Optical transparency of the human cornea and lens

[Sophrosyne]

There are two optically transparent tissues in the human body: the cornea and the lens. But how they achieve this transparency is different, and both in turn appear to achieve this differently than in other optically clear materials like glass.

The transmission of light through glass or other clear solid material is thought to occur because the differences in the energy levels of the molecular orbitals in the material, such as silicone dioxide in glass, are too high for light in the visible range of the electromagnetic range to be absorbed. So the visible-wavelength photons pass through unabsorbed.

But from what I am reading, the transmission of light through the biological cornea and lens of the eye appear to have very different mechanisms. The cornea's mechanism seems to be that the collagen tissue making up the cornea has a perfectly ordered hexagonal cross-sectional arrangement, and the spacing between the collagen fibrils is always kept greater than wavelength lambda/2 to allow the phase of the waves to pass through. But the collagen in the cornea is no different than the collagen that's present in the sclera of the eye as well. The sclera is white and opaque because the fibrils are not regularly arranged as they are in the cornea. They are arranged haphazardly, and that makes all the difference. If the fibrils in the cornea are also disrupted in pathologic conditions, as in injury, trauma, or edema, the cornea will lose its transparency like the sclera.

The lens, the other refractive tissue in the eye, seems to follow yet another mechanism. There is no collagen in the lens. It is made of proteins called crystallins. These are apparently clear proteins. But I have not been able to find the exact mechanism through which they maintain such optical clarity. Regular ordering of fibrils like with collagen are clearly not the mechanism, as these proteins are spread haphazardly in the lens and much more densely packed. But I read in one place that the lambda/2 criterion is also important for these proteins to maintain optical clarity, because when this is disrupted through fluid buildup in the lens or cross linking between these crystallin proteins, the lens loses its optical clarity and a cataract ensues.

So here are my questions:

1) Does anyone know more about this lambda/2 criterion as another mechanism for maintaing optical clarity in materials, independently of the electron orbital energy levels?

2) Proteins are chemically/physically extremely complex structures. It shouldn't matter how they are arranged or spaced. You would think a huge amount of the photons should be absorbed by all the elelectrons in the amino acids of the protein, and the efficiency of light transmission through the biological cornea and lens should be very poor- and yet they are not. Why not?

 

[jim mcnamara]

Re: crystallin Joan Roberts teaches this content regularly: http://photobiology.info/Roberts.html
I would contact her via email if the references at the end of the article plus the article do not help. - Please read the article it is very good. And may dispel some of your assumptions about why proteins can/cannot transmit light, if you read it carefully. Hint - "liquid".

The references are good, too.

 

[Sophrosyne]

Re: crystallin Joan Roberts teaches this content regularly: http://photobiology.info/Roberts.html
I would contact her via email if the references at the end of the article plus the article do not help. - Please read the article it is very good. And may dispel some of your assumptions about why proteins can/cannot transmit light, if you read it carefully. Hint - "liquid".

The references are good, too.

Thanks for the reference. It WAS a good article. But I was a little confused because in one section it said: "The human lens is normally transparent until the age of 40 years. This transparency is a result of the orderly arrangement of protein fibers in the lens normally", but then just a little later it says:"All endogenous or exogenous oxidation denatures the lens proteins, reduces their solubility, and eventually, results in a loss of transparency in the lens, which is known as a cataract."

If something is dissolved, it can't be "orderly arrangement". So which is it that is keeping the lens transparent: orderly arrangement or solubility? The article seems to be contradicting itself.

 

[jim mcnamara]

Soluble component. The lens is flexible, at least in younger adults. It is the reason a five year old can look at her finger print at 5 inches from the cornea, but adults cannot focus objects that close. Reduced flexibility of the lens. Correlated with that is a change in the wavelengths of light transmitted. As you age transmission of the violet end of the spectrum declines.

Think of the orderly thing:
It is a superstructure that is mostly open, like the skeletal frame of a skyscraper. If you could fill a skyscraper skeleton with clear liquid, light would pass through. The liquid in this thought experiment is dissolved protein. Ever see purified human serum? It mostly is water, electrolytes, dissolved proteins. It is largely transparent.

