LONDON When Australian scientists announced earlier this month that they had created a mouse embryo without the use of sperm, they sparked a fiery debate. What, asked the commentators, would the nature of the family be should men not make their traditional contribution? Yet, ironically, Orly Lacham-Kaplan — a male-fertility expert — who developed the technique along with colleagues from the Monash Institute for Reproduction and Development, Melbourne, claimed it was awareness of the infertility problems associated with a low sperm count that led her to explore whether cells other than sperm could fertilise an egg.
Describing the work, which so far is only
It had previously been considered that attempts to fertilise an egg with cells other than sperm (somatic cells, unlike sperm and eggs, which are germ line cells) would be doomed to failure because of the biological nature of sperm and the process of fertilisation.
During normal fertilisation, two sets of chromosomes in an egg separate and one set is ejected in a package called the polar body, leaving a single set to combine with the sperm's single set. Unlike sperm, somatic cells have two sets of chromosomes, making them unacceptable as donors for fertilisation. But by inducing the cellular machinery that ejects the spare set of chromosomes from an unfertilised egg to also eject the additional chromosomes from the donor somatic cell, the team successfully created embryos from eggs of one mouse and somatic cells from another.
The team worked with cumulus cells (those surrounding oocytes), and after inserting these in eggs, they chemically induced the eggs to mimic normal fertilisation by releasing its spare set of chromosomes into a polar body and also made the body cell expel its spare set. When this process was successful the team ended up with two polar bodies and a fertilised egg with one set of chromosomes.
Lacham-Kaplan admits the technique is in its infancy and there are still biological barriers to overcome. For example, the process ejects genetic material of both maternal and paternal origin. "Ideally we hope genes from both mother and father will be retained, as this will mimic normal fertilisation. But it could be that the retained genes are a random mixture", she says.
If this turns out to be the case, it could present a sizeable problem. Some genes retain a 'memory' of whether they were inherited from the maternal or paternal line, and their lineage determines when and if they will be switched on or off. This phenomenon is known as genetic imprinting and it seems to be crucial for embryonic development. Indeed studies in the 1980s showed that mouse embryos created from two sperm nuclei or two egg nuclei develop abnormally and die before birth. More recently, a study published in Science reports that apparently normal looking cloned animals may have variations in imprinted gene expression, and argues the consequences are unknown (Science 2001, 293: 95-97). Lacham-Kaplan says a random separation of chromosomes effecting the expression of imprinted genes could explain why she and her colleagues saw only limited development of the fertilised eggs to blastocysts.
Lacham-Kaplan stresses that more research needs to be done before the technique is hailed a success. "Once the offspring from animals have been born, we need to look at chromosomal distribution to see whether it is normal. We also need to test the normality of the pups and their ability to found a line of healthy offspring for a few generations. If they are normal, it will provide a rethink of cloning and imprinting."