Evolution of Menstruation

[jeskill]

Pharyngula (PZ Myers) has a really interesting post about a recent paper in Bioessays concerning the evolution of menstruation.

 

My favourite line: "I suppose we could blame The Curse on The Fall, but then this phylogeny would suggest that Adam and Eve were part of a population of squirrel-like proto-primates living in the early Paleocene. That's rather unbiblical, though, and what did the bats and elephant shrews do to deserve this?"

 

http://scienceblogs.com/pharyngula/2011/12/why_do_women_menstruate.php?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+PeerReviewOnScienceBlogs+%28Peer+Review+on+Science+Blogs%29&utm_content=Google+Reader

[jimmydasaint]

Reading on a bit in the article, it is quite interesting to read the following:

 

A new paper by Emera, Romero, and Wagner suggests an interesting new idea. They turn the question around: menstruation isn't the phenomenon to be explained, decidualization, the production of a thickened endometrial lining, is the key process.

 

All mammals prepare a specialized membrane for embryo implantation, the difference is that most mammals exhibit triggered decidualization, where the fertilized embryo itself instigates the thickening, while most primates have spontaneous decidualization (SD), which occurs even in the absence of a fertilized embryo. You can, for instance, induce menstruation in mice. By scratching the mouse endometrium, they will go through a pseudopregnancy and build up a thickened endometrial lining that will be shed when progesterone levels drop. So the reason mice don't menstruate isn't that they lack a mechanism for shedding the endometrial lining…it's that they don't build it up in the first place unless they're actually going to use it.

 

So the question is, why do humans have spontaneous decidualization?

 

The answer that Emera suggests is entirely evolutionary, and involves maternal-fetal conflict. The mother and fetus have an adversarial relationship: mom's best interest is to survive pregnancy to bear children again, and so her body tries to conserve resources for the long haul. The fetus, on the other hand, benefits from wresting as much from mom as it can, sometimes to the mother's detriment. The fetus, for instance, manipulates the mother's hormones to weaken the insulin response, so less sugar is taken up by mom's cells, making more available for the fetus.

 

Within the mammals, there is variation in how deeply the fetus sinks its placental teeth into the uterus. Some species are epithelochorial; the connection is entirely superficial. Others are endotheliochorial, in which the placenta pierces the uterine epithelium. And others, the most invasive, are hemochorial, and actually breach maternal blood vessels. Humans are hemochorial. All of the mammalian species that menstruate are also hemochorial.

 

That's a hint. Menstruation is a consequence of self-defense. Females build up that thickened uterine lining to protect and insulate themselves from the greedy embryo and its selfish placenta. In species with especially invasive embryos, it's too late to wait for the moment of implantation — instead, they build up the wall pre-emptively, before and in case of fertilization. Then, if fertilization doesn't occur, the universal process of responding to declining progesterone levels by sloughing off the lining occurs.

 

 

so it might be a war between possible embryo and mother...

pz myers

Edited December 24, 2011 by jimmydasaint

[SeptemberMoon]

That is interesting... I still hate the process, though.

[PSRIt]

so it might be a war between possible embryo and mother...

 

That it is.

[ewmon]

Menstruation is a consequence of self-defense. Females build up that thickened uterine lining to protect and insulate themselves from the greedy embryo and its selfish placenta. In species with especially invasive embryos, it's too late to wait for the moment of implantation — instead, they build up the wall pre-emptively, before and in case of fertilization. Then, if fertilization doesn't occur, the universal process of responding to declining progesterone levels by sloughing off the lining occurs.

Maybe defense, but to me, obviously not against embryos. If human uteri achieved the targeted 100% success rate, we wouldn't be here right now.

 

However → The embryo must penetrate deeply, which the uterus must allow, so I see the thick lining and its periodic sloughing as defense against non-human biological entities penetrating so deeply and compromising the woman's health. Perhaps the embryo requires a think lining that the mother's body cannot properly defend immunologically, so the sloughing of the lining (and anything hiding/growing in it) is an alternative method of defense.

 

In a somewhat likeewise manner, the lining of the lungs, which similarly requires intimate contact with the air, has its method of flushing away undesired biological entities.

[jimmydasaint]

Maybe defense, but to me, obviously not against embryos. If human uteri achieved the targeted 100% success rate, we wouldn't be here right now.

