>>Dr. Ketchum: So once again we’re going to begin with the ovarian cycle. With the
ovarian cycle, the follicular phase is taking place from menstruation to ovulation, which
is day one to day 14. The luteal phase occurs from days 14 to days 28; this is from ovulation
to right before menstruation. You’re going to need to remember that the ovarian cycle
includes the follicular and the luteal phases. So if you think about this, the abbreviations
are OFL, it’s the ovarian follicular luteal phase, okay? For the ovarian cycle this diagram’s
going to be very important, and you want to have this diagram set aside while we’re
going through the next slide. So dealing with the ovarian cycle, you know we’re working
with the ovary and what’s happening or what’s occurring in the ovary with the development
of the ovum. And so the numbers on this diagram that are inside this ovary, one, two, three,
four, and so on, correspond to the numbers on your next slide. When you look at this
flowchart—there are numbers in the flowchart—those numbers correspond to the numbers
that you see here on this chart as well as the numbers on the previous slide, and that
way you can link this information together.
Since we’re working on the menstrual cycle in general, okay, that’s the big picture.
And we’ve got the two cycles; we’ve got a uterine cycle and we have an ovarian cycle.
So when is all this occurring? When does this start in a female’s life? Think back to
the slide on oogenesis. So in oogenesis there were three phases: there was fetal development,
there was ovulation, and after fertilization. So what you should realize is that we are beginning
here at puberty. So this menstrual cycle is what starts to occur at puberty. So at puberty
are we dealing with a primary oocyte or a secondary oocyte, or an oogonia? What are
we starting with if we are beginning this at puberty? So you should say that we’re
beginning with a primary oocyte. Because remember, in fetal development you have the development
of— where meiosis I begins, but it’s arrested once the primary oocyte develops. And so that
meiotic period that’s arrested starts again at puberty. So this primary oocyte is what
we will begin with at puberty. So let’s go through the ovarian cycle first. So once
again, this includes the follicular and the luteal phases. We’re going to start with
the first phase, which is the follicular phase. So we begin with follicles. Remember, a follicle
contains a developing ovum, so the follicle is going to be in this phase called the primordial
follicle. And that’s listed as number one on your diagram. The whole thing is called
the primordial follicle; the developing ovum inside is called the primary oocyte.
Now the follicular phase on this primordial follicle, or rather the follicular cells,
are located on the outside of the primordial follicle. So these are the follicular cells.
The follicular cells start to develop around this oocyte, and you can see them around that
primary oocyte. Those follicular cells are then going to differentiate into another type
of cell called granulosa cells. Once that primordial follicle has those granulosa cells,
we’re going to change the name of the follicle. The follicle is now going to be called a primary
follicle. So this is what you’re seeing in number two. The oocyte is still a primary
oocyte. Now the stage of development, then, is called a preantral stage of development.
Once we have those granulosa cells, they’re going to secrete what’s called the zona
pellucida. Your granulosa cells will develop into the zona pellucida. Now the zone—zona
pellucida is going to differentiate to form what are called the theca cells. So now we
have these theca cells. Now some of these cells don’t go beyond development any longer.
Now remember this is happening at puberty, so we’re talking about a primary oocyte,
right, per month that’s going through this development. Some of your follicles will continue
developing and form a fluid-filled cavity called the antrum. So this is now called a
secondary follicle, and it’s in ear—also called an early antral follicle or the early
antral stage. Some follicles don’t develop beyond this stage either. When the follicle
is either preantral or early antral, one of them is selected to develop to maturity. The
follicle that’s selected to develop to maturity is called the dominant follicle. Development
continues and now what we have is what’s called a late antral stage or a late antral
follicle, and that antrum—look at how much larger it is now on this diagram. Much larger.
So it’s continued to grow. Some of the granulosa cells of this layer will form the corona radiata.
So that’s sort of this rough looking layer here that surrounds that primary oocyte. Then
there’s also the cumulus oophorus. And now after all this happens we have what’s called
a Graafian follicle. All right, once the Graafian follicle has formed, meiosis I is completed.
Remember in the slide on oogenesis when meiosis I was completed, you were left with a secondary
oocyte. So we’re no longer dealing with the primary oocyte, now we’re dealing with
the secondary oocyte. The secondary oocyte is going to be floating freely in the antrum,
all right? So you can see from this diagram where the oocyte—where the primary oocyte
was. It moves in the antrum, this is now a secondary oocyte that’s floating freely.
Now we’re starting the luteal phase. That’s the last phase of the ovarian cycle. So the
Graafian follicle ruptures. So if we move this up just a little bit here, you can see
how the follicle is rupturing. So here’s our secondary oocyte and here’s what’s
leftover. This is the—the entire thing would be called the follicle. All right, so when
that ruptures, the secondary oocyte enters the uterine tube. Remember it’s always the
secondary oocyte that enters the uterine tube. All right, now what’s left of the follicle—the
rest of the follicle? So that gets transformed into what’s called the corpus luteum. So
that corpus luteum is going to secrete estrogen and progesterone, which we’re going to talk
about more later, but always keep in mind corpus luteum secretes estrogen and progesterone.
If the oocyte is not fertilized, the corpus luteum will degenerate and you’re left with
what’s called the corpus albicans. Because you don’t have the corpus luteum, the levels
of estrogen and progesterone then are going to decrease and then that sets the stage for
the follicular phase to occur all over again in the following month.