Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today on the ovarian and menstrual cycle.
Today we are going to be talking about both the ovarian and menstrual cycle, what happens in each of these cycles, and the different hormones that are involved.
We're going to start by talking about the menstrual cycle first. The menstrual cycle is the cycle where oocytes are ovulated and the endometrium of the uterus prepares to receive a fertilized egg. One menstrual cycle is equal to about 28 days. So the cycle is a cycle of about 28 days. Basically, what's happening in this cycle is that the uterus is preparing for a fertilized egg. During these 28 days, the uterus is getting ready. It's prepping itself to receive a fertilized egg. If the oocyte that is ovulated is not fertilized, or if the woman, in other words, does not become pregnant during this cycle, the endometrium that's been prepared-- and the endometrium is just the lining of the uterus-- so this endometrium that's been prepared to receive a fertilized egg is then going to flow out of the uterus through the vagina. And we call this process menstruation.
So during the cycle the endometrium is prepping for a fertilized egg. If the egg does not become fertilized, the endometrium will then flow out of the vagina in a process called menstruation. And then what's going to happen is that the cycle will begin over again. So over those 28 days, the endometrium will start to get rebuilt, get ready to receive another egg, and then if the egg is fertilized, the woman becomes pregnant, then it's all great. If it doesn't, then menstruation occurs again and the cycle repeats again. So that's the cycle of menstruation.
Progesterone and estrogen are the hormones that prep the endometrium for pregnancy. So these are two important hormones in the menstrual cycle. Progesterone and estrogen. Again, help to prep that endometrium.
Menarche is a woman's first menstruation, and menopause marks the end of a woman's fertility. So we have monarchy marking a woman's first menstruation, and then a woman will menstruate through her life until menopause occurs, and then she is no longer able to conceive children, and marks the end of her fertility. So this usually happens around age 15, but it can vary quite a bit from person to person. And then menopause usually occurs sometime around age 50.
In this part of the lesson, we're going to talk about the ovarian cycle. The ovarian cycle of the cycle in which the primary oocyte will mature in the ovary and then be ovulated. We're going to talk about what happens during this process, as a primary oocyte matures and is then ovulated from the ovary.
We're going to start on our diagram by looking at this portion right here. This right here is what we call the follicle. The follicle is basically the primary oocyte, surrounded by this layer of protective cells, which also provides nourishment to that follicle. So at this point meiosis I has been started, but then it is arrested or stopped. So we've had meiosis I that started, but at this point, it is stopped. As this follicle starts to mature, as the oocyte starts to mature, and move through these phases before it is ovulated, it will complete meiosis I. But this point, meiosis 1 has been stopped.
A couple of hormones that are important in this cycle are FHS, or follicle stimulating hormone. Follicle stimulating hormone is released, and what that does is it allows the follicle to grow. GNRH, gonadotropic releasing hormone, plays a role in the production and the release follicle stimulating hormone. So that's its role with that. Follicle stimulating hormone will also increase the expression of luteinizing hormone receptors. So luteinizing hormone and follicle stimulating hormone have this permissive relationship with one another, because follicle stimulating hormone increases the expression of LH hormone receptors.
And LH hormone, in addition to FSH, is released by the anterior lobe of your pituitary gland. And LH regulates the menstrual cycle and egg production. So that's the role of this hormone, is that regulates the menstrual cycle and egg production, and can actually be detected in the urine. So if you're taking a fertility test, for example, it's testing for LH. As the egg gets closer to ovulation, there's a surge in LH levels, and that can be detected by at-home fertility tests.
As the follicle matures, it will continue to go, as I said through, meiosis I, and by the end will complete meiosis I. The zona pellucida will start to form around the oocyte site, which is this thick layer, and then we'll get this buildup of estrogen-rich fluids, and then basically, we're ready for the egg to be ovulated.
What's going to happen during ovulation is that the egg will be released from the ovary. And what's being released, basically, is the polar body here, and the secondary oocyte. So the primary oocyte has gone through and completed meiosis I, producing the secondary oocyte and a polar body.
If this egg is then fertilized, so from the ovary it's going to move up into the fallopian tubes, and if it becomes fertilized, it will then began meiosis II. If it does not become fertilized, it will not begin meiosis II. So meiosis II starting depends on whether or not this egg is fertilized.
The polar body basically, is just this unnecessary material that will later disintegrate, and it's not needed. So this will disintegrate, the secondary oocyte moves towards the fallopian tubes, and then, in that location is either fertilized or not.
Then back here in the ovary, we have our corpus luteum, which forms from remnants of that ruptured follicle. This corpus luteum, basically what it's going to do is it's going to secrete estrogen and progesterone, and that helps prep the uterus for a fertilized egg. So it's helping that uterus prepare to receive this fertilized egg. If the egg is not fertilized, this corpus luteum will start to break down. And when it starts to break down, progesterone and estrogen levels are going to drop.
So as it starts to break down because the egg does not become fertilized, these two hormone levels will drop, and as those two hormone levels drop, it causes the endometrium, or that lining of the uterus, to break down, which then leads to menstruation. So you can kind of see how these two cycles dovetail with each other, and they play a role with one another, the ovarian and the menstrual cycle together.
This lesson has been an overview on the processes that occur during the ovarian and menstrual cycles.
Formed from the remnants of the ruptured follicle which secretes hormones that prepare the endometrium for an embryo.
A female sex hormone that plays various roles in the female reproductive system, including preparing the endometrium for pregnancy.
Follicle Stimulating Hormone; a major hormone of the reproductive system which stimulates ovulation.
The primary oocyte surrounded by nourishing cells in the ovary.
Lutenizing hormone; A hormone released from the anterior pituitary gland and plays a large role in the reproductive system.
A woman's first menstrual cycle.
Marks the end of a woman's fertility in which a woman is no longer able to conceive.
Monthly cycle in which an oocyte matures and is released from a woman's ovary.
The flow of the endometrium out the vagina if the oocyte was not fertilized during the menstrual cycle.
The cycle in which an oocyte matures as the menstrual cycle advances.
The process of expelling the secondary oocyte from the ovary to the oviduct.
A hormone released during the menstrual cycle that prepares the endometrium for implantation.
The oocyte formed after meiosis I has been completed in the ovary.
A protein layer that surrounds the follicle.