Corneal endothelial cells in vivo are inhibited in the G
1-phase of the cell cycle,
1 2 but retain the ability to proliferate.
3 4 5 That these cells retain proliferative capacity but do not divide leads to the hypothesis that the endothelium must be actively maintained in a nonproliferative state. This laboratory has been conducting studies to identify mechanisms responsible for inducing or maintaining mitotic quiescence in these cells. Among the factors that appear to contribute to the nonreplicative status of the endothelium are the apparent lack of autocrine or paracrine stimulation by positive growth factors, age, transforming growth factor (TGF)-β, and contact inhibition. Aqueous humor, which bathes the posterior of the endothelium, does not have significant concentrations of positive growth factors.
6 In addition, although endothelial cells express mRNA and protein for a number of growth factors and their receptors,
7 8 there is little apparent autocrine or paracrine mitogenic stimulation, even after cell loss.
9 Age is a factor, in that cells from older individuals enter the cell cycle more slowly than those from younger individuals and require stronger mitogenic stimulation than their younger counterparts.
5 Specific age-related changes leading to this differential response are currently unknown. Telomere length does not seem to be a limiting factor, because telomere lengths in human corneal endothelial cells appear adequate to permit additional rounds of division before replicative senescence.
4 TGF-β may play a role in inhibiting proliferation in the endothelium. Human corneal endothelial cells in vivo express TGF-β receptor types I, II, and III
10 (RI, RII, and RIII, respectively), which are required for TGF-β-induced signal transduction.
11 12 13 Cultured human corneal endothelial cells synthesize mRNA
7 and protein for TGF-β
8 and cells in vivo are exposed to latent TGF-β2 in aqueous humor.
14 15 In addition, studies using cultured rat
16 and rabbit corneal endothelial cells
17 indicate that exogenous TGF-β1
17 and -β2
16 17 and TGF-β2 in aqueous humor
16 suppress entry into the S-phase. As in many cell types, contact inhibition plays an important role in inhibiting proliferation in corneal endothelium.
18 This appears to be true, even in the endothelium from older human donors. Studies from this laboratory reveal that, in ex vivo human corneas from donors 50 years of age or older, exposure of the intact endothelial monolayer to strong mitogenic stimulation alone does not promote proliferation. However, cells proliferate in response to this stimulation, if cell–cell contacts are broken by mechanical wounding
5 or treatment of the monolayer with EDTA.
19