How are cells of the cornea transparent?

We know that cells have mitochondria, chromosomes and other organelles so that's why they are opaque but can you say how the cells present in the eye cornea are transparent and allow light to pass through?

Most cells are transparent, which is why we can see cells using bright-field microscopy, where the light travels through the cells before you see it. That's how we can see inside onion cells and see structures like onion cell nucleii, moss cells, human epithelial cells and animal mast cells.

The human cornea is made up of five layers, each of which have different structures allowing them to be transparent: the outer and inner epithelial layers are very thin and naturally pretty transparent, and the middle layers have collagen fibres; based on that Wikipedia article, it sounds like there are several hypotheses for why these layers are transparent, but it doesn't look like there's a definitive answer yet.

Corneal endothelium

The corneal endothelium is a single layer of endothelial cells on the inner surface of the cornea. It faces the chamber formed between the cornea and the iris.

The corneal endothelium are specialized, flattened, mitochondria-rich cells that line the posterior surface of the cornea and face the anterior chamber of the eye. The corneal endothelium governs fluid and solute transport across the posterior surface of the cornea and maintains the cornea in the slightly dehydrated state that is required for optical transparency.

Tissue Engineering of Nearly Transparent Corneal Stroma

We determined whether a polyglycolic acid (PGA) scaffold bearing an adherent corneal stromal cell insert could be integrated into the ultrastructure of rabbit corneal stroma without compromising tissue transparency. Stromal cells were isolated from 10 newborn rabbits and expanded by tissue culture. After reaching confluence, the cells were harvested and mixed with nonwoven PGA fibers to form cell–scaffold constructs. After 1 week of culturing, they were implanted into the corneal stroma of female rabbit recipients. Green fluorescent protein (GFP) expression in transplanted corneal stromal cells was monitored at the protein level to determine cellular origin in the reconstructed stroma. Eight weeks after implantation, transmission electron microscopy and histological evaluation were performed on stromal tissue. Insertion of PGA scaffold alone served as a sham control. After stromal implantation, implants gradually became transparent over an 8-week period. During this time stromal histology was gradually restored, as collagen fibril organization approached that of their normal counterpart. GFP-labeled corneal stromal cells were preponderant in the regions bearing inserted scaffolds, suggesting that they were derived from the implants rather than from neighboring corneal stromal cells. No reconstructed stroma developed in regions where naked PGA was implanted instead. We conclude that intrastromal implantation of PGA fiber scaffold implants bearing corneal stromal cells is a useful procedure for corneal stromal tissue reconstruction because, over an 8-week period, the implants become progressively more transparent. Marked losses of translucence during this period combined with restoration of ultrastructure indicate that the implants provide the functions needed for deturgescing initially swollen stroma.

Role Of Stem Cells In Renewing The Cornea

A group of researchers in Lausanne, Switzerland has published a study appearing in the Oct 1 advance online edition of the Journal Nature that shows how the cornea uses stem cells to repair itself.

Using mouse models they demonstrate that everyday wear and tear on the cornea is repaired from stem cells residing in the corneal epithelium, and that more serious repair jobs require the involvement of other stem cells that migrate from the limbus, a region between the cornea and the conjunctiva, the white part of the eye.

The integrity of the cornea, the transparent outer layer of the eye, is critical for vision. Millions of people around the world suffer from partial or complete blindness when their corneas lose transparency. Treatment options involve corneal transplants and, more recently, stem cell therapy. The surface of the cornea is naturally in a state of constant renewal its upper layer, or epithelium, is completely turned over once every 7-14 days. Because slow-cycling stem cells have been found in the mouse limbus, researchers have assumed that these stem cells are the ones responsible for corneal renewal.

The research led by Professor Yann Barrandon, who holds a joint appointment at EPFL and the Lausanne University Hospitals (CHUV), challenges this prevailing opinion that the limbus is the only place where corneal stem cells reside. The researchers demonstrated that the epithelium of the cornea also contains stem cells, and that these cells have the capacity to generate two different epithelial tissues: corneal (covering the transparent part of the eye) and conjunctival (covering the white part of the eye). They demonstrated experimentally that these are the cells activated in everyday corneal renewal. The stem cells residing in the limbus have a different role they are only activated when the cornea is seriously wounded.

