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  • There's a saying that there are two things in life that you're never truly prepared for: twins.

  • And that is in large part because twinning is somewhat rare in our species.

  • Only about three percent of live births involve multiple fetuses.

  • Most of these are dizygotic, or what's commonly called fraternal twins.

  • They occur when a person just so happens to release two eggs instead of one,

  • both of which get fertilized and successfully implant.

  • A smaller percentage of twins are monozygotic oridenticaltwins.

  • Those are the twins that share essentially all of their DNA

  • because they come from a single fertilized egg which later split into two.

  • And both of these types of twins are useful for scientific studies

  • because they can help us sort out the influences

  • of genetics and environmental factors on different traits.

  • But it turns out that twinning is a lot more complicated than just identical or fraternal.

  • And the rarer types of twins suggest that

  • we have a lot more to learn about human development.

  • The namemirror image twinsbasically says it all.

  • They're a type of monozygotic twin

  • where the two people are physical mirror images of each other.

  • Like, if one twin is right-handed, the other might be left-handed.

  • The mirror twin's internal organs might even be on the opposite side;

  • what physicians call situs inversus.

  • So while that degree of mirroring only occurs in about one in every 10,000 people,

  • some degree of mirroring in identical twins is pretty common.

  • Some scientists think this is a hint as to when the twins' embryo actually split.

  • You see, within the first couple of weeks of embryonic development,

  • the cells that will eventually give rise to organs become programmed

  • for the right or left side of the body, a process called left-right asymmetry.

  • During this window, left and right are determined,

  • but the embryonic cells haven't yet migrated

  • to the positions where they will ultimately grow into body parts.

  • So it's thought that full mirror image twins

  • only occur when a single embryo splits after this has happened;

  • sometime around two weeks after fertilization.

  • But there is some debate about this.

  • So, the concept ofidenticaltwins isn't really true;

  • there are always some slight genetic differences between twins.

  • So it's possible that those genetic variations

  • might actually explain mirroring better than when the embryo split.

  • And even if it is a timing thing, the research isn't clear on

  • exactly when splitting will result in mirrored traits like hair whorls or handedness,

  • as opposed to full blown asymmetry of organs.

  • And that's because we can't actually see a human embryo splitting in utero.

  • Ultrasound technology is good, but it is not that good.

  • Though, even if we could see twinning happen,

  • we'd probably miss it for most pregnancies,

  • since it happens during the earliest stages

  • when a person might not even know that they're pregnant.

  • Still, whether it's about when the split happens,

  • or genetic differences between twins, or something else,

  • piecing together the mechanism behind mirroring has the potential to teach us a lot

  • about how our developing bodies determine where our different pieces go.

  • You're probably already familiar with conjoined twins;

  • twins that remain at least somewhat attached at birth.

  • For better or worse, they've received a lot of attention both medically and culturally.

  • Still, we're not sure how conjoining happens.

  • And examining cases more closely has kind of

  • upended everything we thought we knew about twinning.

  • The earliest and still common explanation for conjoinment

  • is that it's a result of incomplete splitting;

  • what's often called the fission theory.

  • This is actually how all monozygotic twins supposedly happen:

  • for whatever reason, one embryo splits into two.

  • And, so the theory goes, if this happens super early,

  • both twins develop their own amniotic sacs and their own placentas.

  • If it happens a little later, they share one or both of those.

  • And if the split is really late, like,

  • two or more weeks into development then you get conjoined twins.

  • It's thought that, for some reason, splits that occur that late fail to fully separate.

  • And that makes a lot of sense intuitively,

  • and there are other reasons to think that conjoinment has to do with late embryo splitting;

  • like, that there are higher rates of mirroring in conjoined twins.

  • But, remember, mirroring might not be a timing thing after all.

  • And many researchers think conjoined twins don't arise from incomplete splitting.

  • Their model, dubbed the fusion theory,

  • posits that the embryo does completely split, and this split occurs a lot earlier on.

  • But then, as the two twins develop,

  • they end up physically colliding and they grow back together.

  • This might explain why most cases of conjoined twins

  • are joined at the chest and often share a heart.

  • See, early on in development, the primordial heart is one of the few parts of the body

  • that isn't covered in the type of cells that will become skin.

  • That may mean that that area is more vulnerable to rejoining.

  • Sorting out which of these two theories is right, or if they both are, at times,

  • would help scientists better understand how and when cells become

  • programmed into different tissues and why twins happen in the first place.

