Biology concepts – twinning, placenta, amnion, monozygotic,
conjoined twins, in vitro
fertilization
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Twins are always a subject of
interest. Quicksilver and
Scarlet Witch are twins in the
Avenger and X-Men series.
Would they be fraternal
(dizygotic) twins or identical
(monozygotic) twins? The
answer – either. Stick around
for a few posts and find out
why.
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How can this be? Cloning humans is a highly technical
affair, with all kinds of molecular biology and developmental biology knowledge
required. It would require a team of scientists with a handful of PhD degrees
….. or …. a woman could just had monozygotic twins.
Twins come in two flavors – monozygotic and dizygotic. You
may know them by other names. A zygote is a fertilized egg, so monozygotic twins come from one (mono)
egg and dizygotic twins come from
two separate fertilized eggs.
Monozygotic twins (MZ) are commonly called identical - they aren’t - but more on that later. Dizygotic twins are known as fraternal, but fraternal
means brother, so this name is bad too.
You see a set of twins on the street. Do you think you would know if they are monozygotic or dizygotic? I bet you can’t.
You see a set of twins on the street. Do you think you would know if they are monozygotic or dizygotic? I bet you can’t.
For many years scientists were sure they could tell the
difference by just looking; sure enough that they used the sight test to enter
twins into studies on genetic traits. When DNA testing became more
common, they found that many of the twins they studied were actually dizygotic.
Science had to throw out forty years of twin research.
We’re going to talk about many exceptions amongst twins, but
perhaps we should start with just what MZ twins are and aren’t, and how they
might come about. First, there is an egg that is fertilized by a male gamete cell
– a zygote is formed. The zygote splits two to three times over the first couple
of days.
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The twinning process isn’t as
simple as monozygotic or
dizygotic. There is also how
many placenta (chorions)
and amniotic sacs to deal
with. If they have one placenta,
they mix blood, so genetic
testing is going to be harder.
Which type they are depends
on when the zygote split
or if there were two
fertilized eggs.
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This how things normally happen, but about 3.5 times in 1000
live births, the zygote or embryo splits and becomes two embryos. This is
possible because the cells of the early embryo are totipotent stem cells; they
have the potential to become any type of human cell. Later on, the cells at
different parts of the embryo are merely pluripotent – they can become several
different types of cells. Even later, the cells begin to differentiate into the
tissues that the fetus will need.
Therefore, those 3.5 embryos that split and become MZ twins
have to split early in development. The placenta (part of the chorion) develops
around day three, and most zygotes don’t split that early; therefore, 75%
monozygotic twins are monochorionic
(they share one placenta and chorion).
To become a dichorionic
set of twins, the split must occur within those first two days, and this only
occurs about 25% of the time. On the other hand, the amniotic sac develops
around day nine. If the split occurs later than day nine, then the MZ twins
will be monochorionic and monoamniotic
(MoMo).
This is also rare because most splits occur before day nine.
The result? – most MZ twins are monochorionic,
diamniotic (MoDi). Can you explain why there can’t be a dichorionic
monamniotic set of MZ twins? Of course you can.
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The top image shows a time
lapse of the blastocyst
emerging from the zona
pellucida – hatching. The bottom
left image shows the
components of the blastocyst before
it hatches. The right bottom
image shows how the zona
can be nicked with a laser to
assist in hatching.
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By this time (8-14 days), the embryo is more than 15-30
cells total, called a blastocyst. It
is hollow and the cells are just starting to assume roles, meaning they are no
longer totipotent. So the split needs to come before the end of that second
week. One theory is that the blastocyst collapses and the cells on each side of
the pinch becomes an individual.
This is the traditional view of when MZ twins form and how
to account for the placenta and anmniotic sac number. But there are other
views. A newer theory contends that the split occurs much closer to
fertilization, maybe in the first couple of divisions after fertilization.
Under this hypothesis, the chorionicity and amnionicity have more to do with
random fusion of separate structures rather than their development based on
time of split. By the way – there’s no evidence for this yet, just some twins
that may not fit the traditional pattern - if, in fact, the recorded timing was correct.
The question you should
be asking about MZ twins is just why the egg splits into two (and very rarely,
three) in the first place. It’s rare, but it has happened throughout history,
so there is a cause. Or causes.