 

However → The embryo must penetrate deeply, which the uterus must allow, so I see the thick lining and its periodic sloughing as defense against non-human biological entities penetrating so deeply and compromising the woman's health. Perhaps the embryo requires a think lining that the mother's body cannot properly defend immunologically, so the sloughing of the lining (and anything hiding/growing in it) is an alternative method of defense.

 

In a somewhat likeewise manner, the lining of the lungs, which similarly requires intimate contact with the air, has its method of flushing away undesired biological entities.

 

I think that the article was looking at the point of menstruation from a meta-cognitive evolutionary standpoint. I take your point that the periodic shedding of the endometrium also protects against potential pathogens, or even against potential damage to the endometrium over the duration of a lunar month. As a consequence, the new 'proto' embryo has a brand new lining in which to implant.

 

However, the researchers were looking at spontaneous decidualisation in animals that generally that do not shed their respective endometria. Why would mice build up a thickened layer when there is no mechanism for shedding it? The researchers proposed a hypothetical mechanism for why this happens in a general evolutionary framework and they came up with the hypothesis that the thickening of the endometrium is a defence mechanism against the greedy embryo.

 

 

I hope this makes sense.

Edited February 18, 2012 by jimmydasaint

[John Cuthber]

I once read the suggestion that the idea was to shed any pathogenic infection.

That makes more sense to me that the idea of an evolutionary battle between the mother and the genetically almost identical embryo.

[jimmydasaint]

I once read the suggestion that the idea was to shed any pathogenic infection.

That makes more sense to me that the idea of an evolutionary battle between the mother and the genetically almost identical embryo.

 

The thing is JC, other animals thicken the lining of the womb and don't shed it like humans. The researchers had a hypothesis which is that the thickened layer provides a 'protection' for the mother when an embryo does implant itself and starts to command nutrients, for itself, from the mother.

 

http://php.med.unsw.edu.au/embryology/images/9/96/Week2_001.mov

[John Cuthber]

Thing is Jimmy, human sexual behaviour is different from many other animals so the potential risk of infection is different.

The other thing is that all embryos demand nutrients from their mothers.

[ewmon]

The thing is JC, other animals thicken the lining of the womb and don't shed it like humans. The researchers had a hypothesis which is that the thickened layer provides a 'protection' for the mother when an embryo does implant itself and starts to command nutrients, for itself, from the mother.

Perhaps the thickening minimizes post-partum bleeding and blood loss when the placenta separates from the uterus at the end of the pregnancy. If my feeble mind remember correctly, most/all of the vascular disruption occurs on the mother's side of the attachment, and mothers can bleed to death. I've never heard of a placenta bleeding post-partum; I've never heard that the umbilicus must be quickly tied off and cut out of concern that the newborn could bleed to death through the placenta.

[jimmydasaint]

Thing is Jimmy, human sexual behaviour is different from many other animals so the potential risk of infection is different.

The other thing is that all embryos demand nutrients from their mothers.

 

Read you loud and clear JC! I am not trying to patronise you. However, back to the main point which is decidualisation should be re-stated. Most primates have spontaneous decidualisation in the absence of an embryo. Mice only have a thickened endometrium when they need it. The authors looked at the degree of invasion of the maternal tissue by the embryo and mammals that menstruate also showed haemochorial invasion of the endometrium. This led to a new hypothesis, which can also include the previous hypothesis about primate sexual behaviour and removal of pathogens.

 

 

Perhaps the thickening minimizes post-partum bleeding and blood loss when the placenta separates from the uterus at the end of the pregnancy. If my feeble mind remember correctly, most/all of the vascular disruption occurs on the mother's side of the attachment, and mothers can bleed to death. I've never heard of a placenta bleeding post-partum; I've never heard that the umbilicus must be quickly tied off and cut out of concern that the newborn could bleed to death through the placenta.

 

Agreed. I don't think your mind is at all feeble, nor did I suggest it. However, I would have read the original article in order to understand the viewpoint of the researchers. It is, after all, just a hypothesis.

Edited February 19, 2012 by jimmydasaint

[John Cuthber]

Fundamentally, if there is a difference between humans and other mammals it can not be due to a characteristic humans share with all other mammals such as the embryo getting it's nutrition from the mother.