To explain this distribution of stem cells and the different roles played by stem cells in different zones of the eye, the researchers propose that the expanding epithelia of the cornea and the conjunctiva act like tectonic plates, squeezing the limbus between them into a kind of equilibrium zone. Due to the constant expansion, stem cells accumulate in this zone. In the event of a rupture in the equilibrium, such as a large corneal injury, these limbal stem cells migrate into the cornea and conjunctiva and differentiate into the appropriate cell type to make repairs.

The limbus is already recognized as a source of cells for corneal stem cell therapy in humans, and this new research indicates that the cornea itself can also be explored as a potential source of these cells. And because cancer has been associated with the presence of adult stem cells, the model also helps explain why transitional zones like the limbus, where stem cells accumulate, are sites where cancer tends to occur more frequently.

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Molecular Biology of Eye Disease, 2015. ed. / John M. Nickerson J. Fielding Hejtmancik. Elsevier B.V., 2015. p. 7-23 (Progress in Molecular Biology and Translational Science Vol. 134).

Research output : Chapter in Book/Report/Conference proceeding › Conference contribution

T1 - Overview of the Cornea

T2 - Structure, Function, and Development

N1 - Funding Information: This work was supported by NIH Grants K12 EY015025 (A.O.E.) and R01 EY016835 (J.D.G.). Publisher Copyright: © 2015 Elsevier Inc.

N2 - The cornea is a transparent tissue with significant refractive and barrier functions. The epithelium serves as the principal barrier to fluid and pathogens, a function performed through production of tight junctions, and constant repopulation through differentiation and maturation of dividing cells in its basal cell layer. It is supported posteriorly by basement membrane and Bowman's layer and assists in maintenance of stromal dehydration. The stroma composes the majority of corneal volume, provides support and clarity, and assists in ocular immunity. The posterior cornea, composed of Descemet membrane and endothelium, is essential for stromal dehydration, maintained through tight junctions and endothelial pumps. Corneal development begins with primitive formation of epithelium and lens, followed by waves of migration from cells of neural crest origin between these two structures to produce the stroma and endothelium. Descemet membrane is secreted by the latter and gradually thickens.

AB - The cornea is a transparent tissue with significant refractive and barrier functions. The epithelium serves as the principal barrier to fluid and pathogens, a function performed through production of tight junctions, and constant repopulation through differentiation and maturation of dividing cells in its basal cell layer. It is supported posteriorly by basement membrane and Bowman's layer and assists in maintenance of stromal dehydration. The stroma composes the majority of corneal volume, provides support and clarity, and assists in ocular immunity. The posterior cornea, composed of Descemet membrane and endothelium, is essential for stromal dehydration, maintained through tight junctions and endothelial pumps. Corneal development begins with primitive formation of epithelium and lens, followed by waves of migration from cells of neural crest origin between these two structures to produce the stroma and endothelium. Descemet membrane is secreted by the latter and gradually thickens.

Behind the scenes of the world's first commercial stem-cell therapy

Graziella Pellegrini explains the 25-year struggle to develop Holoclar, a treatment for blindness.

Last month saw a major landmark for regenerative medicine: the first time that a stem-cell therapy — beside the use of cells extracted from bone marrow or umbilical cord blood — had been cleared for sale by any regulatory agency in the world. The European Commission approved Holoclar for use in cases of blindness caused by burning. The achievement is all the more remarkable because Holoclar was developed by a small laboratory in Italy, a country better known for its lack of support for life sciences — and for its recent tolerance of an unproven stem-cell concoction, marketed by the Stamina Foundation, that claimed to be a panacea for many diseases. Nature talked to Graziella Pellegrini from the University of Modena about how she and her colleagues overcame the many obstacles to take the therapy from bench to bedside.

What exactly is Holoclar and how does it work?

The surface of the cornea — the transparent tissue that sits in front of the iris — is constantly renewed in a healthy eye, to keep it smooth and clear. New corneal cells are generated from a niche of stem cells in the limbus, an area between the cornea and the white of the eye. But if the limbus is destroyed by burning, then the white of the eye grows over the cornea and becomes criss-crossed with blood vessels. This causes chronic pain and inflammation, as well as blindness.

