The key to stem cell renewal

The corneal epithelium constantly renews itself. Until now, scientists believed that it regenerates because of what the limbus does.
“Identification of slow cycling cells (label-retaining cells) in the limbus of the mouse has led to the notion that the limbus is the niche for the stem cells responsible for the long-term renewal of the cornea,” wrote Yann Barrandon, M.D., Ph.D., joint professor of stem cell research and of experimental surgery at the Swiss Federal Institute of Technology Lausanne and the University of Lausanne Laboratory of Stem Cell Dynamics, Switzerland, in the November 2008 issue of Nature. “Hence, the corneal epithelium is supposedly renewed by cells generated at and migrating from the limbus.”
Now, researchers have found that mammalian epithelium itself plays a much larger role in this regeneration process.
“Here we show that the corneal epithelium of the mouse can be serially transplanted, is self-maintained and contains oligopotent stem cells with the capacity to generate goblet cells if provided with a conjunctival environment,” Dr. Barrandon noted. “Furthermore, the entire ocular surface of the pig, including the cornea, contains oligopotent stem cells (holoclones) with the capacity to generate individual colonies of corneal and conjunctival cells.”
Corneal stem cells do not reside exclusively in the limbus; that is the take-home message from Dr. Barrandon and colleagues. At least one ophthalmologist suggests this may have some bearing on making future ocular transplants easier.

A different understanding

Dr. Barrandon analyzed 12 mouse eyes in which the entire circumference of the limbus was cauterized. In effect, this burn caused limbal deficiency.
“The cornea of burned eyes remained transparent for the duration of the experiment (a minimum of four months), and neither corneal ulcers nor stromal blood vessels were observed, strongly suggesting self-maintenance of the corneal epithelium,” Dr. Barrandon reported.
Dr. Barrandon didn’t stop there. He looked at full-thickness central corneal grafts transplanted from one kind of mouse (–gal-ROSA26) to the limbal region of another type (athymic).
“Most of the grafts were rapidly incorporated into the limbal region of the recipient mice, and remained viable for months with no signs of necrosis,” he noted. He administered wounds to these corneas.
“In most cases (47 out of 51) cells quickly migrated out of the grafts onto the denuded corneal stroma and corneal integrity was restored within seven days,” he reported.
Experiments were also performed to ensure that the central corneal transplanted cells were not somehow reprogrammed to a limbal phenotype. In these tests, the central corneal transplants were given to recipients in the same central location. These mouse eyes were then wounded but regained their integrity.
“Transplanted cells often covered the entire distance (5.2 mm) to the opposite corneal side before limbal cells even had time to move out, a further indication of the robust migratory capability of corneal cells,” Dr. Barrandon reported.
The experiments continued, and in all, gave considerable credibility to Dr. Barrandon’s overriding conclusion.
“Together, our results challenge the prevailing opinion that the limbus is the sole niche for corneal stem cells,” he reported.
Nonetheless, Dr. Barrandon found that the limbal region is an important stem cell-containing locale. “The limbal region undoubtedly contains stem cells that are efficient in restoring the corneal surface in the extreme situation of a massive corneal injury or for cell therapy as demonstrated by the impressive clinical results obtained with the transplantation of limbal holoclones,” he concluded.

Non-limbal transplants?

Meanwhile, Mark Packer, M.D., clinical associate professor of ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, said he was pleasantly surprised by the results.
“I was under the impression that only the limbus has stem cells that can replenish corneal epithelium,” Dr. Packer said.
Patients who have had serious ocular surface damage require limbal transplants, for instance, he said.
“The tissue that needs to be transplanted is limbal epithelium where cornea and conjunctiva meet,” he said. “That’s conventional wisdom.”
Perhaps future transplant methods could change based on the findings of this research, Dr. Packer added.
It would be much easier to transplant healthy corneal epithelium than it would be to use limbal tissue, which requires a bigger operation with more, deeper cutting.
“If you can take normal corneal epithelium and transplant that, that would be quite a bit easier,” Dr. Packer said. “I have never heard of that working.”
Perhaps the time has come to study this issue not only in mice and other animals, but also in men.

Editors’ note: Dr. Barrandon has no financial interests related to this study. Dr. Packer has no financial interests related to his comments.

Contact information

Barrandon: yann.barrandon@epfl.ch
Packer: 541-687-2110, mpacker@finemd.com