Let’s do some more embryology. This time – the kidney, which does get pretty complex, so hold on.

We’re starting in the intermediate mesoderm. Along the dorsal body wall, a long elevation is going to emerge called the urogenital ridge. One portion of that ridge is going to make the nephrogenic cord (note the first part of that name) and that is the start of the urinary system. That nephrogenic cord is going to develop into three sets of nephric structures: the pronephros, the mesonephros and the metanephros.

The pronephros is differentiated from the nephrogenic cord and become the pronephric tubules and the pronephric duct. It’s the closest on to the fetal head but it’s also not a big deal. It completely regresses by week 5 and just doesn’t really do anything.

The mesonephros also differentiates inside the nephrogenic cord and becomes mesonephric tubules and the mesonephric duct. The mesonephric duct is also called the Wolffian duct, and you should hold onto that for later. This one comes out of the middle and is a bit more important than the pronephros. It’s a little transitionary and most of it regresses, but mesonephric duct persists and becomes the urogenital sinus.

Then the heavy hitter, the metanephros. It comes from a combination of an outgrowth on the mesonephric duct called the ureteric bud and a condensation of mesoderm inside the nephrogenic cord called the metanephric mesoderm. This starts forming around week 5 and starts working week 10. This, dear reader, is going to be your kidney. Let’s talk about that for awhile.

Okay, so let’s start putting all the parts together. The uureteric bud is going to infiltrate the metanephric mesoderm and start branching. Eventually that is going to form the ureters, renal pelvis, major calyces, minor calyces, and collecting ducts.

The collecting ducts, then, are going to cause the metanephric mesoderm to start changing into metanephric vesicles. Those are going to eventually turn into the renal tubules (they’re S shaped). Those are super important and you can probably guess what they’ll do. Yup, become the whole important line – connecting tubule, distal convoluted tubule, loop of Henle, proximal convoluted tubule and the Bowman’s capsule. Little capillaries called glomeruli are going to jut into the Bowman’s capsule. There is the kidney, complete at birth but it will mature throughout infancy.

Another important thing to note is that the metanephros is located at vertebral levels S1-S2. Where are the adult kidneys? T12-L3. So, they’re going to have to move at some point. So, it’s not so much that the kidney moves as the embryo grows around it at different rates. So, it’s more of a relative ascent, during which the rotate 90 degrees. Originally, the portion of the kidney that has all the vessels going in, known as the hilum, faces anteriorly. As they rotate, they face medially, as we think of them in adults.

Last little note, the blood supply changes as the kidneys move up, too. They sort of just get passed along until the final renal arteries develop at L2. Sometimes those arteries don’t completely disintegrate and we can see them in adult bodies. They’re known as supernumerary arteries.

That’s the kidney! Really, it’s not terrible, but with so many different but similar sounding names, it can be tough to keep track of. Go through it slow once or twice and you’ll be fine. It also helps to learn this at the same time as the reproductive organs, since they’re so intrinsically tied. Maybe we’ll chat about that next week!