  • About ten percent of conjoined twins are considered external heteropagus twins.

  • Essentially, one twin doesn't develop all of their organs or body parts,

  • so they rely on the other twin for survival.

  • This undeveloped or asymmetrical twin may even look

  • more like a mutation than a separate, attached entity.

  • Like, a while back there was a viral video of

  • a pretty cute puppy with a tail that was sticking out of its head?

  • That was probably an external heteropagus twin.

  • Much like other conjoined twins, it's long been assumed that

  • such twins arise when an embryo splits late in the game and doesn't split perfectly.

  • And for some reason, the split is also fairly lopsided,

  • so one twin either doesn't develop well enough to survive,

  • or is essentially part of a person attached to their twin.

  • That kind of wonky fission could explain

  • how you'd end up with a tail sticking out of a puppy's head.

  • But similar cases in people may actually support the fusion theory instead.

  • See, here's the thing: no one has ever seen fission happen.

  • We have never witnessed a human embryo split in two,

  • even though techniques like IVF can involve culturing embryos for almost a week.

  • That means that we've never really confirmed the idea that

  • the timing of embryo splitting makes any difference,

  • and we don't actually know that an embryo can split partially or unevenly and still survive.

  • But we have come pretty darn close to definitive evidence of fusion,

  • thanks to a heteropagus twin case published in 1997.

  • The child described in the study was born

  • with what appeared to be a pair of legs growing out of his chest.

  • But they weren't his. Genetic tests suggest that the legs were from a fraternal twin.

  • Which, if true, means that two distinct embryos fused at some point.

  • Of course, that's just one case.

  • But it does suggest that further research on this kind of twin could help

  • settle the fission/fusion debate, or determine that both of them can happen.

  • Either way, it would provide researchers with greater insights

  • into how developing human cells behave, which could in turn

  • help doctors treat cases where development doesn't go as planned.

  • Sometimes, a heteropagus twin can exist wholly inside its healthy twin, as a fetus in fetu.

  • Okay, I say sometimes, but let's be clear that this is really rare:

  • less than two hundred reported cases have ever been reported.

  • And usually, there's just one, but there is a case report of eleven fetuses in fetu!

  • Not only is there some debate about how fetuses in fetu happen,

  • some researchers don't think it's a twin at all.

  • They believe that a fetus in fetu is actually a highly developed teratoma;

  • a kind of tumor made up of several body tissue types.

  • There are also some that think it stems from a kind of error during development

  • where some stem cells that can become anything divide weirdly.

  • But others say it has to be a true twin, because fetuses in fetu

  • have vertebral columns, and often, other developed body parts like limbs.

  • And of course, everybody here could be right, there could just be

  • lots of different ways to end up with one partially-developed fetus inside of another.

  • That would mean studying fetuses in fetu could help doctors understand

  • why some fetal tumors become problematic while others stay benign.

  • And it could teach us some really cool things about developmental programming.

  • Plus, it could give us further insights into twinning.

  • See, it could be that fetuses in fetu happen

  • because one twin envelops the other at some point.

  • So, kind of like the fusion idea, but all the way.

  • And that actually isn't as far-fetched as it might sound,

  • because there are cases where one twin absorbs part of or all of the other.

  • Now, you may have heard that you could be your own twin without even knowing it.

  • That's based on a phenomenon called vanishing twin syndrome, where,

  • very early on in a multiple pregnancy, one of the twins just kind of disappears.

  • Except, it doesn't really.

  • Doctors now think that vanishing twin syndrome occurs because,

  • when one twin dies spontaneously, it is generally absorbed by the other twin.

  • And by absorbed, I don't mean the tissues are broken down

  • for molecular parts and digested or something.

  • No; whole, living cells get incorporated into the twin's body

  • So one baby is born, and that baby has two people's genomes,

  • making them what biologists call a chimera.

  • And it turns out this kind of merging of cells doesn't just happen when a twin dies.

  • It's also how you end up with chimeric twins:

  • dizygotic twins where one or both has cells with the other twin's genome.

  • Fetal cell swapping might occur in monozygotic twins, too,

  • but since they have essentially the same genome, it's not really notable.

  • When twins are dizygotic, though, the other twin's cells stand out.

  • And researchers have found clear cases of this,

  • like, twins which have two distinct blood cell lines.

  • So some of their red blood cells are type A while others are type O.

  • What's amazing is that this kind of blood chimerism

  • seems to occur in about 8% of dizygotic twins; 21 % of triplets!

  • And the other twin's cells