A team of reproductive biologists from the United States and Japan has succeeded in fertilizing rabbit oocytes with "dead" freeze-dried rabbit sperm. The fertilized eggs continued to develop into embryos, some of which were transplanted into female rabbits. The researchers - from the University of Connecticut, the University of Hawaii, and Hirosaki University - note that rabbit sperm share many similarities with human sperm, so their results suggest that the freeze-drying technique could be used to preserve sperm from humans and many other animal species.
Previously, only freeze-dried sperm from mice had been shown to support embryo development. Mouse sperm, however, are significantly different from the sperm of most other mammals because they do not contribute a cellular organelle known as a centrosome to the fertilized oocyte. The question remains whether the centrosomes in rabbit sperm survive the freeze-drying or whether centrosomes are not essential for embryos of mammals to develop.
Freeze-drying immobilizes rabbit sperm, breaks plasma membranes, and causes fragmentation of the sperm tails. Nonetheless, the chromosomes remain intact in the "dead" sperm. Even after being stored at temperatures above freezing for more than two years, the treated sperm were as capable as fresh sperm at fertilizing rabbit oocytes.
In a paper published in Biology of Reproduction, the team reports that one rabbit pup was born after 230 oocytes fertilized with freeze-dried sperm were transferred to 8 female rabbits. The full-term pup appeared normal but was still-born, a common outcome of single-birth pregnancies in rabbits.
The researchers believe that improvements in their procedure will someday enable freeze-dried sperm of mammalian species to be stored indefinitely at room temperatures.
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