In a bizarre reproductive biology advance, researchers have fertilized mouse eggs with cells from another mouse's body--instead of sperm. The work is the first demonstration that embryos can develop from the combination of an intact egg and a nonreproductive cell. But don't discount the importance of sperm just yet--it's not yet clear whether any of the early-stage embryos could develop further.
Just before normal fertilization, an egg's chromosomes consist of two identical copies--each called a chromatid--of the genetic material. When the sperm enters the egg, the egg's nucleus goes through its final nuclear division and the chromatids separate from one another. One set of chromatids becomes a so-called polar body and is ejected from the cell, while the other set combines with the sperm's half-complement of chromosomes to form a complete genome.
This is no mean feat of genetic manipulation in itself, but it's not all that eggs can do. Earlier experiments with mouse and human cells had revealed that immature sperm that still contain two copies of each chromosome could fertilize an egg and produce live births--the egg is evidently able to expel two extra sets of chromosomes before proceeding with normal development.
Fertility researchers Orly Lacham-Kaplan and Rob Daniels of Monash University in Melbourne, Australia, wondered if normal diploid somatic cells--a cell other than a sperm or an egg that has two copies of each chromosome--could also fertilize an egg under the right conditions. The researchers injected nuclei from cumulus cells (a supporting cell that surrounds developing eggs) into intact mouse eggs, allowed each egg to rest for 2 to 3 hours, and then chemically induced cell division.
Kaplan told the meeting of the European Society for Human Reproduction and Embryology in Lausanne, Switzerland, on 2 July that about half of the 725 eggs survived the injection procedure and 102 of them formed two polar bodies and two pronuclei, each with a half-complement of chromosomes. Of those "fertilized" eggs, 47 developed into clumps of cells called morulae and 13 went on to form blastocysts--a hollow ball of a few hundred cells that normally implant themselves in the uterus. None of the embryos were implanted into the uterus of a mouse, so the team doesn't know whether animals could be born from this technique.
The result is interesting from a cell biology perspective, says cloning pioneer Ian Wilmut of the Roslin Institute outside Edinburgh, Scotland. However, he notes, producing a blastocyst is a long way from producing live offspring--only a tiny percentage of implanted cloned blastocysts survive. "There could still be chromosome damage and breakage," he says, that would interrupt development at a later stage.