Consider the amazing eye.
The cornea is a remarkable tissue that is both alive and transparent due to its unique structure and specialized mechanisms.
Firstly, the cornea is composed of several layers of cells that make up its structure. The outermost layer is composed of epithelial cells, which act as a protective barrier against foreign substances and provide nourishment to the underlying layers. This layer continuously regenerates to maintain the integrity of the cornea.
Beneath the epithelium, there is a layer called Bowman’s layer, followed by the stroma, which constitutes the majority of the cornea. The stroma is made up of collagen fibers that are tightly arranged in a regular pattern, contributing to the transparency of the cornea. These collagen fibers scatter and transmit light rather than absorbing it, resulting in a clear cornea.
Collagen fibers in the stroma of tissues can scatter and transmit light rather than absorbing it due to their physical properties. Here’s an explanation of how this happens:
1. Refractive Index: Collagen, being a transparent protein, has a refractive index similar to that of the surrounding tissue. When light encounters a boundary between two materials with different refractive indices, it can either be absorbed, transmitted, or scattered. Collagen’s refractive index allows it to transmit light to a certain extent without significant absorption.
2. Fiber Arrangement: Collagen fibers are elongated structures that are arranged in a specific pattern within the stroma. The fibers can be densely packed or loosely dispersed, depending on the tissue. This arrangement causes light to scatter as it passes through the stroma, leading to diffusion rather than absorption. The scattering occurs as the light interacts with the irregularities and variations in the density and orientation of the collagen fibers.
3. Size of Fibers: The size and diameter of collagen fibers also contribute to light scattering. If the fibers are smaller than the wavelength of light, they can scatter the light in various directions, allowing transmission and diffusion. The interaction between light and these sub-wavelength fibers leads to scattering without significant absorption.
4. Lack of Pigments: Unlike other pigmented tissues, collagen fibers do not contain specific chromophores or pigments that absorb light. This absence of absorbing molecules contributes to the transparency of collagen, allowing light to pass through and scatter instead of being absorbed.
It is important to note that the exact scattering and transmission characteristics of collagen fibers in the stroma can vary depending on several factors, including tissue type, density, and collagen composition.
Collagen forms long fibers that are tightly packed and arranged parallel to each other. These fibers are composed of individual collagen molecules that are cross-linked together. This fibrous arrangement allows light to pass through the collagen without scattering, resulting in transparency. Collagen is made up of three polypeptide chains that are coiled around each other in a spiral shape, forming a triple helix structure. This structure strengthens its fibrous arrangement and contributes to its transparency. The orderly alignment of the chains minimizes light scattering.
Finally, at the innermost layer of the cornea, we find the endothelium. The endothelial cells function to regulate the fluid content of the cornea, preventing it from swelling and maintaining its transparency. These cells actively pump fluid out of the cornea, maintaining the proper balance of hydration.
Apart from its structural composition, the cornea also possesses mechanisms that enhance its transparency. The absence of blood vessels within the cornea eliminates the possibility of light-scattering caused by red blood cells. Additionally, the regular arrangement of collagen fibers in the stroma and the absence of pigment-producing cells contribute to the cornea’s transparency.
Overall, the cornea is able to maintain its transparency while being alive due to its unique cellular composition, organized structure, absence of blood vessels, and active maintenance of hydration by the endothelial cells. These factors work together to ensure that light passes through the cornea with minimal scattering, allowing for clear vision.
Getting Oxygen
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.
The cells of the cornea receive oxygen through two main mechanisms:
1. Atmospheric Oxygen: During the day, when the eyes are open, the outermost layer of the cornea, called the tear film, is exposed to oxygen in the atmosphere. Oxygen from the tear film diffuses through the cornea, reaching the cells and providing them with the necessary oxygen supply.
2. Oxygen Supply from Limbal Blood Vessels: The limbus is the peripheral region of the cornea where blood vessels are present. These blood vessels supply oxygen to the cells of the cornea, particularly the deeper layers. During the day, the blood vessels in the limbus provide oxygen to the cornea directly. However, at night, when the eyes are closed and the cornea is covered by the eyelids, the oxygen supply from atmospheric oxygen is reduced. In such situations, the oxygen supply mainly depends on the diffusion of oxygen from the limbal blood vessels.
It’s important to note that the cornea has a high oxygen demand, and any decrease in oxygen availability can lead to corneal hypoxia, causing discomfort and potential damage to the corneal cells. This is one of the reasons why contact lenses, which restrict oxygen flow to the cornea, have recommended wearing times and require proper care to ensure the cornea receives enough oxygen.
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!
Fainting from Contact Lenses
I passed out when I tried non gas permeable contact lenses. I was told that some people do, it is not that uncommon. Now I understand why. 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.
Interesting Facts Summarized
From Wikipedia:
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. 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. 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]