[jimmydasaint]

Interesting point. The authors mentioned this genetic and phenotypic heterogeneity in mammals:

 

Within the mammals, there is variation in how deeply the fetus sinks its placental teeth into the uterus. Some species are epithelochorial; the connection is entirely superficial. Others are endotheliochorial, in which the placenta pierces the uterine epithelium. And others, the most invasive, are hemochorial, and actually breach maternal blood vessels. Humans are hemochorial. All of the mammalian species that menstruate are also hemochorial.

Article

 

Additionally, another hypothesis is made. The phenotype, in general, that was stabilised in evolutionary terms, would result in a signalling process involving cyclic AMP. The species that produced progesterone in spontaneous decidualisation of the endometrium also would show an up-regulation of cAMP, with or without the presence of an embryo, but in species that were non-menstruating, there would only be a cAMP up-regulation in the presence of an embryo. The jury is still out I think...

Edited February 19, 2012 by jimmydasaint

[Ceac]

"selfish".. ?

Are you really giving a sense of conscience to biochemistry in THAT level? You might be kiddin'.

[pantheory]

I like the hypothesis that although not as efficient a system as with some other mammals, it has the flushing advantage once a month for pathogens of numerous possibilities, that might otherwise result in infertility or death. For the same reason it might decrease the incidence of transmission of such pathogens to males which might have similar consequences. Like many things in evolution it could be a tradeoff having both advantages and disadvantages.

[DrDNA]

Y'all might find this seminal paper on the loss of estrus and human evolution interesting and perhaps pertinent to your subject.

 

"Loss of Estrus" and Concealed Ovulation in Human Evolution: A Reevaluation.

[pantheory]

Y'all might find this seminal paper on the loss of estrus and human evolution interesting and perhaps pertinent to your subject.

 

"Loss of Estrus" and Concealed Ovulation in Human Evolution: A Reevaluation.

 

Interesting article.

[imatfaal]

Y'all might find this seminal paper on the loss of estrus and human evolution interesting and perhaps pertinent to your subject.

 

"Loss of Estrus" and Concealed Ovulation in Human Evolution: A Reevaluation.

 

Very interesting - but not, I believe seminal or even cited and at a tangent to the subject

[DrDNA]

Very interesting - but not, I believe seminal or even cited and at a tangent to the subject

 

You're right. I got in a rush and pasted the wrong link (that one is a decent review):

I meant to past this one.

Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus?

 

I AM IN THE WRONG FIELD!!

 

Seriously, this one is a great and interesting paper that cites the original literature in this area.

Human oestrus

Edited April 22, 2012 by DrDNA

[differentDemocrat]

lol i guess scientists have nothing else better to study now?

[imatfaal]

lol i guess scientists have nothing else better to study now?

 

What exactly do you think is wrong with studying menstruation? It is a significant hormonal and bodily change to a fair percentage of the population on a regular basis; the study of evolutionary changes to a system can provide vital information about its functioning.

[Athena]

Maybe defense, but to me, obviously not against embryos. If human uteri achieved the targeted 100% success rate, we wouldn't be here right now.

 

However → The embryo must penetrate deeply, which the uterus must allow, so I see the thick lining and its periodic sloughing as defense against non-human biological entities penetrating so deeply and compromising the woman's health. Perhaps the embryo requires a think lining that the mother's body cannot properly defend immunologically, so the sloughing of the lining (and anything hiding/growing in it) is an alternative method of defense.

 

In a somewhat likeewise manner, the lining of the lungs, which similarly requires intimate contact with the air, has its method of flushing away undesired biological entities.

 

 

Wow, that looks like a good argument to me. The average age for diagnosing uterine cancer is 60, when a woman is less likely to be having periods. So I will vote for the thicken wall being a defense, not against her own embryo, but foreign stuff we don't want attached.

[imatfaal]

Wow, that looks like a good argument to me. The average age for diagnosing uterine cancer is 60, when a woman is less likely to be having periods. So I will vote for the thicken wall being a defense, not against her own embryo, but foreign stuff we don't want attached.

 

I did not know that uterine cancer was caused by an outside foreign body - cervical cancer and the link to the HPV are well documented. Endometrial Hyperplasia - excessive growth/thickness of the lining of the uterus is actually a risk factor in uterine cancer rather than being protective