Back To CourseUExcel Anatomy & Physiology: Study Guide & Test Prep
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Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.
Ah, the ovarian cycle... wait! The ovaries cycle - like a bicycle? No, not that type of cycle - more like the cycle that starts, then ends, then starts again, and then ends again, and it just keeps on going and going, kind of like the sun and moon cycle, or maybe the water cycle. You know, the rain evaporates into the clouds, and then it rains, and the rain goes into the lake, and it evaporates back into the clouds - that type of thing. And just like those cycles have multiple parts or phases to them, so does the ovarian cycle - two main phases, to be exact.
The ovarian cycle is about 28 days on average and is split into two parts:
1. The follicular phase, which occurs before ovulation and is the focus of this lesson, takes up days 1-14, with ovulation occurring on day 14.
2. The luteal phase, which occurs after ovulation of the mature oocyte during days 15-28 and which will be covered in a separate lesson.
We can break the follicular phase down into steps, with each step being characterized by a stage of growth. First up is the primordial follicle stage.
To start, all of our oocytes, before they start to mature, are located in what are called egg nests in the cortex part of the ovary. Egg nests are just groups of immature oocytes, or eggs, kind of like a large nursery in the hospital where babies are kept right after they are born.
Each oocyte is surrounded by a layer of follicle cells that would be like the blanket that a newborn is wrapped in. Together, the oocyte plus the follicle cells makes up our primordial follicle. Each primordial follicle hangs out in the egg nests of the ovary until the female reaches that wonderfully awkward stage of life known as puberty.
Once puberty hits, the monthly round of follicle recruitment begins. Each month a subset of primordial follicles are recruited, or chosen, to begin the pathway of growth and development, all with high hopes of becoming the next mature follicle to be released from the ovary and fertilized, hopes of developing into the next generation, a baby. So what happens next?
Once the oocytes are chosen from the egg nests and have developed into primordial follicles, they move on to the primary follicle stage. And - you guessed it - this stage is characterized by the formation of a primary follicle.
It's pretty simple, actually - just like humans have growth spurts around puberty, so do our follicles, or gametes. When the primordial follicles are recruited, they're activated by local hormones in the ovary and they begin to grow. They grow in both size and in the number of follicle cells that surround each oocyte.
As the follicle cells increase in number, they can be divided into two types. The ones closest to the oocyte are the granulosa cells, and those next to the granulosa cells in between adjacent follicles are the thecal cells. Both the granulosa and the thecal cells work together to produce the female sex hormone estrogen.
At this stage, we also have the appearance of another structure, the zona pellucida. This is the lining, or the border, between the oocyte and the granulosa cells. Its job is to help nourish and protect the developing oocyte.
Now, while most all of our primordial follicles recruited during the first step will develop into primary follicles, not all of our primary follicles will continue on down the path to develop into secondary follicles. It's kind of like trying out for a sports team. Say all the girls in your high school tried out for girls' soccer or lacrosse or girls' basketball. Only a few of them are actually chosen to join the team, right? It's the same idea here.
Only a few of the primary follicles are chosen to continue developing and to move on to the next step, the secondary follicle stage.
Follicles chosen to move on will continue to mature further. The inside of the follicle begins to develop fluid-filled spaces that caused the follicle to enlarge. At this point, we have what is called a secondary follicle. And, as only a few primary follicles were chosen to mature further into secondary follicles, only one of these secondary follicles is usually chosen to move on to the next stage, the tertiary follicle stage. You can think of this follicle kind of like the MVP, or the captain of the team. You usually have only one.
Once the MVP of our oocytes is chosen, it continues to grow in size until it reaches maturity. The tertiary follicle stage is characterized by the presence of a large, fluid-filled space inside the mature follicle, or graafian follicle, which is just another term for a mature follicle. You can tell this stage apart from the secondary stage because the space in the middle of the follicle, the antrum, is larger and better-defined at this stage of development then it is during the secondary follicle stage.
Now, growth isn't the only change at this stage. Remember when I told you that the ovarian cycle and oogenesis are occurring at the same time? Well, up until this point, our oocyte has been paused in one of the stages of oogenesis. It's been paused in prophase of meiosis 1. And if you can remember back to your lessons on meiosis vs. mitosis, you will recall that two cell divisions occur during gamete development. You have the meiotic 1 and then the meiotic 2 division.
When our secondary follicle is recruited as the tertiary follicle (or the MVP), hormone levels in the female start to change. The level of one particular hormone, called luteinizing hormone or LH for short, increases. LH causes the pause button on prophase 1 to switch to play, allowing our oocyte to finish meiosis 1, where it produces a secondary oocyte and a smaller polar body.
While it's the end of the line here for the polar body, which doesn't develop any further, the secondary oocyte enters meiosis 2. It continues through meiosis 2 but pauses again once it hits metaphase of meiosis 2. It will then stay paused here for the rest of the ovarian cycle. And once the cycle is complete, our oocyte won't actually complete meiosis 2 unless it's fertilized by a male's sperm.
Wow! That's a lot for one step of the ovarian cycle! But that's probably because we're almost at the end of the follicular phase, so we're kind of putting the finishing touches on our mature follicle, preparing it for the next big step, graduation - well, technically ovulation, but just like graduation leaves high school behind as you enter college, ovulation is the end of the follicular phase and the start of the luteal phase.
At ovulation, the wall of the mature follicle ruptures, releasing the oocyte. The oocyte then travels from the ovary and into the uterine tubes, down the pathway towards the rest of its life. Okay, so maybe our oocyte doesn't go to college or get married, but it does travel down towards the uterus, and there is the possibility that along the way it will meet up with some sperm, become fertilized and then develop into a baby. So ovulation is a pretty big step in the life of an oocyte. You see, it's leaving the ovarian cycle behind. The rest of our ovarian cycle continues within the ovary but without our key player, the oocyte.
But with the end of the follicular phase of the ovarian cycle, we come to the end of our lesson. I guess you will just have to stay tuned to the other lessons to learn about the second part of the ovarian cycle, the luteal phase.
But before you leave, let's go through a quick summary. Before birth, all of our oocytes in our ovary are arranged into egg nests. Each oocyte is surrounded by a single layer of follicle cells, creating a primordial follicle. Once our female enters puberty, the ovarian cycle is activated, and each month, the ovary recruits a subset of primordial follicles from the egg nests to grow and mature.
First, primordial follicles are recruited and activated to develop into primary follicles. Then, some of the primary follicles are chosen to continue to develop into secondary follicles. At this stage the follicles continue to enlarge and fluid starts to fill the center. Next, one lucky secondary follicle is recruited to become a tertiary follicle. The tertiary follicle grows into a mature follicle, completing meiosis 1 and starting meiosis 2. Once our follicle is matured and our oocyte is paused in metaphase of meiosis 2, it is released from the ovary during ovulation, completing the follicular phase of the ovarian cycle.
At the end of this video, you're going to be able to:
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Back To CourseUExcel Anatomy & Physiology: Study Guide & Test Prep
19 chapters | 230 lessons