“ I had seen patients who had starting seeing again after 20 years of blindness: how could I stop? ”

Holoclar treatment can help to reverse these symptoms by adding new stem cells to seed the regrowth of a transparent cornea. But there must be enough surviving limbus in one eye to allow 1 or 2 square millimetres of tissue to be extracted. This tissue is then cultivated on a support made from modified human fibrin (a biodegradable blood protein) under stringent clinical conditions until at least 3,000 stem cells have been generated. The culture, still on its fibrin scaffold, is transplanted into the injured eye, which has been scraped clear of the invading white, and from there stem cells seed the regrowth of a transparent cornea, free of blood vessels, within a year.

How many people will Holoclar be able to help?

Only around 1,000 people annually in the whole of Europe will be eligible: burns victims who have become blind but whose eyes have not been too extensively destroyed.

Was it hard to achieve this working in Italy?

It is always very hard to find research money in Italy. We had to uproot many times. I first started working on the concept of the therapy, with my colleague Michele De Luca, in 1990 when we were post-docs at the University of Genova studying the fundamental biology of epithelial cells — the cells that form the sheets lining organs, and also the skin. In 1996, we moved to Rome to the Institute Dermopatico Immaculate, a hospital run by priests who were highly committed to research and who offered us wonderful facilities and access to patients. But in the end they did not want to support our eye work through to the clinic. So in 2002, we moved to the Veneto Eye Bank Foundation in Venice, which had an epithelial stem-cell laboratory. Then in 2008 we moved again, to the Centre for Regenerative Medicine Stefano Ferrari, which had been newly created at the University of Modena specifically to incubate such types of advanced therapy.

Italy is not supportive of biomedical research. Things might have been easier if we had not had to struggle so much. But I am Italian, and the best way to stimulate me to find a solution is to tell me I can’t do something. And despite the problems, research into advanced therapies does have a history here. One of the world’s first gene-therapy trials — on children with an immunodeficiency disorder — was carried out in Milan.

When did you first start clinical testing?

We published the results of our first two patients — both successes — in 1997 1 . That was proof of principle that the therapy could work. Our major clinical paper, on 112 patients, was published in 2010 2 . Around 77% of the transplants were fully successful, and a further 13% partially successful.

“ Pharmaceutical companies are not much interested in unprofitable rare diseases, so it is important that scientists like ourselves get involved. ”

Why did it take so long to finally get market approval?

Two reasons. First, regenerative medicine was in its infancy, and technologies still needed to be developed to make the treatment more reliable. For example, the limbus includes several different types of cell, and in those early days we had no way of knowing what proportion of the cultured cells to be transplanted were actually the stem cells we needed. In 2001, we discovered a cellular marker that allowed us to do this. Also, the original grafts were very fragile because we had not yet worked out how to culture the limbus cells on a scaffold.

Then, in 2007, new European Union (EU) regulations on advanced therapies came in, which added more — very frustrating — years to the development.

What did those new EU regulations mean for you?

They gave us many years of hell. They were designed to ensure that new types of biologically based therapies were safe, and that they were standardized — so that each patient reliably gets the same type of biological material. These aims are, of course, appropriate and necessary. But at the beginning, this was new territory for everyone, and we quickly learnt that the regulators and we scientists spoke entirely different languages. They didn't understand much about cells and we didn't understand what they really wanted from us. To us, it seemed that the therapy was already there — the clinical data were there and the technology was defined. But it seemed that we had to start from scratch.

We linked up with a pharmaceutical company in nearby Parma that helped us to form a spin-off company, Holostem. The company took on the regulatory challenges and made our product compliant with 'good manufacturing practice' (GMP). Slowly we got to understand what the European Medicines Agency and our national regulators were afraid of, and we began planning experiments to give them the reassurances they needed.

During the many years that you were developing a therapy that could benefit only a few people, and were complying with complicated regulations, the Stamina Foundation was bypassing regulations and treating people in Italy with an unproven stem-cell therapy that it claimed could cure a wide range of diseases. What did you think about that?

Of course, it made me very angry. Stamina should never have happened. It made me realize that the general public and politicians simply do not know enough about stem cells, so people can be exploited. I do a lot of outreach work, and think that scientists need to pay attention to this aspect.

When things got difficult, did you ever think of giving up?

No, never. I had seen patients who had starting seeing again after 20 years of blindness: how could I stop?