Or clear gelatin?
Buy a box at the store and play with it. Dry it is opaque, dissolved transparent. You can denture the gel and watch it become clouded. Kitchen DIY cataract formation model! Hint: rubbing alcohol 99%.

 

[Sophrosyne]

Soluble component. The lens is flexible, at least in younger adults. It is the reason a five year old can look at her finger print at 5 inches from the cornea, but adults cannot focus objects that close. Reduced flexibility of the lens. Correlated with that is a change in the wavelengths of light transmitted. As you age transmission of the violet end of the spectrum declines.

Think of the orderly thing:
It is a superstructure that is mostly open, like the skeletal frame of a skyscraper. If you could fill a skyscraper skeleton with clear liquid, light would pass through. The liquid in this thought experiment is dissolved protein. Ever see purified human serum? It mostly is water, electrolytes, dissolved proteins. It is largely transparent.

Or clear gelatin?
Buy a box at the store and play with it. Dry it is opaque, dissolved transparent. You can denture the gel and watch it become clouded. Kitchen DIY cataract formation model! Hint: rubbing alcohol 99%.

I see. But I would not consider the superstructure of a skyscraper to be soluble. It remains structured exactly because it is all cross-linked and highly organized. And conversely, I would not consider the proteins and electrolytes dissolved in serum to have any kind of structure or organization- if they did, they would not be dissolved. That's why the entropy of things increases when dissolved. These two things, solubility and organization, seem to be mutually exclusive concepts.

 

[jim mcnamara]

I am not getting through. Analogies clearly do not work. Until you get how light goes through the lens, just lose your solubility/organization concept. It is not helpful in the least. The skeleton does let light through. Period. The solubilized (disorganized) protein fraction also let's light through. Period. All at the same time. Disorganized solutions of protein can be transparent at visible wavelengths.

You do realize the lens is tiny.

 

[Ygggdrasil]

In general, cells are opaque not because they absorb light but because they scatter it. Light scattering will often happen where light encounters a change in refractive index, which will usually occur at boundaries between two phases. So, for example, the interface between the aqueous cytosol (fluid inside of the cell) and the lipid-based plasma membrane or the presence of small solids suspended in a liquid (e.g. imagine a fish tank filled with water. You would be able to see through the tank, but you fill the tank with bubbles, you would not be able to see through the tank despite the bubbles not actually absorbing any light).

Cells normally contain quite an extensive network of membranes inside of them, which form the organelles (e.g. endoplasmic reticulum, nucleus, mitochondria) that the cell requires to function. All of these membranes will scatter light and contribute to the opacity of cells and biological tissues. Cells inside the lens lack these organelles and instead are basically just filled with protein, making them much more transparent. As far as I can tell from the quick reading I've done on the subject, these proteins are not organized in any specific way—they are just dissolved in the cytosol at a fairly high concentration. The main purpose seems to be to give the lens the proper index of refraction in order to help the structure focus light.

(As an aside, if you are able to remove the membranes from a piece of tissue, you can render the tissue transparent as researchers from Stanford demonstrated in 2013 by creating and imaging a "see-through brain": )

Proteins, in general, will not absorb visible light. Some of the functional groups on amino acids will absorb UV light (e.g. tryptophan, phenylalanine and tyrosine absorb around 280 nm light), but none absorb in the visible range of the spectrum. Some proteins do absorb visible light but do so because they bind to other components capable of absorbing visible light like transition metal complexes (e.g. iron in hemoglobin) or pigments (e.g. retinal in opsin). So, if you were to have a test tube of a solution containing a protein like crystallin, it should be fully transparent, like looking through pure water. If the protein gets damaged and starts precipitating out of solution as a solid, however, the solution will start to get cloudy (which is essentially what happens in cataracts).

 

[Fervent Freyja]

2. I found: The proteins constituting the major crystallins of the ocular lens are highly ordered and not structured randomly. The amino acids utilized for this structure tend to have higher index of refraction increments. These specific aminos have a higher occurrence of polarizable electrons that determine order by using larger symmetrical polar side chains that help reduce reduce water volume, which allows a denser, more ordered structure to form.

