Welcome back. We're going to talk about more details in how we form sperm, and how we form sex hormones in males. Remember, we said that the testis is going to be the the male gonad. It's going to be found in the testicle. That prgan has two main parts, not only the testis, but the also the epididymis, which is where sperm are stored and where they continue their maturation. In the testis, we have hundreds of seminiferous tubules. These tubes that run in parallel and are coiled. That's what you see filling this testis, these seminiferous tubules. So spermatogenesis is going to happen, the formation of sperm is going to happen, in these tubules. Keep in mind as well that this process is going to be able to happen basically throughout the lifetime of a male once they've reached puberty. Males can remain fertile basically their whole adult lives. If we take a section, if we cut through one of these seminiferous tubules, then we will see, as is on the title slide, these layers of forming sperm. The main cell type that you see in the seminiferous tubule are actually forming sperm. However, mixed in there and surrounding the sperm are Sertoli cells. That's these pink cells. They're the ones in contact with the germ cells. They're the ones responding to FSH, follicle stimulating hormone. They are going to not only be supporting the forming sperm, but also making a protein called androgen binding protein. Which, as its name suggests, helps to bind testosterone to keep it in the area of the sperm. This testosterone helps the sperm mature. The Sertoli cells are also going to convert testosterone to estrogen. This is going to be important in the formation of sperm. Outside of the tubule, we have Leydig cells. These cells are the ones that are respond to luteinizing hormone and produce testosterone. They're also going to be secreting a hormone called DHT, dihydrotestosterone, a much more potent androgen than testosterone. These cells can make DHT because they have an enzyme called 5-alpha-reductase. DHT is very important for the secondary male characteristics such as hair growth and things like muscle mass. DHT will also be important for the prostate. Testosterone is very important for forming the sperm. We need FSH to initiate sperm differentiation, to start the process. We also need testosterone, as well as estrogen, to properly form sperm. You can think about it as the fact that fertilizing an egg is not a very efficient process in terms of the number of sperm that are needed. In normal semen, there's going to be 200 to 400 million sperm per 5 ml. That number will depend on the proliferation rate of the sperm as well as the frequency of ejaculation. Frequent ejaculation will result in lower sperm counts. The number of sperm per ml that determines if a male is infertile is 20 million sperm per ml semen. Once you have less than 20 million sperm per ml of semen, then you're considered infertile. This is, because it takes many, many sperm to be able to fertilize an egg. It's such an inefficient process. Another thing to keep in mind is that the maturation of the sperm takes 90 days. That includes the time when they are sitting in the epididymis, the tubes downstream of the testes, as the sperm mature. They won't even be completely mature until they enter the female reproductive tract. More specifically, enter into the oviduct or the fallopian tube, where they will contact the cells of the oviduct and undergo a process called capacitation. This process makes the sperm able to fertilize the egg. This is in contrast to the female, where it's production of the ovulated egg is so inefficient. In the male case, the sperm production is efficient. It's the fertilization of the egg that requires so many sperm. Then only one is going to end up fertilizing the egg. Let's switch gears a little bit and talk about androgens. We've talked about testosterone which is important for spermatogenesis. It's also going to be important for creating a male during embryonic development, and then also important for feeding back to those kisspeptin neurons. But DHT is going to be important for external virilization, hair growth, things like that that are part of the secondary sexual characteristics. Keep in mind that these are steroids. They're going to have a carrier in the blood. They're going to act to change gene expression in their target cells. They'll be able to diffuse right through the membrane of the cell. Androgens are going to have anabolic actions, meaning that they're going to build structures, in this case, muscle mass. Which is why men have an increased muscle mass compared to women. If we add extra testosterone, that means we're going to reduce GnRH production, which will then cause a decrease in FSH and LH, a decrease in sperm production, which will decrease the size of the testes, because of a decreased sperm count. But the excess testosterone will increase the size of the prostate. The prostate is an androgen-responsive tissue, as we'll see in a minute. So adding excess testosterone can be problematic. It's not a widely used form of birth control, but it will decrease sperm production. However, that increased testosterone will act on androgen-responsive tissues. Keep in mind, because sperm production takes 90 days, that the effect of using a birth control method that affects the production of sperm is not going to be effective until several months into the course of administration of the compound. So we're going to talk just briefly about androgen dependent tissues. One is the seminal vesicles, which are glands that provide a fructose rich liquid that ends up being a large portion of the volume of semen. The prostate gland is also going to contribute an alkaline secretion that's going to be about 20% of semen volume. It's going to be responsive to androgen, such as DHT. Then we're going to have lots of androgen-dependent tissues that are responsible for the secondary sex characteristics. Muscle is androgen-dependent. Facial hair and a deep voice are all androgen-dependent processes. Keep in mind, we're not going to talk about it extensively, but the fact that estrogen is also very important in the male. It occur in tissues that express an enzyme called aromatase, which converts testosterone to estrogen. Bone is one of those tissues that expresses aromatase. It will convert testosterone to estrogen. Estrogen and testosterone are going to be needed to cause the growth of the skeleton, growth of the bones, make you taller. However, it's estrogen specifically that is needed to cause the epiphyseal growth plate to close. Which happens at the end of puberty when you stop growing. This plate will fuse at the end of the bones. , This prevents them from getting any longer. That requires estrogen specifically. So males with an estrogen receptor inactivation, meaning they can't respond to estrogen, or an aromatase inactivation, meaning they can't make estrogen to begin with, they can have growth of the skeleton, because you can use testosterone to make the skeleton bigger. But they can't stop the growth of their skeleton, because they have no estrogen. So they continue to grow throughout their adult lives without treatment because of the lack of estrogen and its role in stopping growth. This is the conclusion of the male system. We've talked about how we're going to make testosterone right next to the spot where we're also forming sperm. How it's going to be important for differentiation of the sperm or formation of the sperm. That we're also going to produce DHT, dihydrotestosterone, which is going to be important for looking like a male. And that we also need estrogen. The example we discussed was in bone, in closing those epiphyseal growth plates.