I am now thinking about other rare diseases in which a genetic mutation causes disorders of the cornea. The idea is to develop therapies for those pathologies without actually correcting the gene, and so reducing any possible risk associated with genetic engineering. As yet, we don’t have a perfect model to work with, so we will be assessing outcomes with several different models.

Another rare disease? Wouldn’t you prefer to develop therapies that will make you rich?

You only have one life, and I believe you should use it to do something useful. Pharmaceutical companies are not much interested in unprofitable rare diseases, so it is important that scientists like ourselves get involved.

Scientists Characterize Proteome Of Human Cornea

Bethesda, MD -- An international group of researchers has characterizedthe proteome of the human cornea. In doing so, they have identified 141distinct proteins, 99 of which had not been previously recognized inmammalian corneas. The details of their findings appear in theAugust/September issue of Molecular and Cellular Proteomics, anAmerican Society for Biochemistry and Molecular Biology journal.

The cornea is the transparent, dome-shaped window that covers thefront of the eye. Although it is clear and seems to lack substance, thecornea is actually a highly organized group of cells and proteins. Itsfunctions include shielding the eye from germs, dust, UV light, andother harmful matter and acting as the eye's outermost lens.

Approximately 120 million people in the United States weareyeglasses or contact lenses to correct nearsightedness,farsightedness, or astigmatism. These vision disorders are often theresult of incorrect curvature or irregular shape of the cornea and arethe most common vision disorders in this country. Other diseases thataffect the cornea range from bacterial, fungal, and viral infections(keratitis) and allergies to various dystrophies including keratoconus.

"Corneal damage and disorders account for several million casesof impaired vision and are second to cataracts as the most importantcause of blindness in the world," explains study author Dr. Jan J.Enghild of the University of Aarhus in Denmark. "Corneal infections bybacteria, fungi, or viruses are common disorders that can lead tocorneal opacification. A group of inherited corneal disorders includinggranular and lattice corneal dystrophies are characterized bydeposition of insoluble and opaque macromolecules in the cornea. Otherdisorders associated with loss of corneal transparency arise fromcornea swelling (Fuchs' dystrophy) or thinning and change of curvatureof the cornea (keratoconus)."

In order to learn more about the cornea and corneal disorders,Dr. Enghild and colleagues characterized the most abundant proteinsfound in the non-diseased human cornea. They identified 141 distinctproteins, 70% of which have not previously been identified in thecornea. This work is the most comprehensive protein study of the corneato date.

"Surprisingly, about 15% of the identified proteins in thecornea are classical blood proteins, which indicate that they originatefrom the blood stream around the cornea and are not produced in thecornea," notes Dr. Enghild. "Our results also showed that proteolysisand post-translational modifications of proteins are common events inthe normal human cornea."

Among the molecules that the scientists identified wereproteins involved in antimicrobial defense, heme and iron transport,tissue protection against UV-radiation and oxidative stress. Severalother proteins were known antiangiogenic factors, which prevent theformation of blood vessels.

The results from this research may open the door to futuretherapeutics for a myriad of corneal disorders. "It is essential toknow the biochemical composition of normal healthy corneas in theeffort to understand the molecular mechanisms behind cornealdisorders," emphasizes Dr. Enghild. "By comparative proteomic studiesof diseased and normal corneas we can identify differences in theexpression profiles that may suggest avenues for therapeuticinterventions. Because the cornea is so accessible, the potential fordeveloping effective drugs for the treatment of corneal diseases isgood. Furthermore, the work is likely to improve the clinicalclassifications of corneal diseases. Identification of the proteinprofile of the normal human cornea may also be very useful in theeffort toward generating artificial corneas for transplantation."

To follow up on their initial research, Dr. Enghild and hiscolleagues have begun proteomic studies of corneas affected by granularand lattice corneal dystrophies, and are also planning on looking atother cornea diseases such as keratoconus and Fuchs' dystrophy.

The American Society for Biochemistry and Molecular Biology (ASBMB)is a nonprofit scientific and educational organization with over 11,000members in the United States and internationally. Most members teachand conduct research at colleges and universities. Others conductresearch in various government laboratories, nonprofit researchinstitutions, and industry.

Founded in 1906, the Society is based in Bethesda, Maryland, onthe campus of the Federation of American Societies for ExperimentalBiology. The Society's primary purpose is to advance the sciences ofbiochemistry and molecular biology through its publications, theJournal of Biological Chemistry, the Journal of Lipid Research,Molecular and Cellular Proteomics, and Biochemistry and MolecularBiology Education, and the holding of scientific meetings.

