BioWorld International Correspondent
LONDON - The plane in which the fertilized mouse egg makes its first division is determined by the point at which the sperm enters the egg, developmental biologists in Cambridge, UK, have found. Their startling discovery, which links the sperm entry point with the future pattern of development of mammalian embryos for the first time, could revolutionize developmental biologists' view of the events immediately following fertilization.
The studies by Karolina Piotrowska and Magdalena Zernicka-Goetz also suggest that the sperm entry position marks the separation of cells that will form the early embryo from those that go on to form surrounding tissues. They describe their experiments in a paper in the Jan. 25, 2001, Nature in a paper titled, "Role for sperm in spatial patterning of the early mouse embryo".
Zernicka-Goetz, a senior research fellow at the Wellcome/Cancer Research Campaign Institute and at the Department of Genetics of the University of Cambridge, told BioWorld International that teams carrying out sperm injection in human in vitro fertilization programs have shown great interest in the work. "It is obviously important to bear this knowledge in mind when doing in vitro fertilization in humans," she said. "At a minimum, it is interesting to remember that the site on the egg where we inject the sperm in humans is probably the area where important future events will occur."
Roger Pedersen, of the Reproductive Genetics Unit at the University of California in San Francisco, commenting on the paper in a News and Views article titled "Sperm and mammalian polarity," said the implication is "sobering."
He writes: "Might [injection of sperm into the egg] be defining an embryonic axis, or even the future body pattern of the child? With this possibility in mind, it becomes urgent to find out exactly how sperm affect mouse patterning."
For developmental biologists in general, however, the finding rebuts the traditional view that the organization of the mammalian embryo is determined only quite late in development, following implantation in the uterus.
"Our finding," Zernicka-Goetz said, "is so important because it comes at a time when researchers are beginning to re-evaluate what the mammalian embryo is doing in its first few days of existence. Previously, it was thought that these days were devoted mainly to making tissues whose role is protective and supportive for the embryo developing in the uterus, but we now know that this is not the case. The early forming tissues appear also to play an important role in specifying the pattern of the future embryo."
Zernicka-Goetz, who put forward a hypothesis several years ago that the pattern of development could be predicted very soon after fertilization, developed a method of tracking the position of the sperm entry point during subsequent development. This involved coating a fluorescent bead with phytohaemagglutin so that it would stick to the egg. The beads remained on the surface of the eggs, and were not engulfed by the cells.
At the point where the sperm has entered the egg, a protrusion forms. For the study reported in Nature, Piotrowska and Zernicka-Goetz placed a bead on the tip of that protrusion and then watched where the bead ended up over the next few days.
There were three main findings. The first was that the sperm entry point defines where the first cleavage plane of the fertilized egg will be. Secondly, following the first division of the fertilized egg, which leads to the formation of two blastomeres (cells), the blastomere that inherits the sperm entry position is the one that divides ahead of its sister. Thirdly, at the blastocyst stage (where the embryo has a fluid-filled cavity attached to a mass of stem cells), the site of sperm entry defines the border between two halves of the embryo - the embryonic part, which contains all the cells needed to form the future body, and the abembryonic part, which gives rise to the tissues that help the embryo implant into the uterus, as well as some tissues that play an important role in specifying body pattern.
"We now want to find out what the molecular aspects of how the cleavage plane is defined are, and what factors cause the blastomere that inherits the sperm entry position to divide ahead of its sister cell," Zernicka-Goetz said. She will also be conducting experiments to find out if the same events occur in mouse eggs if the sperm is injected, as it is in treatment for human male infertility, rather than being allowed to penetrate the egg in the normal way.
She concluded: "This finding is one more example of post-genomic biology. As we enter the post-genomic era, we begin to realize that genes and our knowledge of their sequences do not directly enable us to determine the structure of developing organisms."
The genome functions in a living cell, she said. "The egg, which after all is just a complex living cell, provides the context in which the sperm entry position itself is informative in a way that goes beyond the genomic content of the sperm head. We can now bring all the beautiful power of genomics to bear on this organizational issue, and I think only then will we be able to understand the complexity of this process."