Lecture 25 Flashcards
(16 cards)
Briefly explain the process of ovarian follicle maturation and what happens after ovulation occurs.
A group of follicles will be selected to potentially mature.
Once a follicle starts to mature, it will go through growth phases and there are specific cells that are going to help support the growing ovum.
The theca interna and granulosa cells in the follicle wall are going to be producing estradiol and progesterone, which are important for the maturation of the follicle and development of the uterine lining.
When the follicle is mature, ovulation is going to happen and there will be a surge of luteinizing hormone that is going to peak right before ovulation.
After ovulation occurs, the follicle turns into the corpus luteum (responsible for responding to LH to produce estrogen and progesterone) allowing for the uterine lining to build up and become vascularized so that implantation of the fertilized egg into the uterine lining can occur.
In the event fertilization does not occur, the corpus luteum ends up dying and becomes the corpus albicans (fibrotic tissue)
Explain the pathways that allow for estrogen formation as a result of LH and FSH for females.
LH will bind on the theca interna/interstitial cells -> activating Gas -> activation of adenylyl cyclase -> PKA activation. PKA allows for activation of transcription factors -> transcription of StAR mRNA -> translated into StAR protein. StAR protein sits on the mitochondria membrane, gets activated by PKA and allows cholesterol to enter the matrix -> converts to 5-pregnenolone by P450 SCC -> converted to androstenedione in the smooth ER
In the granulosa cells, follicle stimulating hormone (FSH) will bind to FSH receptors and lead to the production of cAMP through activation of adenylyl cyclase. As a result of this increase in cAMP, this will lead to the activation of PKA -> activate aromatase to convert androstenedione into estrogen. The estrogen produced will be able to enter circulation and affect different organs throughout the body.
Explain the structure of the mature ovarian follicle before ovulation occurs. What needs to happen to this follicle, such that the maturation and the actual ovulation event can occur?
The ovarian follicle right before ovulation will contain a large egg (oocyte) which is surrounded by granulosa cells and theca interna cells. These cells surround the egg to nurture and support it, essentially to prepare it for ovulation.
Maturation: The cells that surround the oocyte are going to respond to LH and FSH. As a result of responding to LH and FSH, these cells are going to stimulate the production of androgens which end up becoming estrogen within the follicle.
The estrogen produced will help the maturation process for this follicle through the selection process all the way through ovulation (when estrogen reaches high enough levels, this will trigger an LH surge which triggers ovulation to occur.
* The follicle determines the timing of ovulation, because it’s the one to produce estrogen. If the follicle is too small, it won’t be able to make enough estrogen. When the follicle is big enough, it will be mature enough to allow for the ovulation to occur.*
When does most of the process of follicle maturation occur?
Before the baby is born in the primordial follicles
- each month a handful of the follicles -> early antral follicle -> graafian follicle -> full maturation process occurs to develop the egg for ovulation
How many oocytes do 6 month female fetuses have? How many do they have at birth? What about the onset of ovulation (puberty)?
6 month old females: 7 million oocytes
Birth: 2 million oocytes
Puberty: 400k oocytes
How many ovulations occur in a female life and when does this process begin and end?
300 ovulations on average from the point of puberty (8-13 y/o) to menopause (40-50 y/o)
Explain the process with estrogen, androgens, progesterone, and inhibins A/B triggered to be secreted, starting with the hypothalamus and ending with the secretion of these hormones. Explain any regulation/feedback that occurs.
Stimulation of hormone release will begin in the hypothalamus (under the influence of kisspeptin, secreted from KNDy neurons from the Kiss-1 gene which secrete additional neurotransmitters as well. This process begins when puberty begins.
Kisspeptin stimulates the hypothalamus to produce and secrete gonadotropin releasing hormone -> stimulate anterior pituitary (gonadotropes) -> produce and secrete LH and FSH at different times
FSH affects granulosa cells -> produce estrogen and inhibin B (negative feedback inhibitor for the secretion of FSH). Estrogen production is partnered with theca interna cell production of androgens so androgens produced in the theca interna is a result of LH and sent to the granulosa cells to become estrogens.
LH affects the theca interna cells to produce androgens for the purpose of estrogen synthesis in the granulosa cells and progesterone synthesis.
Estrogen produced by the follicle will act through a positive feedback mechanism to stimulate both GnRH and LH, creating a LH surge right before ovulation, leading to ovulation. When the corpus luteum is formed, it will then be triggered to secrete estrogen, progesterone, and inhibin A as a result of LH stimulation.
The estrogen and progesterone produced by the corpus luteum will act as a negative feedback inhibitor on the hypothalamus and the anterior pituitary
What are the phases of the menstrual cycle? Explain.
1) Follicular phase: ovarian follicle is developed for ovulation
2) Ovulation: oocyte is released from the matured ovarian follicle
3) Luteal phase: ruptured ovarian follicle becomes the corpus luteum
4) Menstruation: endometrial lining sheds if a fertilized egg does not implant
5) DO IT ALL AGAIN
progesterone, estrogen, LH, and FSH will be higher in their respective phases
The hormones that are released during the menstrual cycle do not just affect the ovaries and uterus, but also what?
Breast tissue
- every month, females grow breast tissue which can diminish over time
During the menstrual cycle as well as lactation, how are the secretion of GnRH, FSH, LH, dopamine, and prolactin regulated?
1) Negative feedback loops:
- inhibin A and B are going to inhibit anterior pituitary secretion of FSH.
