Abstract:
This review presents actual knowledge about energetic, ionic, osmotic and gaseous control of fish sperm motility and its duration. Right after they are activated, fish spermatozoa of most species swim for a short period of time, in the range of one to several minutes. What determines the activation process? Is it due to the new ionic, gaseous and/or osmotic environment? Why is the duration of motility so short? Is it resulting from a fast exhaustion of energy stores (ATP, ADP, AMP, PCr) combined with the above-mentioned ionic/osmotic stress leading to morphological alterations? The motility criteria (flagellar beat frequency, head displacement velocity, flagellar waves morphology, etc.) used to characterize fish sperm movement and sperm flagella will be described. Most parameters change very rapidly during the brief motility period of fish sperm. Then will be considered the main environmental factors, ionic and/or osmotic signals, responsible of the activation of fish sperm motility. Then the metabolic compounds involved in cell energetics will be considered as their concentrations also rapidly change during the motility phase. An additional feature will then be discussed concerning the mechanisms by which fish sperm cell can be revived for a second motility round at the end of the first motility period. A model is proposed to explain how external osmolarity can control internal ionic composition, the latter being the key factor controlling flagellar activity.Scientists have long known that some fish are able to switch their sex, either spontaneously or when exposed to steroids. This led them to suspect that a subset of the population of cells in male fish that normally become sperm, called spermatogonia, might be stem cells that have the potential to become either sperm or eggs.
To test their hypothesis, the researchers isolated spermatogonia from the testes of adult rainbow trout and transplanted them into newly hatched trout of both sexes. In male hatchlings, the transplanted cells developed into sperm, while in females they developed into eggs.
The scientists are currently looking into whether eggs could be transformed into sperm. Other researchers have successfully produced sperm from mice stem cells.
Sushi on demand
The technique could be used to rapidly breed inbred strains of domestic or research animals with desired genetic traits, the researchers write.
Tokyo University's Goro Yoshizaki, the study's principal investigator, told LiveScience the technique could be used to establish surrogate breeding programs for animals such as the blue fin tuna, a fish popular for sushi and sashimi in Japan and elsewhere. Populations of the fish are declining worldwide and adult blue fin tunas are expensive and difficult to keep in captivity.
One solution, Yoshizaki said, is to isolate the spermatogonia from adult male tunas and transplant them into both male and female hatchlings of a smaller fish species, such as mackerel. Once transplanted, the germ cells would differentiate into sperm and eggs. The mackerel would then be allowed to reproduce normally, but they would produce tuna instead of mackerel.
The technique could also be used to help save endangered fish species, Goro added. A single male endangered fish could be used to repopulate an entire species.
"Even if the species is extinct, we can restore them by transplanting them into closely related species," Goro said.
