Video Transcript
What usually happens when a zygote
develops into two embryos? (A) This question is misleading; a
zygote cannot develop into two embryos. (B) They develop into fraternal
twins, each with a separate placenta. (C) They develop into fraternal
twins that share a placenta. (D) They develop into fraternal
twins with a shared amnion. (E) They develop into identical
twins that share a placenta.
An egg can be fertilized by a sperm
cell in order to produce a fertilized egg, or a zygote. The individual cell of this zygote
can then become an embryo by dividing into two cells, then four, and so on. This embryo can then implant into
the endometrium of the mother to form a pregnancy. Eventually, the embryo develops
into a fetus that exchanges nutrients and waste products with the mother through the
placenta, as we can see here. This is what happens when a single
fetus forms, but what about twins?
This can happen different ways, but
one way is when a single zygote splits to form two separate embryos. This split usually occurs between
eight and 12 days after fertilization. Because these embryos are both from
the same zygote, they are identical. These can develop into monozygotic,
or identical, twins. Monozygotic twins have two
identical fetuses and often share the same placenta as we can see here. Less frequently, two placentas can
form. Another type of twinning can occur
when two separate zygotes are fertilized to form nonidentical, or fraternal,
twins. But this isn’t what the question is
referring to.
In the question, we are asked what
usually happens when one zygote develops into two embryos. The answer to this is given by
answer choice (E). They develop into identical twins
that share a placenta.
