filter and make urine. So, we're going to zoom in on this portion now and look at it.

All right, so now we're going to zoom in on these collecting tubules and look at two portions: the collecting tubule, right? And then all of this actual metanephric mesoderm that's around it. Okay? So, when we zoom in on this portion, this is your metanephric mesoderm, right? But we kind of give it a little special, like weird name because technically it's capping this collecting tubule. We call this a metanephric mesoderm cap. That's stupid, but that's what they call it. Then what is can, like, what is actually in communication, if you will, with this metanephric mesodermal cap? This is your collecting tubule, one of them, right?

Now, what the collecting tubule does is the cells of the collecting tubule start secreting lots of growth factors, and it starts influencing this metanephric mesodermal cap and really starts causing it to proliferate, proliferate, and condense into kind of one singular kind of structure, if you will. Okay? So, it really starts proliferating and condensing. So, after you release a lot of these, what are these chemicals that you're releasing here? A lot of growth factors. It triggers the proliferation and condensing of this metanephric mesoderm, and this now turns, what I'm going to abbreviate, this is your collecting tubule, and now this metanephric mesoderm cap becomes what's called the metanephric vesicle. Then the metanephric vesicle, there's going to be a continuous release of growth factors. The metanephric vesicle then starts kind of becoming S-shaped, kind of like a little bit coiled. And when it does that, it makes this kind of metanephric tubule, if you will, right? And then what happens is you start having a connection between the collecting tubule and the metanephric tubule. And once they fuse, they start to begin this production of the nephron.

So, here we're going to have the collecting tubule, the metanephric tubule. When it connects to the collecting tubule, we now give this kind of structure here a very special name. We call this the distal convoluted tubule. So, the metanephric tubule then becomes the distal convoluted tubule. We have that distal convoluted tubule, right? That we formed from that metanephric tubule, collecting. It's connecting to the collecting tubule. It's going to continue to keep growing and replicating and growing and coiling and becoming S-shaped. As it does that, look at what happens next here. Here we still have our distal convoluted tubule here. We have our collecting tubule. But look what it started to make now. This next structure here at the end, it makes. So, we had the distal convoluted tubule. This becomes your proximal convoluted tubule. So, we started with the metanephric vesicle, metanephric tubule, DCT, and then DCT makes the PCT, or the proximal convoluted tubule. The next thing that happens here is that from the proximal convoluted tubule, it starts to kind of change its conformation, its configuration, if you will. So, we still have the collecting tubule. We have our distal convoluted tubule. From the distal convoluted tubule, we made our proximal convoluted tubule. And then off of that, it starts to make this cup shape. And this cup shape is called your Bowman's capsule. Very similar to what the mesonephric tubules did. Oh, we're getting there. We're really getting there.

Then here's the next step. Okay, I know there is another structure. If you guys know your kind of microanatomy, there's, you're like, "Well, where's the loop of Henle?" Something else has to happen before we make the loop of Henle. If you guys remember, we had that aorta that was coming down the length of the embryo. If we just zoom in on a portion here, here's a portion of the aorta at the level of, very interesting, the common iliacs. Because where do we say that the metanephros developed? Do you guys remember? In the pelvis. So, at the pelvic region is your common iliac arteries. So, you've got to remember here because you commonly think, "Oh, it's the renal arteries." The kidneys, the actual mature kidneys, develop in the pelvis and then ascend upwards. So, technically, we're down in the pelvis. So, this is your abdominal aorta. This is your right common iliac. This is your left common iliac. Off the common iliacs, I'm only looking at one, there would be the same thing on this other side. Coming off that left common iliac, you're going to have arterials, and then you're going to have the capillaries and all that stuff like that. But guess what it does? It makes your glomerular capillaries. Guess what the glomerular capillaries interact with? Your Bowman's capsule. Guess what the Bowman's capsule is developed from? The PCT. Guess what the PCT is derived from? The DCT. And guess what the DCT is actually kind of stimulated from? The collecting tubule. And then guess what? We're starting to make the urine. Why? Because if you remember, blood will move through the iliac system, through the glomerular capillaries, and get filtered across this Bowman's capsule into the proximal convoluted tubule, go to the distal convoluted tubule, into the collecting tubule, and then down all those structures to drain into the cloaca, right? But something else has to happen.

