Washington, July 10 : Scientists have announced the development of uman embryonic stem cells (hESCs) from a single cell, or blastomere, of a 4-cell stage embryo.
The researchers believe that their findings might make it possible in the future to produce hESC lines at an earlier stage without destroying the embryo.
Blastomeres are formed in the very early stages of embryonic development. About 24 hours after fertilisation the egg divides into two cells.
The division into four cells occurs after 48 hours. After 96 hours, at the morula stage, the fertilized egg has divided four to five times. During this time the size of the embryo does not increase, so the cells become smaller and smaller and they are strongly attached to each other which makes them more difficult to manipulate.
At this early stage important decisions are taken: inner cells will become the foetus (including germ cells) and outer cells will become trophoblast (the outermost layer of the embryo that attaches to the wall of the uterus and serves as a nutritive pathway).
There was, until now, uncertainty about which stage of early development the blastomeres ceased to be totipotent, i.e. able to develop into all cell types of the body.
"Previously, scientists have been able to derive hESC lines at the 8-cell stage, but success rates were variable and it was necessary to culture them by mixing with established hESC lines," said Dr. Van de Velde
"We have been able to derive hESCs at an earlier stage of embryonic development, and without the need for co-culture with established hESC lines. Now we have derived a second hESC from one cell of a 4-cell stage embryo. Given the complex nature of earlier attempts, we were pleased that we could develop a technique that seemed simple and was also reproducible," he added.
The scientists used mature eggs donated by couples being treated at the University's IVF centre. Embryos were obtained after ICSI using sperm from a consenting donor.
Three, good quality 4-cell stage embryos were split into 12 single blastomeres and allowed to grow in vitro to produce twelve morulas that were cultured in the conventional way for hESC derivation.
From these twelve, one resulted in a stable hESC line. The scientists concluded that at least one cell was pluripotent.
These new data confirm their recent report that at the 4-cell stage the cells are equal and totipotent.
"Now we will try to derive four hESC lines from the same embryo in order to compare the potency capacity of all four cells," said Dr. Van de Velde.
The work could have major ramifications for preimplantation genetic diagnosis (PGD), by enabling the biopsy of one cell from a 4-cell stage embryo, allowing the remaining three cells to develop into a blastocyst (five day embryo) which could be transferred into the uterus and develop into a healthy baby.
"We need to determine whether the removal of one cell at the 4-cell stage impairs the capacity of the embryo to develop into a healthy child in comparison with the removal of one/two cells at the 8-cell stage," he said.