“A few hours after fertilization the cell, with its cargo of chromosomes, embarks on a remarkable journey. It travels along the fallopian tubes toward the womb.
Then it starts to divide: first in two, then four, then eight, each cell an identical replica of the other.
This rapidly multiplying collection of cells is called a blastocyst, and it’s here that nature’s most remarkable reproductive anomaly can sometimes occur.
It’s an event so mysterious, it wasn’t witnessed until recently.
Several days after conception, the blastocyst spontaneously splits in two. Each new blastocyst is composed of cells with identical sets of chromosomes, and they carry the same arrangement of genes along their length. The two blastocysts now have the potential to develop into twins.
Exactly how and why the blastocyst splits is one of reproductive science’s greatest mysteries, and it wasn’t until 2007, during a lab study of 26 embryos, that embryologists witnessed this event for the first time.
The blastocyst’s outer membrane is called the zona pellucida. The lining inside is called the trophoblast. Just one cell layer thick, the trophoblast will develop into the placenta.
The inner cell mass, filled with embryonic stem cells, will become the fetus.
Over the course of four days, embryologists noticed that a junction between cells in the trophoblast membrane would regularly weaken, letting liquid leak out, and causing the trophoblast to collapse like a balloon full of water. Then it would repair itself.
But in two of the eggs they saw something for the very first time. After the membrane collapsed, the inner cell mass divided in two, each clump sticking to a different side of the trophoblast as it re-inflated. If these clumps were to develop in utero, they would grow into identical twins.”
~from “Twins in the Womb,” by Nt’l Geographic.