In order to maintain lens transparency, the crystallins must resist the formation of light scattering aggregates. This seems to be done with them being heat-shock treated, allowing structural integrity to be maintained at higher than typical temperatures and prevention of phase transitions caused by temperature changes, especially at this tiny thinness- I assume heat treated is implying that the folding patterns are highly conserved and the resultant shape is resistant to aggregation. We find these have much more reliable thermodynamic stability than typical proteins.

Other controlling, conserving variables conducive to maintaining an ordered structure that supports a high refractive index are that: the hydrodynamics aide in controlling protein folding by it's high density in intracellular crowding of crystallins within the intracellular fluid that help to stabilize the structure, guide the formation of heat-treated, long lasting crystallins, and also serves to keep the proteins bound in solution. I also interpret that this lens structure also acts as a filter for rejecting specific photons in the UV spectrum, significantly improving the efficiency in transmission of light within the visible spectrum.

 

[Sophrosyne]

In general, cells are opaque not because they absorb light but because they scatter it. Light scattering will often happen where light encounters a change in refractive index, which will usually occur at boundaries between two phases. So, for example, the interface between the aqueous cytosol (fluid inside of the cell) and the lipid-based plasma membrane or the presence of small solids suspended in a liquid (e.g. imagine a fish tank filled with water. You would be able to see through the tank, but you fill the tank with bubbles, you would not be able to see through the tank despite the bubbles not actually absorbing any light).

Cells normally contain quite an extensive network of membranes inside of them, which form the organelles (e.g. endoplasmic reticulum, nucleus, mitochondria) that the cell requires to function. All of these membranes will scatter light and contribute to the opacity of cells and biological tissues. Cells inside the lens lack these organelles and instead are basically just filled with protein, making them much more transparent. As far as I can tell from the quick reading I've done on the subject, these proteins are not organized in any specific way—they are just dissolved in the cytosol at a fairly high concentration. The main purpose seems to be to give the lens the proper index of refraction in order to help the structure focus light.

(As an aside, if you are able to remove the membranes from a piece of tissue, you can render the tissue transparent as researchers from Stanford demonstrated in 2013 by creating and imaging a "see-through brain": )

Proteins, in general, will not absorb visible light. Some of the functional groups on amino acids will absorb UV light (e.g. tryptophan, phenylalanine and tyrosine absorb around 280 nm light), but none absorb in the visible range of the spectrum. Some proteins do absorb visible light but do so because they bind to other components capable of absorbing visible light like transition metal complexes (e.g. iron in hemoglobin) or pigments (e.g. retinal in opsin). So, if you were to have a test tube of a solution containing a protein like crystallin, it should be fully transparent, like looking through pure water. If the protein gets damaged and starts precipitating out of solution as a solid, however, the solution will start to get cloudy (which is essentially what happens in cataracts).

Thanks for that explanation! This makes a lot of sense. The reason for the opacity of tissues like heart, liver, etc... it is not that the protein structures are necessarily absorbing light, but because, like you said, of scattering, and also because of pigmented compounds in them like myoglobin, cytochromes, etc... which absorb at specific wavelengths and give the different organs and tissues their distinctive colors.

This also helps explain, to some extent, what is happening in the cornea as well. The cornea has very few cells in it. Most of it is just collagen (unlike the lens, which is mostly crystallins) arranged in highly ordered parallel lamellar arrays. The amino acids in the collagen are not absorbing in the visible wavelength, so light passes through.

But that still leaves the question of why the collagen in the cornea needs to so perfectly arrayed to remain optically transparent, but the crystallins in the lens don't (apparently) have to be. I was reading about this magic distance of the collagen fibers not getting closer than lambda/2 of the wavelength in the cornea before losing transparency. But the protein arrangement in the lens is NOT regularly arranged like in the collagen. I have been trying to find the supposedly organized structure of crystallins which Jim McNamara here has been mentioning, and I haven't yet been able to find it. I do understand what he is talking about though in terms of solubility. If anyone has references on this, I would be interested. The lens cells just seem to get rid of their organelles and stuff their cytoplasm with these crystalline proteins, not necessarily in any organized manner. There seems to be something quite different at the molecular level between collagen as a structural protein in the cornea, and crystallins as structural proteins in the lens. One needs to have a very regular arrangement to remain transparent, while the other apparently does not.