Corneal Regeneration Laboratory

The Corneal Regeneration laboratory focuses on the cornea, an organ that provides a visual portal to the world. The connective tissue of cornea (stroma) is extremely tough, and transparent to light. It also presents a significant biological barrier to infection. Globally, millions of patients have corneal opacification due to disease or trauma, hence vision loss. Our work focuses on the biological processes that produce and maintain the unique tissue of corneal stroma as well as the pathological changes that occur during injury, wound healing, scarring and diseases. We explore new designs to reverse the scarring process or replace the scarred cornea with bioengineered corneal tissue. Our lab has reported the use of stromal keratocytes and stromal stem cells to restore corneal transparency. These cell-based treatments produce tissue identical to that of the transparent corneal stromal tissue in animal models of corneal injury. We are developing GMP compliant Standard Operating Procedure for clinical trials in patients with corneal scarring. We are also actively investigating the mechanism by which the stem cells induce tissue regeneration, including exosomes, cytokines, and microRNAs.


In most people, the cornea is somewhat oval in shape, and it is thicker at the edges than it is at the center. The cornea sits above the iris and the lens.


Although it is very thin and transparent, the cornea is made up of five separate tissue layers.

  • Epithelium: These cells produce a thin, glistening “skin” layer on the outer cornea.
  • Bowman’s layer: Also called Bowman’s membrane, this thin tissue layer is made up of collagen cells that provide structure to the cornea.
  • Stroma: The thickest layer of the cornea, the stroma is composed of collagen cells.
  • Descemet’s membrane: This very thin layer of cells provides some elasticity to the cornea's structure.
  • Endothelium: A single layer of cells on the innermost part of the cornea, the endothelium maintains the cornea’s crystal clearness.

Anatomical Variations

Congenital (present at birth) abnormalities of the cornea do occur, and they usually cause the cornea to be cloudy instead of clear. When these abnormalities occur, they often appear in conjunction with other medical conditions, including:

  • Congenital brain abnormalities
  • Heart defects
  • Abnormalities of craniofacial (head and face) development
  • Inherited corneal defects

What kinds of corneal surgery might I have?

If you need surgery to treat a disease or disorder in your cornea, you might have:

    Descemet's stripping endothelial keratoplasty (DSEK) – DSEK is another option for people who need a cornea transplant for cornea swelling. DSEK is also called the "sutureless corneal transplant," since it does not require stitches. DSEK has some advantages over traditional corneal transplant. These include faster vision recovery, a stronger eye, lower risk of rejection, and lower risk of conditions that can be caused by stiches, such as astigmatism and infections.

For DSEK, the eye doctor removes or "strips" damaged endothelial cells and places a partial corneal transplant in the eye. As with traditional corneal transplant, this cornea is usually donated from an eye bank. The eye doctor then injects an air bubble into the eye to attach the partial corneal transplant to the surface of the person's original cornea. There are no stitches needed to attach the partial transplant. The natural pumping action of the endothelial cells helps create a suction that keeps both the original and transplanted layers of cornea together. If you have this surgery, it is very important that you use an eye shield and stay in a face-up position for one to two days afterward. This will help the transplant attach to your cornea.

For this treatment, doctors can use an excimer laser to eliminate the outermost thin layers of diseased corneal tissue and etch away the surface problems associated with many corneal dystrophies and scars. Healthy tissue can then grow over the new, smooth surface. The laser is controlled with a computer, and damage to surrounding areas is minimal or non-existent. Sometimes the doctor will also use a medicine called mitomycin-C after the PTK treatment to reduce the risk of a side effect called corneal haze.

While recovery from a corneal transplant takes months, recovery from PTK takes only days. Vision can come back very quickly, especially if the problem was in the top layer of the cornea. The PTK procedure has a good success rate, and is especially useful for people with inherited disorders, whose scars or other corneal problems harm vision by blocking the way light hits the retina. Because PTK reshapes the stroma of the cornea, a person might need a different glasses or contact lens prescription after this procedure. This is because PTK can change a person's refractive error.

Watch the video: How does laser eye surgery work? - Dan Reinstein (January 2022).