The menstrual cycle Flashcards
The menstrual cycle
Occurs roughly every 28 days.
Includes events in the ovaries (ovarian cycle) and the uterus (uterine cycle).
The Ovarian Cycle
Follicular Phase: Lasts about 13 days. Primary follicle matures and secretes estrogen. Under control of FSH.
Ovulatory Phase: Day 14. A secondary oocyte (note this is only after Meiosis I) is released from the ovary, as a result of increased LH from anterior pituitary. (ie. Ovulation)
Luteal Phase: Lasts about 14 days. Full formation of corpus luteum (exists for about 2 weeks) in the ovary that secretes estrogen and progesterone.
Uterine Cycle
Note that the hormones released during the ovarian cycle direct the procedure in the uterus.
Menstruation: Lasts about 5 days. Result of degradation of corpus luteum annd drop in estrogen and progesterone. Causes sloughing of the endometrial lining.
Proliferative Phase: Lasts 9 days. Estrogen from the follicle drives proliferation of the endometrium. Ovulation occurs here.
Secretory Phase: Lasts about 14 days (occurs after ovulation), estrogen and progesterone further develop the endometrium. Includes the secretion of glycogen, lipids and some other materials. If no pregnancy, menstruation will be triggered again. Secondary oocyte is traveling down the uterine tube at this point.
Hormone cascade in the menstrual cycle
GnRH from hypothalamus > LH and FSH release from Pituitary
FSH Stimulates granulosa cells and thecal cells to develop during follicular phase and secrete estrogen. Estrogen initially inhibits GnRH, LH, and FSH, but when it peaks at the end of the phase it has a positive effect on LH.
This positive effect on LH, causes a surge in LH that stimulates ovulation and then causes the follicle to become the corpus luteum.
When there is no pregnancy, estrogen and progesterone act to heavily inhibit GnRH, FSH, LH. When LH secretion drops it causes degradation of corpus luteum and leads to menstruation.
If estrogen and progesterone levels are kept artificially high what is the consequence?
There would be no ovulation.
If these artificially high levels are suddenly removed it results in menstruation.
Endometrial shedding
Result of drop in estrogen and progesterone
Why do estrogen and progesterone drop?
Corpus luteum degenerates
Why does the corpus luteum degenerate?
Drop in LH
Why is there a drop in LH?
Inhibitory effect of estrogen and progesterone.
Ovulation should be inhibited when…
Pregnancy occurs.
This is done by maintaining high estrogen and progesterone levels, that inhibit LH and therefore prevent ovulation.
Note though that the corpus luteum only degenerates if fertilization has not occured.
How does pregnancy proceed if LH hormone cannot remain high, but LH is necessary for the maintenance of the Corpus Luteum?
If an egg is fertilized, it is implanted in the endometrial lining. A placenta forms, with the CHORION, as part of it developing from the zygote.
hCG is secreted by the chorion and takes the place of LH in preventing degradation of the corpus luteum.
Estrogen and progesterone stay elevated.
*hCG is used to confirm pregnancy.
Secretion of the Chorion
Human Chorionic Gonadotropin. hCG which takes the place of LH in maintaining the corpus luteum.
Hormonal state during menopause
High LH and FSH, but low estrogen and progesterone as a result of the lack of follicle cells.
During the secretory phase of the endometrial cycle what is occuring?
Estrogen and progesterone are being secreted from the corpus luteum.
Secondary oocyte is in the uterine tube.
Initial process of fertilization
A secondary oocyte is ovulated and enters the uterine tube (has corona radiata and zona pellucida). It can remain fertile for about a day like this.
Sperm if deposited near the cervix will be diluted and activated of CAPACITATED (loss of inhibiting factors in the sperm). Will swim to secondary oocyte.
Fertilization
Fusion of the spermatozoan with the secondary oocyte (still only having undergone one round of meiosis).
Acrosome reactoin
Sperm penetrates corona radiata and binds and penetrates the zona pellucida.
Acrosome
Head of sperm with many hydrolytic enzymes.
Acrosomal process
After penetration of the corona radiata, the acrosomal process penetrates the zona pellucida.
Bindin in the acrosomal process, helps bind the receptors in the zona pellucida.
How quickly does meiosis occur post-insertion of sperm nucleus enters the secondary oocyte?
Twenty minutes.
Produces an Ootid and a polar body. Ootid rapidly matures into an ovum.
After fertilization
Sperm and ovum nuclei fuse, and the cell is now considered a Zygote.
Fast block to polyspermy
depolarization of the egg plasma membrane that prevents fusion of other spermatozoas.
Slow block to polyspermy
Influx of Ca2+; also known as the cortical reaction.
results in:
Swelling of the zona pellucida and plasma membrane
Hardening of the zona pellucida
Egg activation - increases metabolic activity and protein synthesis
Slow block to polyspermy
Influx of Ca2+; also known as the cortical reaction.
results in:
Swelling of the zona pellucida and plasma membrane
Hardening of the zona pellucida
Egg activation - increases metabolic activity and protein synthesis
Embryogenesis
Occurs within 36 hours of fertilization.
However, proceeds slowly.
Cleavage
The first stage of embryogenesis, results in a ball of cells called a MORULA.
Note that the cell divisions that occur in this initial stage are primarily in S and Mitotic stages, as the size does not get much larger than the zygote was intitially.