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I know that in human body ovulation takes place at only 1 ovary every month. But development of a follicle depends on FSH levels in the blood, and since both the ovaries are exposed to the same blood supply, how does it make sense for development of follicle to be seen only in one ovary? Aren't both ovaries exposed to same FSH levels every month?

I am not able to figure out the solution to this anomaly and would appreciate some answers.

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    $\begingroup$ I don't know the answer to this but it could simply be that once an ovary has ovulated it enters some sort of refractory period so that it ends up skipping a cycle. $\endgroup$ – Alan Boyd Aug 19 '17 at 10:08
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    $\begingroup$ @AlanBoyd - Ovulation isn't democratic. Each ovary has an equal chance of ovulating every cycle. I don't know why that is, but it is. $\endgroup$ – anongoodnurse Aug 19 '17 at 11:38
  • $\begingroup$ @anongoodnurse I had got the idea that they alternated. $\endgroup$ – Alan Boyd Aug 19 '17 at 13:00
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Each ovary has hundreds of thousands of follicles. And, of course, each one of those follicles is exposed to the same blood supply.

Therefore, your question...

Why isn't ovulation seen in both ovaries together?

... is the same of:

Why isn't ovulation seen in two follicles (regardless the ovary)?

All follicles are stimulated by FSH and they indeed start to mature at the same time. However, in a given cycle, normally only one follicle fully grows1. What we have here is a competition among follicles.

Guyton is not my favourite book on physiology, but it suggests an explanation for this:

Only One Follicle Fully Matures Each Month, and the Remainder Undergo Atresia:

After a week or more of growth — but before ovulation occurs — one of the follicles begins to outgrow all the others; the remaining [...] developing follicles involute (a process called atresia), and these follicles are said to become atretic. The cause of the atresia is unknown, but it has been postulated to be the following: The large amounts of estrogen from the most rapidly growing follicle act on the hypothalamus to depress further enhancement of FSH secretion by the anterior pituitary gland, in this way blocking further growth of the less well developed follicles. Therefore, the largest follicle continues to grow because of its intrinsic positive feedback effects, while all the other follicles stop growing and actually involute.

This process of atresia is important, because it normally allows only one of the follicles to grow large enough each month to ovulate; this usually prevents more than one child from developing with each pregnancy. The single follicle reaches a diameter of 1 to 1.5 centimeters at the time of ovulation and is called the mature follicle.

Therefore, the reason why we don't see both ovaries ovulating at the same time is the same reason why we don't see one ovary ovulating twice at the same time: the fastest growing follicle inhibits the growth of the other follicles (in the same ovary or in the contralateral ovary, it doesn't matter).

Besides that, it also explains the pattern of ovulation: in a given cycle, the left and the right ovary have equal chances to ovulate. Thus, the ovaries don't take turns like this (L is left and R is right):

L → R → L → R → L → R → L → R → L → R → L → R → L → R → L → R → L → R etc...

The real pattern is probably random:

L → L → R → R → L → R → R → R → L → R → L → R → L → L → R → L → L → L ETC...

According to Ecochard (2000):

It is concluded that in normally fertile women, the cycle length and the hormonal profile are independent of the, most probably random, site of ovulation. (emphasis mine)

Sources:


1Note that, for answering this question, we will (wrongly) assume that there is no polyovulation in humans. However, as polyovulation does occur in humans, ovulation can sometimes be seen in both ovaries.

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  • $\begingroup$ Unrelated: "Guyton is not my favourite book on physiology...". Why so? Also, what book would you recommend instead? (I use Guyton to clear physiology questions that I have...if you know if a better book, I'd love to hear it from you!) O:) $\endgroup$ – paracetamol Aug 19 '17 at 15:12
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    $\begingroup$ My favourite is Ganong (the old editions). It doesn't mean that I recommend it, it's just a matter of personal choice: I like Ganong better. $\endgroup$ – user24284 Aug 19 '17 at 15:13
  • $\begingroup$ I see! Is there any particular reason for your preference? Is it...well...more lucid? or very detailed? or something? (I'm still a high-schooler, but I'm inclined to choosing the medical field. Someone recommended I keep a copy of Guyton handy. It's a good book, but I won't mind being recommended a few more) Sorry for bugging you again! ^_^ $\endgroup$ – paracetamol Aug 19 '17 at 15:22
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    $\begingroup$ Ganong gives more detailed explanations at the biochemical level. However, it's harder to read if you don't understand physiology. On the other hand Guyton is easier to read, and covers way more ground. $\endgroup$ – user24284 Aug 19 '17 at 15:50
  • $\begingroup$ So u guys are basically saying the whole left right theory is incorrect and that it is a random process $\endgroup$ – Adithya Eshwarla Aug 20 '17 at 0:52

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