Roles of stem cells in the eye

Chicago-Adult stem cells are specialized cells for particular tissues that persist throughout life, whereas embryonic stem cells give rise to any cellular type in the body and are present only in the early embryonic stage.

A stem cell is one that can self-renew and differentiate.

“The best model of this is asymmetric division in which a stem cell divides asymmetrically and gives rise to a stem cell and a more differentiated cell,” said Ali R. Djalilian, MD, from the University of Illinois Eye and Ear Infirmary, Chicago.

Beside embryonic and adult stem cells, the induced pluripotent stem cells are adult cells that are forced in the laboratory to become embryonic-like stem cells.

In adult tissue, stem cells are important for maintaining tissues with regular turnover and regenerating tissues after injury.

“The corneal epithelium would require a reservoir of stem cells because it undergoes regular turnover,” Dr. Djalilian said. “The corneal epithelium and stromal keratocytes sustain injuries and undergo regeneration and this may also apply to the corneal endothelium.”

The corneal epithelial stem cells maintain the corneal epithelium, he said.

Evidence for the location of the stem cells in the limbus is substantial.

In 1971, Davenger and Evanson reported that the epithelium grows from the limbus toward the central cornea. In 1986, Lavker and Sun found the limbal basal epithelium is least differentiated and has slow-cycling cells.

In culture, the epithelium from the limbus has much higher proliferative potential, and when limbal cells are damaged or compromised, keratoplasty fails.

In the limbus, the limbal palisades (invaginations of the epithelium into the stroma) are the anatomic landmarks that correspond to the stem cells.

Stem cells have also been identified in the limbal corneal stromal close to the epithelium.

“They display characteristics of mesenchymal stem cells,” Dr. Djalilian said. “In culture, the stem cells give rise to cartilage and neuronal type cells; their role in maintaining and regenerating the stroma is unknown, but they seem to support the limbal stem cells.”

It is unknown if the endothelial stem cells are true stem cells or progenitor cells, as there seems to be an ability to activate these cells to proliferate, he said.

“The limbal zone is an important area for the corneal stem cells that contributes to the maintenance of these cells,” Dr. Djalilian said.

The importance of the limbal niche-the microenvironment that helps support the stem cells and includes the extracellular matrix and the supporting cells (keratocytes/mesenchymal cells, nerves, vasculature, and immune cells)-is essential to the niche functioning, and the tear film, he said.

Another important point, Dr. Djalilian said, is that stem cells can change fate under the right conditions.

“After a limbal transplant, recipient cells can partially replace the donor cells,” he said. “This suggests that restoration of the limbal niche can promote repopulation by host (stem) cells.”

Stem cells are important in the future because they will facilitate long-term gene delivery to the ocular surface.

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