So long, and thanks for all the fish

Fresh fish have clear corneas

I was at a fish market recently, looking to pick out something for dinner. Those in the know about fresh fish have been taught to purchase ones with clear eyes and avoid those that have cloudy corneas.  Among other characteristics of fresh fish, such as a good ocean-like smell, a clear cornea is a sign of freshness while a cloudy cornea means that the fish is old.  Why is that?

Behold the corneal endothelial cell

The cornea is an optically clear structure at the front of your eye that is responsible for a large portion of focusing light as it enters your eye. It is composed of five layers.

The eye in cross-section (Photo credit: Wikipedia)

From front to back, they are the: epithelium, Bowman’s layer, stroma, Descemet’s membrane, and the endothelium. The innermost endothelial cells serve as a bilge pump for the cornea, constantly pumping water out of the cornea so that it can stay clear. Without functioning endothelial cells, your cornea would become swollen with water and be cloudy.

5 layers of the cornea

Even after you (or a fish) dies, the endothelial cells will live on for a while.  That is why a fish’s cornea will remain clear for a few days after it dies… the endothelial cells are still pumping away — removing water from the cornea and keeping it optically clear.  After a while, without nutrients or a living body to sustain them, the endothelial cells die and the cornea becomes cloudy.  Thus, clear corneas are a sign of fresh fish.  Those that have been caught more than a few days prior will have dead endothelial cells and white corneas.

Organ donation

So the good news about endothelial cells is that they can live for a few days after you die. That means that there is time for them to be transplanted to a lucky donor. Corneal transplantation is a vision-restoring surgery that has high success rates in the proper patient. So in addition to saving up to eight lives, organ donors can also save people’s vision.

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(My apologies to Douglas Adams for using the title of his book. And yes, I am an organ donor.)

Related articles

• A letter of thanks to my organ donor (kevinmd.com)
• Using Stem Cells To Repair A Damaged Cornea (medicalnewstoday.com)
• Biosynthetic corneas help restore light–and sight (news.cnet.com)