Urothelium and Urethra

00:00 Let's briefly now look at the ureter, shown here on the right hand side. It's a muscular tube. It passes the urine down to the bladder. It's lined by what we call a transitional epithelium or a urothelium.

00:18 And this persist this type of epithelium all the way down through the bladder, and to some degree, through the urethra. It's called transitional epithelium because it changes its appearance. When the bladder and the ureter is distended during flow of the urine, the epithelium changes its shape. And because of that, it's called transitional, and I'll describe that in a moment. But it's just a muscular tube. Here is a section of the bladder on the right-hand side. Have a look at the diagram of the male bladder indicated here. It's a bag for storing urine.

00:59 And on the right-hand side, you see sections through the mucosa of the bladder when the bladder is empty, and the mucosa and the epithelial cells, in particular, are very thick. It's a very thick stratified layer. On the left-hand side is this epithelium of the bladder, and it's typical of the epithelium of the ureter as well, stratified epithelium. Notice on the surface of these cells, there are some eosinophilic stains. These are plaques.

01:33 And what you see here on the right-hand side is the bladder when it's distended, and the surface cells flattened out from the more cuboidal shape that you see at the surface on the left-hand side in more a relaxed bladder. Hence the name transitional epithelium, but also gets the name urothelium. Those plaques are very, very important, because what they do is they prevent water and salts going back, out of the bladder back into the body. They form a seal and they're impermeable, which is very important when you're storing rather toxic substances and components of the blood you feel that you don't want in the body. You want to eliminate in the urine. And finally, the male and female urethra. In the female, it's short. The urothelium persists for a while and then it changes to a non-keratinized stratified squamous epithelium. In the male, the urethra is long because there are three different parts, the prostatic urethra, the membranous urethra, and the penile urethra. There's a little star next to the urethra, the membranous urethra in the male because that's where urothelium changes from typical transitional epithelium I've described to stratified squamous epithelium throughout the penis and then beyond towards the skin. Sometimes in the urethras you find mucus secreting glands, which are probably there just to protect the epithelium from perhaps an acidic urine.

03:21 So let's just summarize now what the functions of these components are.

03:28 The kidney has a very important structure of the nephron, which consists of the glomerulus filtration apparatus. And all the tubules that you will learn in physiology have a very important role in filtering our blood, getting a filtrate plasma based solution, and it then gets rid of products that we don't want to retain, or excessive products, and passes it into the urine. The ureter conveys that urine to the bladder, the bladder stores that urine and make sure things don't leak back into the system, into the body. And the urethra is the tube whereby we eliminate urine from the bladder in both the male and the female. So I hope you now understand histology of the major organs of the urinary system. And thank you very much for listening to this lecture.