So, then we got to do another thing. So, here we have the proximal convoluted tubule, the distal convoluted tubule, the collecting tubule, your Bowman's capsule, and then you have your glomerular capillaries coming off of your common iliacs. Then what happens is that the actual tissue, the metanephric mesoderm between the PCT and DCT, starts to grow downwards and form a loop. Now you finally get your loop of Henle. So, we're kind of going in an order of how the nephron is developing. But what have we made? When we start here with the glomerulus, the Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, we've made our nephron. So, we made our nephron from the metanephric mesoderm. And we made the collecting system from this point all the way down from the ureteric bud. Beautiful.

All right, so we've kind of now formed our nephron. And again, the nephron was the Bowman's capsule, the PCT, the loop of Henle, the DCT, right? We made that from again, what? Metanephric mesoderm. Collecting system all the way down to the ureter was developed from the ureteric bud. Now, we started with the blood feeding via the glomerular capillaries coming from the abdominal aorta. But what portion of the abdominal aorta? The common iliacs. So, blood was feeding into this actual kidney because we've now made a kidney, the actual mature type of kidney at this point. Blood was feeding into it via what structures? Again, your common iliacs. But guess what happens? The common iliac arteries start kind of breaking down. They start degenerating. And as that happens, the kidneys start ascending. So, the kidneys will start kind of moving upwards. As the kidneys move upwards and they lose their vascular supply, they move kind of into the upper quadrants of our abdomen. And then what happens is blood vessels from the top part of the aorta, and we'll go over the development of the vascular system in another video, but the aorta will develop these lateral branches that'll move and invade the actual kidney. And what are these arteries that they will become? The lateral branches of the aorta. This will become the renal arteries. Pretty straightforward, right? This will become your renal arteries.

So, we have now completely gone through the entire process of making the collecting system of the kidney and making the actual nephrons, the structural and functional unit of the kidney. But guess what? We did not finish yet. We've made our kidney by representing all that brown tissue, that's our nephron. We made the collecting system with the collecting duct, the minor and major calyces, the renal pelvis, and the ureter. But guess what we haven't done yet? We haven't actually talked about how the cloaca becomes the bladder and the urethra. That's the next step.

In order for us to do this, again, by week 10, by week 10, we have actually kind of taken over. The metanephros takes over the job of the mesonephros. If you guys remember, we had that mesonephros here, right? And that mesonephros was getting blood from the aorta. But again, that starts breaking down, and the mesonephros no longer will be making any urine. Who will be making it? The metanephros. So, now what we have to do is understand the next step here.

What I want us to understand is how we now take the mesonephric duct and this ureter, and how that empties into the cloaca at this point. So, again, what is this structure here? This was that ureteric stalk, but it becomes the ureter, right? But again, technically it's the ureteric stalk, but it will become the ureter. And then this right here is your mesonephric duct. This was the structure that was draining urine from the mesonephric tubules before week 10, making urine. Both of these empty into what is this structure here? The cloaca. And again, the cloaca is getting urine from this structure and getting poop from the hindgut, emptying it out via the allantois.

What happens is the next step is that imagine this kind of connecting portion here, right? This connecting portion where they both kind of empty into the cloaca. This is going to get absorbed into the cloaca. So, as this gets absorbed into the cloaca, imagine this portion here where they join is disappears, gets absorbed into it. We're in this step here. So, now if you look here, what is this structure emptying into the cloaca? The mesonephric duct. And then what is this structure over here? This is going to be the ureter. Eventually, they're going to fuse in with the cloaca. And then what happens is the mesonephric duct will actually kind of move a little bit more backwards. And then the ureters will move a little bit more towards the sides. Now, technically, this portion here where they're kind of joining is going to be what's called the vesicular trigone or the trigone of the bladder. It's represented by the uh, the urethra, as well as the two ureters where they enter into the bladder.

