If you talk to me very often, and perhaps even if you don't, then you know that until recently I have been swimming in a thick, molasses-like sea of frustration. The reason for my erstwhile annoyance? My mice were not breeding. Without breeding, I could not get samples, and without samples, I could not do science. And lack of science makes for a very sad Elizabeth.
Well, one dissection two weeks ago, three last week, and two more today are confirmation that the dry spell is over and I now have more samples than I could ever, possibly, use (seriously. I showed one of my labmates my box-o-samples and he looked at me with an overwhelmed, pitying gaze). I mean, really, who makes 120 RNA libraries from 60 genetically identical individuals anyway? Grad students? Damn.
The bright side (apart from being on the road to data-ville) is that I saw some really cool things. Last week, I was pulling apart the yolk sac to get to the embryos, and two heads popped out. Turns out there were two embryos in there, sharing the same placenta and yolk sac. "Cool," I thought. "Monozygotic twins!" I hadn't seen that before in my dissections, of probably a few hundred mice. I showed Cheryl, the postdoc I'm working with who has seen a lot more mouse embryos than I have, and she had the same reaction. "Cool! I've never seen that before."
So I did a little bit of digging, and it turns out that monozygotic twins (naturally occuring ones, not artificially separated ones) are very very rare in mice. "Huh," I thought. "Nifty."
One week later, this time as I'm removing the embryo/placenta/yolk sac from the womb, I see two yolk sacs with embryos inside attached to the same placenta. "Whoah," I thought. "That's really weird. Twice in two weeks?"
And when we were subdissecting the placentas to prep them, Julie found another placenta that seemed to have been the product of two that fused at some point in development. She had never seen anything like it -- any of it -- and she has dissected thousands and thousands of mice.
It seems to me that there are two separate phenomena possibly occurring here: the first being a 'fused' placenta leading to two embryos in their own amnions and yolk sacs, and a placenta that still has the remnants of the fusion: a bright red line of giant cells in the middle showing the bifurcation. I would be willing to believe that this was in part a product of competition, possibly, in a large litter (the litters today had 10 and 12 pups respectively, which is on the big side but certainly not unheard of). Something like, two adjacent placentas are so close together in a large litter that they sort of grow into each other. This makes sense in the respect that the placentas that we saw with multiple yolk sacs were larger than usual and showed some remnants of the former separate structures. The second phenomenon is the 'monozygotic twinning' resulting in two embryos in their own amnions but sharing a yolk sac (or possibly two yolk sacs depending on when the cleavage occurred?) and a unitary placenta without any remnants of fusion. From what I've read, this seems like it would be much rarer?
One of the confounding issues being that mice have litters: multiple birth is the norm rather than being a relative exception. Only 1% of people have a twin, identical or fraternal. Just about every mouse has a few siblings that were born from the same litter. So in one way, 'twinning' in mice is much more common than it is in people.
On the other hand, about 1 in 500 people have an identical twin. (Most of them share a placenta but not an amnion: 60-70%. Some have separate placentas: about 36%. And sharing an amnion is even rarer: about 2%.) This is a lot harder to measure in mice (after all, lab mouse strains are all inbred and mostly have the same exact genome -- what does it mean to be an identical twin when all of your siblings and for that matter your parents have the same exact genome that you do? And how can you tell which siblings resulted from cleavage at a very early stage and which from multiple identical eggs being fertilized by multiple identical sperm?) But this is the phenomenon that seems to be utterly, excruciatingly rare. So much so that they can't use mice as a platform for studying monozygotic twinning in people. Sadly I can't access this article (just the abstract! Boo!) but the gist I can get is that out of 2000 mice born, none were monozygotic. In people, if you looked at the same number you would expect to find about four identical twins, or two sets. And of course there personal evidence like the reactions of Cheryl and Julie to the stories: neither of them had ever seen something like this before, and each have dissected thousands of mice.
On the other hand is this article (1965! a classic!) Which studies a strain of mice with, apparently, rather high rates of monozygotic twinning: they saw 9 pairs of 'identical' twins in 114 litters or about 500 mice. The problem with this paper being that the determination of 'identical' was based on cosegregation of 10 markers: 9 mutations and gender. And their values are barely statistically significant when you account for the fact that certain mutations and combinations of mutations kill the mice. So that's not very conclusive evidence of any monozygotic twinning at all in my opinion. The more recent paper which looked at 2000 mice (a different strain) used a more rigourous genetic panel and found that of 35 possible pairs (which fits with the 9 in 500 rather beautifully), none were truly identical.
But at base I'm less interested in what the probability is that any two mice from the same litter are genetically identical (almost 100% given the inbred strains I mentioned earlier). I'm more interested in the phenomenon of two embryos sharing the same placenta, either due to a hybrid placenta caused by fusion or due to cleavage at some early stage ala 'monozygotic twinning'. So it behooves me to look for embryological studies -- ones that examine the embryo at about the same time as I am looking.
I haven't found much to speak to sharing-a-placenta, and the rates thereof in mice. But possibly its rarity in the literature has something to do with its rarity in nature (in lab? is lab nature for a lab mouse?). Nothing to do but keep looking.
(Side note: turns out rabbits have conjoined twins, at especially high rates in a certain strain. As in, four partial duplicates in 250 or so animals in the study I saw. Siamese rabbits!)
Next time something more interesting to people who aren't me? Who knows. Probably not.