Mesenchymal stem cells (MSCs) are recognized as one of the most important types of stem cells, with one of the longest and most clinically advanced histories. Although bone marrow is the usual source from which MSCs are derived, they are also known to exist in many other types of tissue such as umbilical cord and placental blood, menstrual blood, peripheral blood, muscle and teeth.
Now researchers in India have discovered MSCs in the limbus of the human eye. After examining the epithelial cells of limbal tissue from patient biopsy samples collected during eye surgery, the scientists identified cells with a spindle-like morphology. The cells were then characterized for a number of parameters including morphology and immunophenotyping, and upon expansion in the laboratory the cells were found to possess a phenotype similar to that of MSCs that are derived from bone marrow. Most significantly, the cells were found to be capable ot differentiating into adipocytes (fat tissue) and osteocytes (bone and cartilage tissue).
Why would stem cells which are capable of differentiating into cartilage and bone exist in the eye? The answer to questions such as this may shed light not only upon the role that these cells play in limbal stem cell transplants but also on the role of MSCs in wound healing and regeneration in general. Especially in ocular injuries and surgeries, the identification of various subsets of stem cells in the human limbus that are similar to bone marrow-derived MSCs is particularly relevant. Already by 1999, a publication in the New England Journal of Medicine (Tsubota et al.) had reported 70 corneal epithelial stem cell transplants in which a statistically significant improvement in vision had been observed.
The presence of MSCs in the eye represents what the discoverers refer to as cells that are “unique to the adult stem cell niche.” Scientists are now investigating further the precise properties of these unique cells, for additional clues into the nature of molecular and cellular regenerative mechanisms.