Fertilization Really Isn't Magic!

         The creation of a new life first requires the fusion of an egg with sperm. This initiates a cascade of events that drives the division of the newly fertilized egg. Though this phenomenon is considered to be a fundamental part of biology, there are many aspects of fertilization that are unknown. For example, the mechanism for sperm-egg partnering has been nebulous for many years. In 2006, a breakthrough was made in which a protein called Izumo1 was discovered on the surface of mouse sperm. Inoue and colleagues at Osaka University in Japan found that male mice lacking this protein were sterile. Interestingly, sperm lacking this protein were observed to bind to an unfertilized egg, but they were unable to undergo fusion. These results led the group to conclude that Izumo1 was necessary for sperm-egg recognition, but it also raised a new question surrounding the identity of Izumo1's binding partner on the surface of the egg.

         A new report in Nature has proposed an answer to this question. A study from Dr. Gavin Wright's laboratory at the Sanger Institute in Cambridge, England, have identified a protein on the surface of the egg called Juno. This protein binds to Izumo1, and it is essential for egg-sperm recognition since fertilization does not occur in female mice lacking this protein. An intriguing facet of this report is the disappearance of Juno protein from the egg surface after fertilization. This occurrence may be responsible for limiting eggs to one fertilization event since it takes place within forty minutes of fertilization. These findings led Wright's group to conclude that Juno is the long-lost partner of Izumo1, and the presence of these proteins in humans indicates that identical interactions occur during the inception of human life.

        Both these studies raise a new set of questions. How does binding of Izumo1 to Juno lead to the surface of the egg becoming more permeable? Which signaling events are taking place in the cell to remove Juno from the egg surface after fertilization? More work will have to be done to fully understand all events occurring during the fusion of egg and sperm. It will also be useful to determine whether a reduction of either protein in humans can serve as an underlying cause of infertility. Conversely, it may be possible that an increase in Juno protein on the egg surface can facilitate fertilization. It's a wonder that these protein-binding events spur the wonderful journey that we call life! 

A fertilized human egg after in-vitro fertilization.
Courtesy: Advanced Fertility Center of Chicago; http://www.advancedfertility.com/embryos.htm