In vivo visualisation of murine corneal nerve fibre regeneration in response to ciliary neurotrophic factor

The aim of this study was to examine the murine subbasal nerve fibre plexus (SNP) regeneration altered by surgical dissection. Investigations in the mouse model addressed the regeneration capabilities of the SNP, and the influence of local ciliary neurotrophic factor (CNTF) application on the regeneration process. In preliminary experiments, the healthy mouse cornea was monitored using in vivo confocal laser-scanning microscopy (CLSM) from the age of 8-52 weeks, to reveal and rule out the age-dependent changes in SNP. Nerve fibre density (NFD) was determined with the semi-automatic nerve tracing program NeuronJ. No quantitative or qualitative changes in NFD were detected in untreated animals over time; mean NFD in mice aged 8 weeks (28.30 ± 9.12 mm/mm2), 16 weeks (29.23 ± 7.28 mm/mm2), 30 weeks (26.31 ± 8.58 mm/mm2) and 52 weeks (26.34 ± 6.04 mm/mm2) showed no statistically significant differences between time points (p > 0.05). For regeneration studies a circular incision through corneal epithelium and anterior stroma of minimum 60 μm depth was generated with a custom-made guided trephine system to cut the subbasal corneal nerves in adult mice. The corneal nerve pattern was monitored and NFD was measured before and up to 8 weeks after surgery. Animals were divided in three groups each comprising 6 mice. The CNTF group received eye drops containing CNTF (25 ng/ml) 3 times daily for 3 weeks, whereas the control group received no further medication. In the sham group the same treatment schedule was applied as in CNTF group, using vehicle. The regenerating subbasal nerve fibres sprouted out of stromal nerves within the cut and additionally regrew over the scar rim from outside. They showed parallel orientation but were thinner than before incision. Whorl patterning was observed after 4 weeks. All three groups revealed a marked NFD reduction starting at one week after incision, followed by continuous recovery. After 8 weeks the NFD reached 23.5 ± 2.4 mm/mm2 (78% of baseline), 21.9 ± 1.6 mm/mm2 (73% of baseline) and 29.2 ± 3.4 mm/mm2 (93% of baseline) in the control, sham and CNTF group, respectively. By comparison with control and sham group, the CNTF group demonstrated significantly higher NFD at every observation time point. The mouse cornea provides a practicable animal model for in vivo CLSM monitoring of corneal nerve behaviour over time and following injury. Non-penetrating trephination generated a severe reduction in the NFD of the SNP, but murine corneas recovered to pre-injury NFD levels within 8 weeks. Local application of CNTF served merely to temporarily accelerate the recovery of NFD.