Hello, were going to start talking about the male reproductive system. Keep in mind that what were going to be focusing on is the hypothalamus pituitary gonad access, axis, so a lot of this is going to be familiar to you from the endocrine system. In this slide we have a view through the testes which is going to be the male gonad that's going to form sperm, and here you can see a section through a seminiferous tubule that has the epithelial cells of the semeniferous tubule's sertoli cells, which we'll be talking about as well as many different types of differentiating sperm cells. And here in the lumen of the tubule, you can see these black, kind of flecks, which are nuclei of forming sperm. Before we start talking specifically about the male system we're going to talk about general principles, and one is sexual differentiation. So how do you become a male or female? And that requires actually several steps, and unfortunately several steps that can go wrong. So when, during embryo genesis, then, germ cells are starting, to form in males and females. Primordial germ cells. Whether we're going to start to form, s, sperm versus egg is going to be determined on the chromosomal sex, which is going to be XX forming a female and XY forming, a male. And on the Y chromosome, the expression of a gene called SRY is going to be, required to form a testes. Once the decision of having a testes versus an ovary is determined based on XX versus XY, then that will be the gonadal sex. So that cannot, ne, sometimes the gonadal sex does not match the chromosomal sex. So for instance, if the SRY gene is mutated on the Y chromosome, then an ovary, will form even though the person has a chromosomal male sex of XY, they can have a gonadal sex that is female. Once you've established which gonads you're going to have, the gonad is going to be the source of hormone which is going to determine your phenotypic sex. Basically what do you appear to be, and so that requires that the gonad is producing the proper hormone and that you have the proper receptors for the hormone in order to have the correct phenotypic sex based on the gonadal sex and then based on the chromosomal sex. So an example of this is that you need the SRY gene so that you can form a testes, but you also need the testosterone receptor in order to appear to be a male. Whether or not you have the test normal testes, you still need to be able to form testosterone and to express the testosterone receptor to appear, as a male. So there's many steps that are required in order to form, properly in terms of gender. We're going to now move to talking about that hypothalamus pituitary gonad axis, and we're going to call it the common axis because it's amazing how similar it is between males and females. Where we're going to have, production of gonadotropin releasing hormone from the hypothalamus. And you know from Dr. [UNKNOWN] that release is going to have to be pulsatile, so that we don't down regulate the GnRH receptors in the pituitary. So we're going to form GnRH in the hypothalamus. That's going to cause production of two different hormones in the pituitary, luteinizing hormone and follicle stimulating hormone. Follicle stimulating hormone, when it's released by the pituitary is going to act on the sertoli cells. The ones that are contacting the sperm. They will ,that will cause them to promo, and will promote the formation of the sperm. Sertoli cells will also form inhibin, which then will negatively regulate FSH production. LH will act on cells that are outside of those seminiferous tubules. Act on cells that aren't in direct contact with sperm, called leydig cells. And it will cause leydig cells to produce testosterone. Which will be required for forming sperm and will also act in a negative feedback manner on the pituitary and the hypothalamus. So, we'll talk about, when we get to the female reproductive system, how similar this is, in the female as well. So Inhibin is going to affect FSH secretion, and then sex hormones, whether your male or female, are going to affect GnRH, LH and FSH production. Let's focus on this left portion of this diagram for right now, the male half of it. Where this previous axis that I've already shown was confusing because the GnRH neurons themselves, do not have receptors for testosterone, estrogen, or progesterone. But yet it was known that testosterone negatively regulated GnRH production. And so somewhat recently it's been shown why that is, or how this system can still work. And it's because there is a second set of neurons that are the ones that actually have the hormone receptors. And these are called kisspeptin neurons because they make a neuron transmitter called kisspeptin. Which then binds GRP54 receptors on the GnRH neurons. So in the male, when testosterone is formed, testosterone binds the kisspeptin neurons and causes a decrease in kisspeptin release, which then causes a decrease in GnRH release by those neurons in the hypothallamus. So the kisspeptin neurons explain why testosterone has its effects even though GnRH neurons do not have testosterone receptors. And we talk about this, this system in the female. This system is also going to be important during puberty. So, kids, express very low levels of sex hormones like testosterone and estrogen. But yet, that small amount of estrogen or testosterone still results in very low GnRH production. And so, this is because at this time, the hypothalamus and presumable the kisspeptin neurons are very sensitive to the sex hormones. However, as the puberty approaches, somehow, and it's not completely understood how, the neurons in the hypothalamus become less and less sensitive to the sex hormones so that, that same level of testosterone will allow more GnRH to be produced. Which will then cause more testosterone to be produced. Until, and that will continue, the hypthalmus becoming less and less sensitive to sex hormones until we get to the adult amount of sensitivity. And so that's what's are going to be, changes that are going to occur during puberty to allow you to have the adult, axis basically of hypothalamous pituitary gonad. So, we've talked, really, only about, cases that are very similar between males and females. Where we're going to have, the formation of germ cells, and the synthesis of sex hormones which are going to happen in the male and female gonad. And we're going to have secretion of GnRH from the hypap, hypothalamus which is going to control FSH and LH from the pituitary. Again, both in males and females, and that, that FSH and LH are going to, be what's responsible for producing the germ cells of the male and female and the sex steroids. And then, the sex steroids will feedback negatively through kris, kisspeptin neurons on FSH and LH. They'll be one exception in the female that we'll talk about. And, so, these sex steroids are going to be required for, for, fertility, for, bein able, being able to form the gonads, but also for appearing as a male or a female for the secondary sexual characteristics.