The short answer is science doesn’t know yet, but it has
many hypotheses. The rate of MZ twinning has been stable over the years and
doesn’t show a predilection for a race or area of the world. These all point to
MZ twinning having no genetic basis. So what might be causing it?
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Which twins are monozygotic
and which are dizygotic?
Sometimes it isn’t easy to
tell. This is why 40 years of
twins research before genetic
testing had to be thrown
away. By the way, Mary Kate
and Ashley Olsen on the top
are fraternal twins. The two
women below are identical.
One smoked most of her adult
life, the other didn’t.
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The increase in in vitro fertilization (IVF)
therapies in the last decades has paralleled a slight increase in MZ twins.
This isn’t because of more than one embryo implanted in IVF - those would be
dizygotic twins, we’re talking an increase in MZ twins only here.
IVF therapies involve more than just implanting embryos. The
reproductive system has to be made ready for the zygotes, and this means giving
the lady lots of hormones. Might this throw off the balance and promote zygote
splitting? Could be, but there are other possibilities.
The age of the embryo when implanted makes a difference.
Five-six day embryos have a higher splitting rate than those that are implanted when they are only 2-3 days old. However, both age group embryos had a
higher rate of MZ twinning than did control embryos from natural
fertilizations. So perhaps we need to look further.
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For in vitro fertilization. The eggs are removed from the
ovary, and each in turn is
held in place by a slight suction
on a pipet (bottom right).
The egg is pierced and one male
gamete cell is injected
through the zona pellucida and into
the cytoplasm. Then, several
are allowed to divide a few
times and then they are
deposited into the uterus.
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Then again, the idea that MZ aren’t linked to genetics may
be wrong. There have been observations of MZ twins running through the males in
some families, perhaps through a y-linked gene. Another study has documented seven families with higher than normal rates of MZ twinning.
It is through these mechanisms, or others we have no idea
about yet, that an embryo might split to become MZ twins. But we have been
assuming that the split embryo becomes two individuals. What if that doesn’t
happen? What if the split embryos remain attached to one another? You get conjoined twins.
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Chang and Eng were the most
famous of the Siamese twins.
In fact, they are why we call
conjoined twins Siamese. They
became rich plantation owners
in the American South, but
lost all their money when the
North won the Civil War. They
were each married and had 21
children total. There are
twins in the descendants (no conjoined), but neither Chang
nor Eng had twin children themselves.
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There have been famous examples of conjoined twins, sometimes called Siamese twins because of the most famous pair – Chang and Eng of Siam in the 1800’s.
Conjoined twins can also be called imperfect twins, because the embryo split imperfectly. If the split occurs very late, perhaps after day 12 post-fertilization, the rate of conjoined twins might be increased. At this point the cells are already past the point of starting to differentiate, so they are becoming tissues and organs. The ones that split become organs on their own, but the cells that are shared will become organs and structures that the twins will share. In all, we don’t know the risk factors for creating conjoined twins, but it is interesting to note that Latin American women have a much higher rate of conjoined twins than anywhere else in the world. I can't imagine why.
Conjoined twins are described based on where they are
joined. The majority are joined at the chest (called thoracopagus, pagos is from Greek = something joined) or the
buttocks (pyopagus). The number and
extent of organ and structure sharing can vary greatly, but thoracopagus twins
often share a heart, while pyopagus twins might share a lower GI tract.
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Dr. Ben Carson was the lead
surgeon in the successful 1987
operation to separate
occipital craniopagus conjoined twins.
He has also been lauded for
his hemispherectomy method to
stop seizures and
craniofacial reconstruction in
achondroplastic dwarfs. Now
he is running for president.
What one has to do with the
other, I have no idea.
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Needless to say, there are many more papers describing them
and their problems than there are solutions and treatments. Surgery is getting
better, but it depends on what they share. What we have described are the
equally conjoined twins. But sometimes, one twin dominates over the other. One
will be smaller, weaker, and less able to survive. This is called a parasitic twin. Taken to the extreme
……. Well, that will have to wait for next week.
For more information or
classroom activities, see:
Monozygotic twins –
Zona pellucida –
Chorion and placenta –
Amnion –
Conjoined twins -
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