What I want you to remember here for right now is that the mesonephric duct, we don't care about that with respect to the urinary system. The reason why is we'll talk about this within the reproductive system. The mesonephric duct will become a part of your ejaculatory ducts, right? The vas deferens, the epididymis, the seminal vesicles, the common ejaculatory ducts, stuff like that. And the females have become the uterine tubes, part of the uterus, stuff like that. So, we don't really care about that. What we do care about is the ureter emptying into the bladder. But this does not look like a bladder. So, guess what happens? The next thing that has to happen is we got to make this cloaca look like a bladder and urethra.

So, what happens is here's your cloaca, okay? You develop kind of a little like a septum in between the cloaca, right? It's called the urorectal septum. And what that urorectal septum does is it starts to separate the cloaca into two distinct parts. So, again, what is this little septum here called? It's called the urorectal septum. And what it does is it separates the cloaca into two parts: an anterior portion, which is going to move this way, and a posterior portion, which is going to move this way. The anterior portion is what's going to become the bladder, okay? And then the posterior portion is going to be what's uh, the anal canal. So, if you imagine here, imagine the urorectal septum comes in here, and it starts actually kind of working its way backwards and eventually will kind of separate the hindgut away from the cloaca. So, that if you come down here, as the urorectal septum kind of comes backwards, here would be your hindgut. It would kind of move into its own little space here, which would become the anal canal. And then anterior to this, what is this going to be? This is what's going to become the bladder and the urethra.

So, now we've separated this, and what we actually call this anterior portion besides the bladder and the urethra, we actually call the urogenital sinus. Now, the urogenital sinus actually has three distinct parts. Okay? This pink part is the most proximal, in other words, it's closest to the ureter. So, here's the ureter. This portion here, the proximal portion of the urogenital sinus, is going to be what becomes the bladder. It's pretty straightforward, right? This middle portion here is actually going to be come the urethra. Okay? And there's actually an important thing to distinguish here. When we talk about the urethra in females, it's just called the female urethra. But in males, there's actually three parts of the urethra: there's the prostatic urethra, the membranous urethra, and the penile urethra. The prostatic and membranous urethra is made by this middle portion. But the penile urethra is made by this bottom-most distal portion. So, we're actually going to put here P for penile urethra. And then here for this blue portion, I want you to remember that this is, we'll put PR for prostatic and membranous urethra.

So, now we have our ureter emptying into the urogenital sinus, which as it starts to continue to grow and grow and mature, this portion closest to the ureter will become bladder. The middle portion will become the prostatic and membranous urethra of males, or just urethra in females. And the distal portion of males will become the penile urethra. So, now come down here to the bottom.

So, now to put all of this into perspective: the brown portion, this is going to be making the nephron. This is where the nephron was coming from. That was from the metanephric blastema or mesoderm. This entire collecting system from the collecting tubules all the way down to the ureter, this was coming from the ureteric bud. But this was your collecting system. We went through all of that, right? Then the mesonephric duct, again, we said don't worry about that, but that will kind of come here in the back, and that'll make a lot of the ejaculatory duct and some of the actual ducts of the male and female reproductive system. But again, the ureter will connect to what we said was the cloaca, right? And we said that the cloaca will actually get separated by the urorectal septum and separate into a posterior anal canal and an anterior urogenital sinus. The anterior urogenital sinus, forming from proximal to distal, will become the bladder and the urethra.

The other thing that I want you guys to remember was we said that there was a structure that drained the cloaca. What was that called? The allantois. The allantois. What I do want you to take away at the end of this is it becomes another structure called the urachus. And then the urachus will actually start becoming a ligament, right? And it becomes what's called the median umbilical ligament. So, that's an important thing to take away as well is that the structure that was draining the cloaca was called the allantois. And to remember here that the allantois becomes the urachus, and then the urachus actually becomes an umbilical ligament called the median umbilical ligament. So, now we have completely covered the development of the urinary system.

All right, ninja nerds, in this video, we talked about the development of the urinary system. I hope it made sense, and I hope that you guys did like it. All right, engineers, we love you. Thank you. And as always, until next time. [Music]