From left to right: Panteleimon Rompolas, MBA, PhD; Olivia Farrelly; Vivian Lee, MD.
By Rebecca Salowe
Scheie Vision Annual Report 2021
A recent collaboration between the ophthalmology and dermatology departments at the University of Pennsylvania revealed new knowledge about the localization and function of stem cells in the cornea. This study, led by Panteleimon Rompolas, MBA, PhD, Assistant Professor of Dermatology, with collaborator Vivian Lee, MD, Assistant Professor of Ophthalmology, was published in Cell Stem Cell.
The cornea, which is the transparent layer forming the front of the eye, can easily become scratched or injured. Though initially painful, these injuries—which typically affect a layer called the corneal epithelium—are able to heal remarkably quickly. “This replenishment and regeneration after injury is made possible by stem cells, which are residents in these tissues,” said Dr. Rompolas. “Stem cells not only replace cells as they naturally die off, but are also activated in settings such as wound healing.”
Like many skin diseases, corneal diseases are often characterized by abnormal cell proliferation and differentiation, which are the inherent roles of stem cells. Thus, it is essential to understand how stem cells support these activities in a healthy cornea—and what conditions can cause their activity to go awry, leading to disease.
Dr. Rompolas previously investigated a similar question in skin, publishing a paper in Nature describing the biological behavior of cutaneous stem cells in vivo. “Given the similarities between the skin and eye epithelia, in terms of barrier function and histological organization, curiosity about corneal stem cells naturally arose between our groups,” he said.
“Another motivating factor in our partnership was the long tradition of collaboration between dermatology and ophthalmology here at Penn,” added Dr. Lee. “In fact, the existence of corneal stem cells was first discovered by the current Chairman of Dermatology, Dr. George Cotsarelis.”
The main goal of this collaboration was to capture stem cell dynamics during corneal maintenance and regeneration. To investigate this question, Drs. Lee and Rompolas used a cutting-edge imaging technology called two-photon microscopy to monitor the activity of corneal stem cells in intact eyes of live mice.
“Two photon microscopy is ideal for imaging cells in intact live organs, such as the cornea,” explained Dr. Rompolas. “The cornea is thin and relatively transparent, so detailed images of individual cells can be obtained and their activities captured in real-time, even when imaging the eye of a live, breathing mouse.”
The study identified and characterized two distinct compartments of stem cells in the cornea. The team was the first to discover these two compartments of functionally diverse stem cell populations, which have unique roles in repairing and replenishing the corneal epithelium. These two compartments are defined as outer and inner limbal stem cells. While inner limbal cells help to support corneal homeostasis, outer limbal cells are more involved in corneal regeneration after injury.
“Revelation of this unique organization of corneal stem cells provides us with the understanding that not all stem cells have the same function or capacity,” said Dr. Lee.
“An assumption that there is one homogenous population of stem cells, instead of two, could lead to indeterminate conclusions,” added Dr. Rompolas. “However, by now knowing that there are two populations, a more precise approach to study the individual populations separately could lead to robust data.”
One unexpected finding from the study was that when corneal cells underwent terminal differentiation, they had an unconventional, “centrifugal” 3D trajectory. “This behavior appears to be fundamentally different to all other stratified epithelia, but its functional significance remains to be determined,” said Dr. Rompolas.
These results have important implications for treating corneal diseases. Ocular trauma and diseases of the cornea are major causes of blindness worldwide. However, the main treatment methods of corneal grafts and transplants are limited by the lack of resources globally, such as surgical expertise and donor tissue. Additionally, corneal transplants can be met with complications such as rejection and infection.
Because ocular surface diseases predominantly manifest through abnormal stem cell proliferation and/or differentiation, the findings of this study are key to understanding how stem cells coordinate and interact with their surrounding microenvironment to support tissue homeostasis. “Now armed with that knowledge, more informed therapeutic strategies can be developed for the variety of conditions that stem cells can treat,” said Dr. Lee.
“By understanding the mechanisms that regulate stem cells, one can harness this information to investigate new therapies, such as tissue regeneration,” added Dr. Rompolas.
References
Farrelly O, Suzuki-Horiuchi Y, Brewster M, et al. Two-photon live imaging of single corneal stem cells reveals compartmentalized organization of the limbal niche. Cell stem cell. 2021;28(7):1233-1247.