Development of the Kidneys

Back to ANHB2212 stuff!

Recognise the pattern of kidney development as a reflection of vertebrate development

This is basically the whole "ontogeny recapitulates phylogeny" thing that I mentioned a long time ago (and is a very useful idea for understanding how organs develop).

The intermediate mesoderm, which develops into kidneys, is segmented into divisions called "nephrotomes." These nephrotomes become three different types of kidneys. In the neck, they become the pronephros, in the thorax and abdomen they become the mesonephros and in the pelvis they become the metanephros. Ultimately only the metanephros becomes the kidneys in the human, but the other two can be found fully functioning in some other species.

Pronephros

The first "kidney," the pronephros, is never functional in humans and degenerates by day 26. However, it's important in freshwater fish. Fresh water is more dilute than the body fluids, so there is a risk of too much fluid moving into the body via osmosis. Hence, the pronephros has to keep removing all of that excess water, while conserving salts.

The pronephros forms when a cavity develops in the intermediate mesoderm, and balls of blood vessels from the aorta (or aortas, as the embryo starts with two aortas that eventually fuse) start bulging into this space. These balls of blood vessels are the glomeruli of the pronephros. Initially water filters through directly into the coelom, but afterwards the intermediate mesoderm cavity joins up with other segments to form a pronephric duct through which water can drain.

Mesonephros

The mesonephros is pretty much the opposite of the pronephros. Instead of getting rid of as much water as possible and conserving the salts, the mesonephros' job is to conserve the water and get rid of the salt. Many saltwater fish have a mesonephros.

The mesonephros has a smaller glomerulus than the pronephros, in order to produce less filtrate. It also has a larger system of tubules to ensure that more of the fluid gets reabsorbed. The tubules connect to a mesonephric duct, which drains into the cloaca (a cavity at the end of the digestive tract for excretory and genital products).

In humans, the mesonephros appears in week 4 and produces urine (which at this stage isn't so much to get rid of waste but to increase the amniotic fluid). The thoracic segments begin to regress at week 5, but the remaining segments retain their function until around week 12. The mesonephric duct and paramesonephric (para = next to) ducts eventually go on to form parts of the reproductive system- I'll go into detail later. (They did also get a brief mention in one of my PHYL2001 posts, if you're impatient.)

Metanephros

The metanephros is what actually becomes our kidneys.

Development of the metanephros actually starts with the mesonephros. The mesonephros forms ureteric buds which spout from the mesonephric ducts. These ureteric buds induce the formation of the metanephros from surrounding intermediate mesoderm. The ureteric buds also bifurcate repeatedly, forming the calyces and connecting ducts of the kidneys.

The kidneys begin to function around week 12, around the time when the mesonephros stops functioning. Once again, the urine produced here doesn't get rid of waste- it increases the volume of amniotic fluid, which the fetus drinks. (Yeah, hate to break it to you but you used to drink your own piss.)

Understand the fate of the mesonephric duct in males and paramesonephric duct in females

Primitive vertebrates would release their eggs and sperm straight into the coelom before entering the cloaca through small pores. In humans and other higher vertebrates, eggs are still released into the coelom, but are picked up by the paramesonephric ducts. Sperm are a bit different though, as the testes develop direct connections with the mesonephric ducts.

Initially, both males and females have both ducts. In the female, however, the mesonephric duct degenerates, and the two paramesonephric ducts become the fallopian tubes, fusing distally to become the uterus and vagina. (Failure to fuse results in a bicornuate uterus, which is basically like two mini uteruses stuck together.) In the male, the opposite happens: the paramesonephric duct degenerates, and the mesonephric duct forms the epididymis, vas deferens and seminal vesicles. For more information on how hormones cause all of these changes, take a look at my PHYL2001 post.

The gonads originally start in the upper abdomen, but they don't stay there. They are connected to the labioscrotal fold by the gubernaculum, which grows much more slowly than the rest of the fetus, causing the gonads to be dragged down. In males, this pulls the testes into the scrotum, and in females, this pulls the ovaries down next to the uterus.

While we're talking about things moving around, let's talk about the kidneys moving around! The kidneys begin to ascend around week 6, though sometimes one or both kidneys may fail to ascend, resulting in a pelvic kidney. As the kidneys ascend, the metanephric ducts elongate to become ureters. As the kidney ascends, new segmental arteries are gained from the aorta and arteries lower down are lost, so the kidney appears to "climb a ladder." Hence, sometimes vestigial arteries are found on the kidneys.

Another thing that can happen is that the kidneys can fuse together, forming a "horseshoe kidney." Horseshoe kidneys can't ascend past the inferior mesenteric artery- they get stuck.

Normal kidneys eventually reach their final position with the tops of the kidneys around T11/12 (right kidney is lower because of the liver) and rest on the psoas muscle. The diaphragm is behind the upper half of the kidneys, and so the kidneys move with breathing. As for the arteries and all that, the renal vein is most anterior, followed by the renal artery and then the ureters. The left renal vein is longer as it has to cross the aorta.

And that's pretty much it for that topic! Next we'll be talking about the pelvis, which is probably my weakest topic in this unit >_>