Copyright 2001, all rights reserved
Q: In the case of identical twins, what triggers a single zygote to result in two embryos? What is the probability of having identical twins?
A:Your first question has me stumped. I've asked medical experts from various universities and hospitals. The general consensus is: we don't know.
Identical twins do not run in families and, therefore, twinning is not related to genetics. Comparisons among different ethnic groups rule out an ethnic cause-nodifference. Only mother's age correlates with twinning: the older the mother, the greater the likelihood.
I can mentionhow and when two embryos form instead of one even though we don't exactly know what triggers the events. We have some hunches though,which I'll mention as we discuss the timing. Twinning is abnormal in humans. Things can go wrong at different times during the embryo's development.
During the first four days after fertilization, the huge fertilized egg (zygote) splits into many smaller cells suitable for building the eventual baby. Sometime afterthe fourth day about 30 of these smaller cells are hanging around. The spaces between them have filled with pools of clear fluid and gather together to form ahollow cellular ball, called a blastocyst. An inner cell mass forms inside. This is the stuff that an embryo will develop from.
Now consider the approximate time sequence (from fertilization) of when things can go wrong and what kind of twins result.
You can see this happening in the photograph of Figure 1. Michael Tucker, PhD., F.I. Biology, captured a hatching event with theincredible photograph. The shell (a gelatinous capsule called the zona pallucida) is the fuzzy layer around the blastocyst. The innercell mass (the proto-embryo) is just starting to hatch through the hole in the shell.
Figure: [Michael Tucker, IVF.com] Hatching blastocyst
When the whole blastocyst passes through its shell, the inner cell mass can fragment into two (or very rarely three) clumps of cells. The two cell clumps form into complete embryos, which become identical fetuses, and eventually twin babies. The twins aremirror images of each other. Dr. Kliman's daughters are mirror-image twins, one having a set of moles on her right side and theother with the same set of moles on her left.
The reason age contributes to the likelihood of a woman having twins is that the enclosing shell (that the blastocyst hatches out of) isharder in older mothers. It's more likely, therefore, that the inner-mass will break into clumps as it hatches out of a tougher shell.
The second part of your question is relatively easy. About three in every 1000 deliveries results in identical twins. This average seems to be the same worldwideand the reasons are uncertain.
(Answered by April Holladay, science correspondent, May 9, 2001)
Further Surfing:
Yale U: Behind every healthy baby is a healthy placenta
U of Pennsylvania: Basics of embryo development
Children's Hospital of Iowa: Twins, a parents' guide
