Hello, we are going to start talking about the male reproductive system.

Keep in mind that what we are going to be focusing on is

the Hypothalamus-Pituitary-Gonad 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 though a seminiferous tubule

that has the epithelial cells of the seminiferous tubule.

So totally cells, which we'll be talking about as well as many different types of

differentiating sperm cells.

And here and the lumen of the tubule, you can see this

black kind of flex, 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 embryogenesis,

then germ cells are starting to form in males and females,

primordial germ cells whether we're going to start to form

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 deform a testes.

Once the decision of having a testes versus an ovary is determined

base on XX versus XY, then that will be the gonadal sex.

So that cannot, sometimes the gonadal sex does not match the chromosomal sex.

So for the 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 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 re required in order to form properly in terms of gender.

We're going to now move to talking about that hypothalamic–pituitary–gonadal

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. Jakoi, that release is going to have to be pulatile,

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, that will

cause them to promote the formation of the sperm.

Sertoli cell will also form inhibin,

which then will negatively regulate FSH production.

LH will act on cells that are outside of the seminiferous tubules.

Act on cell that aren't indirect contact with sperm called leydig cells and

it will cause leydig cell 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 the female reproductive system how similar

that is in a female, as well.

So inhibin is going to affect FSH secretion and

then sex hormone whether you're male or

female are going to effect 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 neurotransmitter 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 hypothalamus.

So, the kisspeptin neurons explain why testosterone has its effects

even though GNRH neurons do not have testosterone receptors and

we'll talk about 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 result in very low GnRH production.

And so this is because at this time, the hypothalamus and

presumably, 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 and

that will continue the hypothalamus becoming less and

less sensitive to sex hormones until we get to the adult amount of sensitivity.

And so, that's what are going to be changes that are going to occur

during puberty to allow you to have the adult access basically

of hypothalamus 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 hypothalamus, which is going to

control FSH and LH from the pituitary, again, both in males and females.

And that FSH and LH are going to be what's responsible for

reproducing the germ cells of the male and female, and

the sex steroids, and then the sex steroid will feedback

negatively through kisspeptin neurons on FSH, and LH.

There'll be a one exception in the female that we'll talk about.

And so the sex steroids are going to be required for fertility for

being able to form a gonads, but also for appearing as a male or

female for the secondary sexual characteristics.

Hypothalamus-Pituitary-Gonad Axis

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