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- Let's meet testosterone. We produce chemical signals in our bodies that allow one part of our body to communicate with other parts of our bodies, and these signals are called hormones. Testosterone is one of these hormones. So testosterone is produced by our testes here. So let me draw a zoomed in view of one testis. And after the testes make testosterone in cells that they have called Leydig cells, that testosterone is transported to other parts of the body via the bloodstream. So I'm drawing red blood cells in red, and I'll show you the little molecules of testosterone in green. And these blood vessels carry testosterone all over the body so that it can carry out all of its biological functions. So let me just point out that testosterone isn't only produced in men. It's actually produced in men and women, but males past the age of puberty, about 12 or 13 years old, have about seven or eight times the testosterone that women of the same age as them do. So we still refer to it as the primary male sex hormone. And to clarify, I'm just calling it a sex hormone because it's produced primarily by some of the male sexual organs, i.e., the testes. So I've told you that it's made in the testes and that it travels around in the blood vessels to every part of the body, but once it gets to its target, what exactly happens? How does it work? So to show you this, I'll blow up a little cell here in the thigh. So here you have a cell, and up here, or lining the cell or just beside the cell, you have that bloodstream that the testosterone is floating around in. And so I'll draw back in those red blood cells, and let's just say the blood is going that way, and here's our testosterone. So when testosterone sees a cell that it wants to enter, it crosses through the cell membrane, which is sort of the cell's barrier to things outside the cell, almost like a selectively permeable gate, and once the testosterone gets inside the cell, it sort of meets this carrier protein, and the carrier protein is something that will bind the testosterone and take it to its next destination within the cell. So we'll draw that carrier protein as a taxi. We have testosterone sitting right here in the backseat of the taxi, and then that carrier protein just drives it a little short way to something called the nucleus. And the nucleus contains our DNA, and those are the blueprints of how we're made. So in red I drew a specific part of our DNA called a gene, and our DNA is made up of millions of genes. So genes are little segments of the DNA, and some hormones interact with our genes to change our characteristics. So that's exactly what testosterone does. It jumps out of the cab, once it gets into the nucleus, and it sort of binds onto that gene. I'll just label that that's a gene. And when testosterone binds the gene, it's able to influence its function. So, for example, if the gene told the body to make more muscle, then testosterone interacting with that gene would increase the rate of muscle production, and, in fact, that is one of testosterone's functions, and we'll touch on that a little bit later. Now, one more note on this topic. Depending on the type of cell that testosterone enters, it can be converted into a different hormone before going to the nucleus to interact with a gene. So the two possibilities are dihydrotestosterone or DHT and estrogen, and that may sound a little counterintuitive, because I think most people think of estrogen as a female-only hormone, but actually males need estrogen as well, and we have a little bit of it, and it comes from testosterone. And even in the female, testosterone is converted to estrogen, and that's how estrogen is made. It just happens a lot more often in the female. Having said that, in men only about 7% of testosterone is converted to DHT, and less than 1% is converted to estrogen. So it's primarily testosterone that's exerting all of the hormonal effects. You're probably wondering what some of the roles of testosterone are. So let's cover those. So testosterone actually starts working when you're really young, when you're still in development in your mother's womb, and what it does there is it induces your reproductive organs to differentiate into male reproductive organs. Because when males and females start out in development in the womb, they have the same precursor reproductive structures, and so the presence of testosterone actually pushes the reproductive organs to turn into masculine ones. Another really important thing testosterone does is, you remember how we said that testosterone is made in the Leydig cells here, and the Leydig cells are one of the cells in the testes? When testosterone starts to be released in higher quantities from those Leydig cells, once you hit puberty around age 12 or 13, that testosterone signals other cells in the testes to start the process of making sperm, a process called spermatogenesis. After sperm production starts, having a baseline level of testosterone in the testes keeps sperm production going. So these are the major functions that testosterone has on the male reproductive tract. But testosterone actually has a lot of other functions around the body. It's responsible for what we call secondary sex characteristics, and these are just physical traits that we typically think of being masculine or feminine. For example, it stimulates the growth of facial hair, armpit hair, pubic hair, and hair on your arms and legs and your chest, just to name a few areas. It's also responsible for the deepening of your voice that happens as you develop as a male, and that happens because testosterone induces growth of the voice box or the larynx, and some people know that as the Adam's apple. That's why you see a prominent sort of bulge in the throats of males. Another thing it does is it induces male-pattern fat distribution. And so, that's sort of fat distribution around the central part of the body. It also has some structural affects on our bodies. So it stimulates the process of anabolism, and what that means is taking smaller components in our bodies, like proteins, and building them up to bigger components or aggregates of those components, like muscles. So muscles are basically a big aggregate of proteins. That e.g. just means for example. Testosterone also stimulates bone growth. So it'll make your bones bigger in diameter and longer, but it also stimulates the termination of bone growth once your bones can't grow anymore. So those are the major secondary sex characteristics that testosterone stimulate. It affects your behavior, increasing your sex drive, and has been thought to possibly cause higher levels of aggression. Testosterone can increase the number of red blood cells that we have, and it does that by stimulating our kidneys to produce another type of hormone called erythropoietin or EPO, and EPO's role is to cause the creation of more red blood cells. So that all sounds pretty awesome, right? Well, except for maybe the possible link to aggression. But it turns out there's a limit to the testosterone that you want floating around in your blood at any given time. If you have too much, bad things can start to happen. It's been theorized that the prostate has cells that are stimulated by DHT or dihdryotestosterone. Do you remember that metabolite of testosterone we talked about earlier? And so, the idea is that they grow bigger when there's too much DHT around and increases the risk of developing prostate cancer. The jury's still out on that one, though, but one thing we do know is that male-pattern baldness, that is, baldness on the top of the scalp, is actually promoted by excess DHT in the blood. So this tells us that we don't want too little or too much testosterone traveling around in our blood and affecting our cells. So, how do we control how much testosterone is in our body? Well, it turns out that testosterone, to a large extent, self-regulates how much is produced. But how can it do that? It actually does that through something called a feedback loop, and this concept of the feedback loop is by bar how most hormones work. So again, a feedback loop is a method of self-regulation. So an example of a feedback loop would be something like a thermostat. Let's say you set the temperature in your room to a nice 25 degrees Celsius. So let's say it's 20 degrees in the room now. The thermostat will sense the temperature of the current air in the room and heat the room another five degrees, until it gets to 25 degrees. And once it's sensed that 25 degrees has been reached, it will turn off the thermostat. And testosterone in the body's regulated in a similar way. There's a part of the brain called the hypothalamus that acts similar to that thermostat. It's actually sometimes referred to as the body's thermostat, because it regulates most of our feedback loops, and the testosterone feedback loop is one of them. So the hypothalamus will sense the amount of testosterone that's floating through your blood. If it's not enough testosterone, the hypothalamus will send a signal to another gland in the brain, called the anterior pituitary, and that actually just sits right underneath the hypothalamus, and then the anterior pituitary sends more hormones to the testes that increases their testosterone production. Conversely, if there's too much testosterone in the blood, the hypothalamus will sense that, and it will stop sending signals to the anterior pituitary, and then the anterior pituitary will stop sending signals to the testes. And so, when that happens, you actually get a decrease in testosterone. So overall, this negative feedback loop is used to control the amount of testosterone in the blood, and that control of blood testosterone levels is actually referred to as testosterone homeostasis, homeostasis meaning to remain constant. And that's testosterone.