Stephen G. Saupe - Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; (320) 363-2782;

Gink and Go Study Reproduction

 Setting:  Gink and Go are sitting at a table studying biology.  Gink lets out a primal scream.

Gink: Aaagh!  I don’t understand this stupid stuff that Saupe was saying about the hormonal regulation of the female reproductive cycle.
Go: Now calm down.  It seems complicated but it’s just a nifty cycle with a variety of feedback mechanisms.
Gink: The only feedback I want right now is some pizza.
Go: Get you mind off food for once so we can study this together.
Gink: Whatever.  I don’t understand how the cycle starts and stops and starts and stops and starts and…..
Go: Enough....let’s begin with GnRH.
Gink: You mean gonadotropal releasing hormone?
Go: See, you know more than you think.  GnRH is produced by the hypothalamus and triggers the pituitary gland to produce FSH and LH.
Gink: I remember now – and then FSH and LH stimulate a follicle to develop in the ovary.
Go:  Right.  A primary oocyte that was patiently waiting in a follicle continues its meiotic division as the follicle enlarges. 
Gink:  You mean it was haploid already?
Go:  Not quite – the follicles contain primary oocytes that are resting in prophase I of meiosis.  They don't finish the meiotic division until the follicle begins to develop.  In fact, it still isn't complete even when the egg is released on ovulation. The oocyte, now a secondary oocyte, is released from the follicle in prophase II of meiosis - it doesn’t complete meiosis until the sperm penetrates the egg.
Gink: Cool – it’s a little slow, just like me!  But where does estrogen fit?
Go: Good question – one of the roles of estrogen is to stimulate the growth of the endometrium.
Gink:  What’s the endometrium?
Go:    The lining of the uterus. Estrogen stimulates endometrial growth during the first half of the cycle.
Gink You mean days 1 to 14?
Go:    Right – the estrogen stimulates the cells to divide.  
Gink: It’s like a fertilizer for endometrial cell growth.
Go: That’s a good analogy.  As the follicle enlarges it produces more and more estrogen.  By day 14 the levels are so high that it causes the pituitary to release a surge of LH.
Gink:  LH Surge sounds like a tasty caffeinated soft drink.
Go:  Get serious.
Gink:  It's a joke.  You science nerds would say that the estrogen causes a large production of luteinizing hormone which, in turn, triggers ovulation.
Go: And did you also know that LH is what a woman can measure in an ovulation test kit?
Gink: Really?  So if she wants to know when she is ovulating she just measures the amount of LH in her urine?
Go: You bet.  Now, do you remember what happens to the follicle after it releases the egg?
Gink:   I think it collapses and becomes the corpus luteum.
Go:  Exactly, and it begins to produce progesterone.
Gink: What does progesterone do?
Go: Good question.  Progesterone turns off the hypothalamus so it doesn’t release GnRH.  Since there isn’t any GnRH the pituitary doesn’t release FSH or LH.  
Gink:   And that means no more follicles develop because the body is waiting to see what happens to the "bun in the oven."
Go:  I'm impressed – do you remember what else the progesterone does?
Gink:  It has something to do with the endometrium.
Go: It sure does – it helps to maintain the endometrium.  Progesterone stimulates glands to secrete materials into the endometrium to prepare it for embryo implantation.
Gink:   The progesterone is like a stabilizer for the endometrium.
Go:  Another good analogy.  Now let’s talk about what happens if fertilization and pregnancy don’t occur.
Gink:  By day 28 the corpus luteum stops producing progesterone.  This turns the hypothalamus back on to make GnRH, which then stimulates the pituitary to release FSH and LH to cause another follicle to develop.  
Go:  Also, since there’s no progesterone to stabilize and maintain the endometrium, it sloughs off and bleeding begins.
Gink: But what if the bun gets buttered?
Go:  Cute….if fertilization occurs, the embryo begins to grow and divide and it implants in the uterus on around day 21.
Gink: But why doesn’t the endometrium slough off?
Go: Good question – its because the chorion layer of the embryo produces another hormone, human chorionic gonadotropin, abbreviated hCG, that stimulates the corpus luteum to continue producing progesterone.
Gink:  So the hCG is important because it maintains the uterine lining and ultimately prevents anymore follicles from developing.
Go:  Excellent – you’ve got it.  Ultimately the corpus luteum stops functioning and releasing progesterone, but this is no big deal because the placenta takes over the role of progesterone factory.
Gink: Since hCG comes from the embryo, I’ll bet it would make a good test for pregnancy.
Go: You’re brilliant – that’s exactly how the pregnancy test kits work.  Saupe would be proud of you.
Gink: Do you expect me to care what that maple syrup-making weirdo thinks about me?  Let’s go watch some TV.  I think "Fantasy Island" is on.

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Last updated: April 17, 2008    � Copyright by SG Saupe