Characterization of Human Transition Zone Reveals a Putative Progenitor-Enriched Niche of Corneal Endothelium

Abstract

The corneal endothelium regulates corneal hydration to maintain the transparency of cornea. Lacking regenerative capacity, corneal endothelial cell loss due to aging and diseases can lead to corneal edema and vision loss. There is limited information on the existence of corneal endothelial progenitors. We conducted ultrastructural examinations and expression analyses on the human transition zone (TZ) at the posterior limbus of corneal periphery, to elucidate if the TZ harbored progenitor-like cells, and to reveal their niche characteristics. Within the narrow TZ (~190 μm width), the inner TZ—adjacent to the peripheral endothelium (PE)—contained cells expressing stem/progenitor markers (Sox2, Lgr5, CD34, Pitx2, telomerase). They were located on the inner TZ surface and in its underlying stroma. Lgr5 positive cells projected as multicellular clusters into the PE. Under transmission electron microscopy and serial block face-scanning electron microscopy and three-dimensional (3D) reconstruction, the terminal margin of Descemet’s membrane was inserted beneath the TZ surface, with the distance akin to the inner TZ breadth. Porcine TZ cells were isolated and proliferated into a confluent monolayer and differentiated to cells expressing corneal endothelial markers (ZO1, Na+K+ATPase) on cell surface. In conclusion, we have identified a novel inner TZ containing progenitor-like cells, which could serve the regenerative potential for corneal endothelium.

Regenerative capacity of the corneal transition zone for endothelial cell therapy

“…The TZ width distribution has been shown to vary in different anatomical locations. In orientation-marked corneas, the nasal TZ width was found to be significantly thinner (a ratio of 0.8 compared to the TZ in the superior quadrant, and 0.9 in the inferior and temporal TZ) [56]. Similar findings were also reported by Breazzano et al [69].…”

“…As most of these studies were conducted using Caucasian donor corneas from middle to old age (with the mean CEC density > 2000 cells/mm 2 ), it is as yet not known if there is any association of TZ dimension with donor age and corneal health. Our recent analysis comparing two age groups, ≤ 60 years old (n = 6) versus > 60 years old (n = 11), showed no statistically significant age-related changes; however, the sample size was small [56]. Hence, we cannot exclude any TZ variation, especially in young donors.…”

“…Interestingly, the TZ is devoid of CEC and TM fibers [67,68]. This region is demarcated with the inner border adjacent to the PE as shown by a transition from regularly arranged tightly packed PE cells to more polygonal cells with irregular sizes inside the TZ [56]. The outer border is marked by the anterior non-filtering portion of TM with beam inserts and bridges.…”

“…The outer border is marked by the anterior non-filtering portion of TM with beam inserts and bridges. Several studies using SEM had reported the average width of human TZ ranges from 80 to~200 μm, using random, block area to 360°whole rim measurements [56,68,69]. As most of these studies were conducted using Caucasian donor corneas from middle to old age (with the mean CEC density > 2000 cells/mm 2 ), it is as yet not known if there is any association of TZ dimension with donor age and corneal health.…”

Prospects and Challenges of Translational Corneal Bioprinting

“…This pumping activity prevents the overhydration (edema) of the stroma, facilitating corneal transparency [ 10 ]. Although progenitors are suspected to reside in the posterior limbal area [ 11 , 12 ], adult human corneal endothelial cells (hCECs) do not proliferative in vivo. Aging, trauma or iatrogenic factors cause cell loss, leading to corneal edema and deterioration of vision ( Figure 2 E,F).…”