Development of the Kidneys | Renal Embryology | Development of the Urinary System | Embryology

Hello and welcome to Byte Size Med. In this video, we're going to talk about  the stages of renal development. First let's look at what we need to know in the  anatomy and the structure.

So there are two kidneys in the abdomen and each kidney has millions of  nephrons. The kidneys have an excretory portion and a collecting portion. The excretory portion  of the nephron starts from the glomerulus, then the proximal convoluted tubule, the loop of Henle,  and the distal convoluted tubule.

The collecting system is the collecting ducts and tubules, which  form the minor calyces, then the major calyces, the renal pelvis, which continues down as the  ureter, which is going to empty into the bladder. The kidneys develop from the intermediate  mesoderm and the portions of this mesoderm that condense to give rise to the kidneys  are the nephrogenic cords. There are two kidneys, so they're bilateral.

They extend from the  cervical end to the sacral end of the embryo. So if we assume this is the back of the embryo,  it's from the neck end up to the sacral end. There are three stages of development.

The pronephros,  then the mesonephros, and the metanephros. They're one below the other. So the pronephros  is at the cervical region, the mesonephros is between the thoracic and the lumbar region  and the metanephros is lower lumbar and sacral.

The first stage is the pronephros. We're  now at week four. The pronephros is very primitive, just some cell clusters and tubules,  and the pronephric duct, this duct opens into the cloaca, part of which is going to form the  bladder.

But for now it's still the cloaca. So the pronephros can't function, it  degenerates. So we're just going to remove that and now we're on to stage two, the mesonephros.

This appears late in the fourth week. They have  mesonephric tubules, which are like primitive versions of renal tubules, and these are associated  with capillaries. What's that?

That's a glomerulus, that means they can perform filtration,  and they drain into the mesonephric duct, also called the Wolffian duct. Again that opens into  the cloaca. So this stage can perform filtration, so it's functional, and it does the work of the  kidneys, until the final kidney takes over.

So up to around 12 weeks. These tubules, they  start degenerating as well, from the top down. By the fifth week, the metanephros forms. 

Now this is going to form the actual kidney. It starts functioning towards the 12th  week, it takes over from the mesonephros. From the lower portion of the  mesonephric duct, there's a bud that grows.

This is the ureteric bud. Now this is going  to form the collecting system of the kidney. The lower part of the nephrogenic cord has the  metanephric blastema.

The ureteric bud induces differentiation of the mesoderm in this region,  and that's going to form the excretory portion. First let's look at the ureteric bud. The caudal  part of the bud, that's going to form the ureter.

So that leaves the cranial part to form the rest.  So it branches and forms those collecting tubules. The first few set of branches will form the major  calyces and the next set will form the minor calyces.

So we're done with the collecting system.  We've got the calyces, the pelvis, and the ureter. Now on top of each of these collecting tubules, the  metanephric mesenchyme forms a cap.

This is going to form the excretory portion of the kidney. The  ureteric bud induces differentiation of this cap. There's formation of a renal or a metanephric  vesicle.

So these vesicles, they become S-shaped, and one portion of this S gets associated with  capillaries. So that's going to form the glomerulus. The rest of the S becomes the renal tubule.

They  lengthen and they form the proximal convoluted tubule, the loop of Henle, and the distal convoluted  tubule. So we finally got a nephron. So the ureteric bud induces the metanephric blastema, and that in  turn induces the ureteric bud.

So the ureteric bud forms the calyces, the renal pelvis, and the ureter.  So everything from the collecting duct downwards. And the metanephric blastema forms the  excretory portion, from the glomerulus to the distal convoluted tubule.

As gestation  continues, the number of nephrons increase, because ultimately the kidneys have millions of  nephrons, remember? So the kidneys have developed in the pelvic region, but the kidneys ultimately are  going to be in the abdomen. And that's because with body growth, they ascend.

So initially their blood  supply is from branches of the abdominal aorta, but finally the renal arteries come  directly off the abdominal aorta. So during foetal life, when the kidneys function,  they form urine. That drains into the cloaca and then into the amniotic fluid.

And that  is how the kidneys develop. Three stages pronephros, mesonephros, and metanephros.  If this video helped you, give it a thumbs up and subscribe to my channel.

Thanks for  watching and I'll see you in the next one!