— ADVERTISEMENT —
Yanagimachi showed the ICSI method could work in hamsters in 1976 and applied it to other animals after it was used successfully in 1992 to produce human babies.
Although it is largely successful, he said he and Morozumi found the production rate might be improved by removing the acrosome, a caplike structure covering the sperm's head, before the injection.
The acrosome can be removed easily and quickly with a chemical or mild detergent, Yanagimachi said.
Enzymes in the acrosome help the sperm penetrate the outer membrane of an unfertilized egg (oocyte) in a natural fertilization process, but the acrosome itself never enters the egg, the researchers explained.
In the fertility treatment, sperm with acrosomes are injected directly into the eggs.
"It is rather amazing that the eggs tolerate the enzymes and develop into an apparently normal offspring," Yanagimachi said.
However, he said, some scientists have speculated that the acrosome could damage a developing embryo if injected.
Morozumi said only 30 percent of eggs injected with human sperm develop successfully, and only 40 percent of women undergoing the procedure go home with babies.
In their investigation, Yanagimachi and Morozumi injected sperm with and without acrosomes into unfertilized mouse eggs.
They found that sperm cells with intact acrosomes caused significant damage and sometimes death to the eggs. They said the damaging effects appear to be due to the enzymes.
Yanagimachi said ICSI success rates have not been high for animals because sperm of some animal species have very large acrosomes, and injection of such sperm could kill the egg.
"Fortune has smiled on human ICSI," he said, explaining that human eggs are relatively large (about one-tenth of 1 millimeter), and a human sperm cell has a small acrosome.
"If the human egg was much smaller or the sperm had large acrosomes, ICSI in humans could have been problematic from its inception," Yanagimachi said.
Still, only four out of 10 couples have babies when injected with sperm with acrosomes, Morozumi said.
Morozumi has worked the past year at the UH Institute of Biogenesis Research, headed by Yanagimachi in the John A. Burns School of Medicine.