- Estrogen and progesterone inhibit LH from the anterior pituitary.
- Estrogen, progesterone, LH inhibit GnRH secretion from the hypothalamus.
- Dopamine inhibits prolactin
2) Positive feedback loops (depending on the time of the month):
- estrogen has positive feedback on prolactin
- right before ovulation, estrogen causes a positive feedback loop for GnRH and LH, meaning estrogen produced as a result of LH will increase GnRH and LH concentrations, which will increase estrogen. It is the huge increase in LH that causes ovulation to occur.
- While estrogen inhibits GnRH and LH secretion as well, it’s only before ovulation when estrogen stimulates GnRH and LH secretion.
1) For females, when does the onset of GnRH secretion occur?
2) What hormone allows for the secretion of GnRH in females? Explain.
3) How does this differ from male onset of GnRH secretion?
1) Onset of GnRH secretion occurs when puberty starts
2) Kisspeptin allows for the secretion of GnRH. There are specific neurons which become activated that enable the secretion of kisspeptin. There are additional neurons which secrete additional neurons in order to stimulate GnRH neurons to produce and secrete GnRH.
3) Outcome is the same in both males and females but there are additional neurons that become activated in females during puberty, allowing them to activate GnRH neurons to lead to GnRH production and secretion which lead to FSH and LH secretion. The difference is that FSH and LH which will trigger ovulation and estrogen/progesterone production.
What is the whole purpose of the hormonal events of the menstrual cycle? What happens in the event that the end goal of the menstrual cycle is not successful?
The purpose is to fully mature the ovarian follicle in hopes of pregnancy.
When ovulation occurs, there are a few things that your body also tries to prepare for. Not only is your body trying to get the egg matured and sent into the fallopian tubes, but your body is also trying to make the environment of the uterus and the fallopian tubes ready as well. The reason for this is so that the egg can get transported and that the environment is not too hostile, allowing for both conception and fertilization of the egg to be able to occur.
This allows the fertilized egg (zygote) to be able to divide and invade the uterus and implant itselt.
The uterine lining at this point has to be thick enough for the zygote to attach properly to allow pregnancy to occur. However, in the event that the end goal of the menstrual cycle is not successful, then the body tries again.
Explain what concentrations of hormone are highest during the follicular phase, ovulation, and the luteal phase of the menstrual cycle. What happens after the luteal phase in the event that fertilization of the ovum does not occur?
In the follicular phase, there will be an increase in GnRH secretion as a result of decreased concentrations of estrogen and progesterone, causing increased secretion of both FSH and LH. These 2 hormones work to select/develop several ovarian follicles. The follicles produced will end up secreting their own estrogen and have negative feedback effects. This causes GnRH, LH, FSH to begin leveling off. At this point, estrogen will begin stimulating regrowth of the endometrial lining.
In addition to estrogen, inhibin B secretion will play a role in FSH secretion inhibition as estrogen levels begin to increase.
When the follicles mature enough, they will begin secreting more and more estrogen, such that estrogen levels reach a threshold to result in positive feedback on GnRH, LH, and FSH levels. The surge in LH is important because it induces ovulation to occur so that ovum is released from the ovary into the fallopian tubes.
After ovulation occurs, LH will stimulate the ruptured follicle to form the corpus luteum and secrete progesterone and estrogen. This is called the luteal phase. Increasing levels of progesterone are going to have a negative feedback effect on GnRH, LH, and FSH, causing levels to dwindle.
Inhibin A concentrations will also begin to increase to inhibit further FSH secretion. During the luteal phase, estrogen and progesterone levels will remain high, such that this helps thicken, vascularize, and maintain the endometrial lining in hopes for implantation.
In the event that fertilization of the egg does not occur, the corpus luteum loses stimulation from LH and production of progesterone and estrogen ceases. As a result of this, the endometrial lining sloughs off, allowing GnRH, FSH, and LH to increase once again so that the cycle can continue.
Each month, several ovarian follicles are recruited late during what phase of the last cycle. Only one of them becomes a mature follicle. What happens to the remaining follicles that were selected? Why is it that multiple are selected, such that only one becomes matured?
Recruited late during the luteal phase of the last cycle
Only one gets selected because this allows for extra genetic variation. The rest of the ovarian follicles end up dying.
Why isn’t the endometrial lining kept thick all the time? Why is it that the endometrial lining sheds and rebuilds as a result of low and high estrogen levels?
It’s because it would be too energetically costworthy to maintain the uterine lining at any given point. By cycling through periods of shedding and rebuilding, this allows the endometrial lining to build only in preparation of ovulation, fertilization, and implantation. However, if these events do not occur, then the uterine lining is not needed to be kept thick and well vascularized.
It’s also better to get rid of older cells to prevent the risk of cancer.
Explain the process of fertilization and implantation of the zygote starting with ovulation.
When the ovarian follicles have been fully matured, this will cause a LH surge which allows the ovulation event to occur. If the ovum does end up becoming fertilized into a zygote, it has to travel all the way through the fallopian tubes into the uterus where it will begin cell division. Fertilization can occur at any point while the ovum is traveling through the fallopian tubes. The blastocyst will then implant itself into the thick uterine lining to access the mothers blood supply. Initially, it is the hormones that are produced by the corpus luteum that are going to be responsible for supporting the blastocyst. If the blastocyst implants properly and begins dividing properly, what happens is that the placenta will begin to grow. It is then the placenta will begin to produce the progesterone, estrogen, in addition to hCG which are needed to support the growing embryo.
