Scientists Create ‘See-Through’ Fish + Human Cornea + Contact Lenses and Fainting + REM + the Echidna

By | February 10, 2008

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Scientists have created ghostly transparent fish to make human biology clearer. The feat has been achieved with zebrafish are genetically similar to humans and are already in widespread use as models for human biology and disease.

The new see through fish allows scientists to directly view its internal organs, and observe processes like the spread of tumours and blood production after bone-marrow transplant in a living organism, say researchers at Children’s Hospital Boston.

Source: Telegraph
Previously: Scientists Create Transparent Frogs (sept.2007)

Wild. As many artists know, human skin is already slightly transparent. You can see blood vessels under it, and so on. Could this more transparent skin be given to humans? Is there a transparent person in a secret lab in some place like Dulce, New Mexico? One part of our body is already transparent: our corneas.

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eye’s optical power.[1] Together with the lens, the cornea refracts light, and as a result helps the eye to focus, accounting for approximately 80% of its production to 20% of the lens focusing power.[2] The cornea contributes more to the total refraction than the lens does, but, whereas the curvature of the lens can be adjusted to “tune” the focus depending upon the object’s distance, the curvature of the cornea is fixed.

The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid at the outside and the aqueous humour at the inside and also from neurotrophins supplied by nerve fibres that innervate it. In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5 mm – 0.6 mm in the center and 0.6 mm – 0.8 mm at the periphery. Transparency, avascularity, and immunologic privilege makes the cornea a very special tissue. The cornea is the only part of a human body that has no blood supply, it gets oxygen directly through the air.

In humans, the refractive power of the cornea is approximately 43 dioptres, roughly two-thirds of the eye’s total refractive power.[3]

Ah ha! So that’s why I felt like I was suffocating and why I passed out when I tried non gas permeable contact lenses! My eyes told my brain that there was no oxygen in the room, so my brain shut my body’s blood flow down to avoid brain damage! I’ve wondered about this for years.

My theory: In some people the cornea’s oxygen metabolization includes a feedback loop wired to the brain as an oxygen sensor.

The see-through cornea is living tissue and to stay alive, it needs oxygen. The cornea is fairly oxygen hungry, but due to sflt-1, “a free-floating receptor for vascular endothelial growth factor A,” the cornea lacks blood vessels to feed it oxygen. This allows it to be clear.

a word on oxygen: air contains 21% of it if you are at sea level, and less if you are on an intercontinental flight. The concentration of oxygen at the cornea reduces to 8% – 15% if wearing contacts and to some 7% – 8% if eyes are closed, which is the minimum allowable value before cornea damages set in. Strenuous activity raises this minimum. BBC

From this I infer that the cornea gets 1% to 7% less oxygen when the eyes are closed. How does the central anterior cornea get its oxygen when your eyelids are closed? The best answer I’ve found so far is that eyelids don’t stay completely closed and some oxygen may come from the back of the eyelids. I suspect that our rapid eye movement every 90 minutes while we sleep helps deliver oxygen to the cornea. Rapid eye movements come from watching something in our dreams.
This may mean that corneal oxygen is one reason we (and other mammals) dream! Interestingly, echidna are mammals which do not seem to have rapid eye movements. (Dophins do have REM and besides, they are always half asleep. Imagine being half in a dream world all the time!)

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The echidna is a monotreme, an egg laying mammal. Monotremes, which include the platypus, diverged from the placental and marsupial lines very early in mammalian evolution. The platypus, however, has very active REM. The echidna eye is more like the reptilian eye. Reptiles do not have REM. How do reptile corneas get oxygen at night?

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