Fish face some serious osmotic challenges; freshwater fish soak up water while those in the sea lose water to the salty surroundings. Luckily, fish can be flexible in their uptake or excretion of ions; some species switch their gills' ion pumps when they move between fresh and salty water. But how do embryos that don't yet have working gills cope with salinity changes? Junya Hiroi and his colleagues already knew that the yolk-sacs of tilapia embryos contain ion-pumping cells. Now, they have discovered that tilapia embryos deal with salinity challenges by changing the ratio of different ion-pumping cell types in their yolk-sacs (p. 2023).
Hiroi explains that three main ion transport proteins regulate a fish's internal salt levels. To see where these transporters are located in tilapia yolk-sac cells, he used antibodies labelled with three fluorescent colours,each of which binds to a specific ion transporter. Each antibody's fluorescent colour shows up wherever its specific ion transporter is present in a cell.`This was the first time that we could see all three transport proteins at the same time in one cell,' Hiroi says.
Watching the appearance or disappearance of these fluorescent stains over time would allow Hiroi to see how the number of yolk-sac ion-transporting cells changed as the embryos coped with salinity stress. Hiroi first incubated some tilapia eggs in freshwater and others in seawater. After a few days he moved the freshwater embryos to seawater, and vice versa. To see how the embryos' yolk-sac cells changed over time, he incubated the yolk-sacs with the three fluorescent antibodies and examined the stained yolk-sacs under a microscope before he transferred the embryos, and repeated this process 1, 2 and 3 days after transfer.
Hiroi was able to classify four different cell types based on the combinations of ion transporters that he could see in the cells. In the freshwater embryos he saw three cell types, which he labelled type-I, type-II and type-III. When Hiroi moved embryos from fresh to seawater, he saw that the number of type-III cells decreased while type-IV cells appeared and grew in number. This suggests that type-IV cells are ion-secreting cells that help the embryos survive in salty water by pumping out salts. In contrast, seawater embryos started out with many large type-IV cells and only a few type-I and type-III cells. When Hiroi moved seawater embryos to freshwater, he saw that type-IV cells disappeared, type-III cells became more numerous, and type-II cells appeared and also multiplied. Hiroi concludes that type-II cells are freshwater-specific ion absorbers, which scavenge ions to replace lost salts when embryos are in freshwater.
The patterns of mysteriously appearing and disappearing type-III cells suggest an exciting possibility; type-III cells might transform into type-IV cells when freshwater embryos suddenly find themselves in seawater. `But we can't say that this is definitely happening,' Hiroi admits. He hopes to take a closer look at the function of type-III cells once antibodies are developed that can distinguish between absorptive and secretory isoforms of one of the ion transporters found in type-III cells.