Development of Kidney – Urinary System and Kidney Development

by John McLachlan, PhD

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    00:00 These are arranged in a kind of ladder-like step manner down the length of the embryo. They come in various phases which are called the pronephros, the first nephros, the mesonephros, the middle nephros, and then the metanephros, the true kidney at the end. And it’s the metanephros, the true kidney that will persist and be functioning in the child and adult after birth.

    00:26 Pronephros is something which diminishes very rapidly. Indeed, it may not even be present in human embryos at all, but is present in some of our ancestors. The mesonephros is consists of the mesonephric duct, which runs down the length of the body. Now as you can see in the middle diagram, it will insert into the cloaca. Now, this is the hindgut region.

    00:49 So it’s inserting into the cloaca, and cloaca, incidentally, is the Latin word for a sewer.

    00:55 So you can see it’s a relatively appropriate kind of term. There are a number of divisions, and number of these tubules, as I say in a ladder-like manner, running down the body.

    01:05 Down towards the tail end close to the cloaca, we can see what’s called the ureteric bud.

    01:11 This is a bud that grows out from the mesonephric duct into the surrounding mesoderm. As we’ll see, what happens is that the surrounding mesoderm begins to condense roundabout them.

    01:23 If we look at a transverse section through the mesonephros, you can see that it has now become really quite complicated, and we can see the mesonephric duct that’s running down the body, of course, to insert into the cloaca at the hind end. Then now the tubules become extremely complicated. It’s still making a close association with blood vessels which have come from the aorta. Now, the mesonephros never produces true urine. It is handling fluid, but of course, all the waste products in the embryo are actually handled by the mother via the placenta. Now we mentioned ureteric bud and here we’re looking at it in a vision of the embryo seen from the side. We can see that ureteric bud developing down towards the tail end of the embryo. Incidentally, note how far down the ureteric bud is that kidneys in the adult are not in this relative position, and therefore, there will have to be a shift in relative position of the kidneys between the time of their first development and the time when the baby is born. So, the ureteric bud grows out, and it is from a mesodermal structure, but other mesoderm begins to condense roundabout them. Looking at this in a slightly larger view, you can see that we’ve sketched in the mesoderm condensing around the origin of the ureteric bud. The bud itself and the surrounding mesoderm will give rise to different components of the kidney, which are readily recognizable in sections through the adult kidney.

    02:59 Gradually, although initially, the ureteric bud is coming off from the mesonephric duct and the mesonephric duct is inserting into the cloaca as we’ll see shortly, the mesonephric duct will separate from the developing ureter, the connection from the ureteric bud, and they will have independent origins into the bladder and the lower part of the urethra respectively. In this slide in a slightly larger view, we can again see the difference in the insertions between the mesonephric duct and the definitive ureter of the ureteric bud as it forms the metanephric kidney. So initially, the ureteric bud is coming off from the mesonephric duct, and inserting jointly into the urogenital sinus but subsequently, they will insert separately to the mesonephric duct in males, will insert into the urethra, and the ureteric bud as ureter will insert into what will be the urinary bladder.

    04:00 Here we can see the differentiation of the metanephros of the true kidney and what we’ve indicated here is that different parts of the kidney will come from these two different sources.

    04:11 So the original ureteric bud will give rise to the ureter, it will give rise to the renal pelvis, to the major and minor calyces of the kidney, and also to the collecting ducts.

    04:23 The other parts of the kidney will come from the mesoderm which condensed around these structures. Here’s the same image again, so you can see the entire sequence of development from the original formation of the ureteric bud through the collection of the mesonephric mesoderm roundabout it, and to finally the final adult form of the kidney as represented in the final diagram. Now, there are a number of prenatal changes in kidney position, as I mentioned. Initially, the kidneys are far down the body towards the tail, they are pelvic.

    04:56 It often describes that they are going to shift cranially, that’s towards the head towards the adult position, and moving up by about week nine after fertilization.

    05:07 During this shift, which is often described as the ascent of the kidneys, obviously, their blood vessel supply will have to change. Initially, they’re being supplied by arteries which normally supply the legs, and these are the common iliac arteries. But as they shift their relative position, they begin to attract blood vessels from other nearby blood vessels, and till finally, normally, they’re supplied by branches from the aorta. Now, this process shows lots of variation. It’s quite possible that a kidney might have more than one major blood vessel supplying it. Of course, that in turn would be important for our surgeons operating on the kidneys subsequently if they are coming across an unexpected major artery that would pose a serious challenge. Although this process is described as a shift, you must not imagine that the kidneys are zooming up through the body. Really what’s happening is that there’s relative growth going on, and they’re shifting their position in accordance with this relative growth. If you imagine the curved embryo, the back grows faster than the inner part of the curve, and therefore, structures which are closer to the center tend to shift relatively as the back grows away from them although it is not a radical migration of structures through the body in the way that you might imagine. So here, this image is shown again in a larger scale. You can see that the kidney is shifting, and in particular, note the changes in blood vessel supply that are present as a result of these shifts in kidney position.

    About the Lecture

    The lecture Development of Kidney – Urinary System and Kidney Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Development of Kidney – Urinary System and Kidney Development

     John McLachlan, PhD

    John McLachlan, PhD

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