 

[Sophrosyne]

2. I found: The proteins constituting the major crystallins of the ocular lens are highly ordered and not structured randomly. The amino acids utilized for this structure tend to have higher index of refraction increments. These specific aminos have a higher occurrence of polarizable electrons that determine order by using larger symmetrical polar side chains that help reduce reduce water volume, which allows a denser, more ordered structure to form.

In order to maintain lens transparency, the crystallins must resist the formation of light scattering aggregates. This seems to be done with them being heat-shock treated, allowing structural integrity to be maintained at higher than typical temperatures and prevention of phase transitions caused by temperature changes, especially at this tiny thinness- I assume heat treated is implying that the folding patterns are highly conserved and the resultant shape is resistant to aggregation. We find these have much more reliable thermodynamic stability than typical proteins.

Other controlling, conserving variables conducive to maintaining an ordered structure that supports a high refractive index are that: the hydrodynamics aide in controlling protein folding by it's high density in intracellular crowding of crystallins within the intracellular fluid that help to stabilize the structure, guide the formation of heat-treated, long lasting crystallins, and also serves to keep the proteins bound in solution. I also interpret that this lens structure also acts as a filter for rejecting specific photons in the UV spectrum, significantly improving the efficiency in transmission of light within the visible spectrum.

Whoa! Now this sounds like a VERY good explanation- I just have to wrap my head around it a little. I am not sure I am quite understanding everything you just said there. Do you have any references/links for this information?

 

[Ygggdrasil]

But that still leaves the question of why the collagen in the cornea needs to so perfectly arrayed to remain optically transparent, but the crystallins in the lens don't (apparently) have to be. I was reading about this magic distance of the collagen fibers not getting closer than lambda/2 of the wavelength in the cornea before losing transparency. But the protein arrangement in the lens is NOT regularly arranged like in the collagen. I have been trying to find the supposedly organized structure of crystallins which Jim McNamara here has been mentioning, and I haven't yet been able to find it. I do understand what he is talking about though in terms of solubility. If anyone has references on this, I would be interested. The lens cells just seem to get rid of their organelles and stuff their cytoplasm with these crystalline proteins, not necessarily in any organized manner. There seems to be something quite different at the molecular level between collagen as a structural protein in the cornea, and crystallins as structural proteins in the lens. One needs to have a very regular arrangement to remain transparent, while the other apparently does not.

I don't know much about the cornea specifically, but just reading from the Wikipedia article on the corneal stroma, it seems like how the cornea remains transparent is not fully understood:

Corneal stroma (also substantia propria): a thick, transparent middle layer, consisting of regularly arranged collagen fibers along with sparsely distributed interconnected keratocytes, which are the cells for general repair and maintenance.[9] They are parallel and are superimposed like book pages. The corneal stroma consists of approximately 200 layers of mainly type I collagen fibrils. Each layer is 1.5-2.5 μm. Up to 90% of the corneal thickness is composed of stroma.[9] There are 2 theories of how transparency in the cornea comes about:

1. The lattice arrangements of the collagen fibrils in the stroma. The light scatter by individual fibrils is canceled by destructive interference from the scattered light from other individual fibrils.[11]
2. The spacing of the neighboring collagen fibrils in the stroma must be < 200 nm for there to be transparency. (Goldman and Benedek)

https://en.wikipedia.org/wiki/Cornea#Layers

 

[Fervent Freyja]

Whoa! Now this sounds like a VERY good explanation- I just have to wrap my head around it a little. I am not sure I am quite understanding everything you just said there. Do you have any references/links for this information?

I'm trying to tell you that the structure of the ocular lens is highly organized, almost comparable to that of the cornea. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3892301/