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Barbara Maria Braunger, Bahar Ademoglu, B'Ann T Gabelt, Julie A Kiland, Elizabeth A Hennes-Beann, Kevin G Brunner, Paul L Kaufman, Ernst R Tamm; The search for adult stem cells in the trabecular meshwork outflow pathways of the primate eye. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5671. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The number of cells in the trabecular meshwork (TM) outflow pathways declines with age and glaucoma. It is unclear, if undifferentiated adult stem cells exist, whether they are able to compensate for the loss of differentiated TM cells. Adult stem cells in the body typically reside in defined niches, divide rarely, and retain proliferation markers such as 5-bromo-2'-deoxyuridine (BrdU) for a prolonged period of time. Here we searched for cells with long-term BrdU-retention in the primate TM outflow pathways.
Four young adult cynomolgus monkeys were daily treated with BrdU (30 mg/kg body weight) for 6 weeks. Two animals were sacrificed immediately thereafter (group1) while the two other animals were sacrificed four weeks after BrdU treatment (group2, long-term BrdU retention). Following enucleation, the eyes were embedded in paraffin and analyzed for BrdU-positive cells by immunohistochemistry. The number of BrdU-positive cells was quantitatively analyzed for the different parts of the TM, the scleral spur, the operculum region and along the inner surface of the cornea in region of Schwalbe’s line, and Schlemm’s canal (SC) endothelial cells. Double labeling experiments were performed to clarify the identity of the BrdU-positive cells.
The number of BrdU-positive was significantly smaller in group 2 than in group 1. In both groups, BrdU-positive cells were found to be evenly distributed the different regions of the TM outflow pathways. In the SC endothelium, the number of BrdU-positive cells was significantly higher than in all other areas, both in group 1 and 2. BrdU-positive cells of the SC endothelium had characteristics of differentiated vascular endothelial cells and were immunoreactive for the endothelial marker CD31. No significant differences in the relative numbers of BrdU-positive cells were found for the other areas.Moreover, we found no evidence for an accumulation of BrdU-positive cells in a specific niche.
There is evidence for the presence of cells with long-term BrdU retention in the primate TM outflow pathways. The cells are not confined to a specific niche, but are rather evenly distributed throughout the different areas of the outflow pathways. So far it is unclear if TM cells with long-term BrdU retention are indeed undifferentiated adult stem cells, or are rather slowly dividing differentiated TM cells.
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