Investigating cone outer segment renewal rate

"It is known from prior in vitro experiences with animals that there is a photoreceptor renewal process, including the formation of new discs at the junction between inner and outer segments and disc shedding followed by phagocytosis at the end of the photoreceptors," she said.¹ "However, in vivo observation of this process is still very challenging."

Dr Kroisamer from the department of ophthalmology and the team of Prof. Pircher at the Center for Medical Physics had already studied and published results on bright reflection spots (BRS) found in the outer segments of some cones and assumed that these might be defects within the disc packing.^2,3 "So, the aim of our most recent study," she continued, "was to investigate temporal changes of these BRS over time."

To image the cone photoreceptor mosaics of the study group, Dr Kroisamer and colleagues used a custom SLO/OCT imaging device (developed in Prof. Pirchers group).^2,4 "We imaged the cone mosaic at about 4° eccentricity from the fovea every 24 hours up to a total of 96 hours," she said. "Our measurements were taken at the same time each day and subjects followed their normal daily routine between measurements."

Once all the measurements had been taken, the team gathered the results and considered those for analysis that had four consecutive images of good quality. "We manually counted all BRS that showed motion in at least three of the four frames and measured the position of each spot at a given time," Dr Kroisamer added.

The team imaged 12 healthy volunteers and was able to obtain images of good quality in five subjects of different ages and with different refractive errors. From these images, it was possible to see BRS progressively moving from the inner segment (IS)–outer segment (OS) junction to the end tips of the cone photoreceptors.

"From the dynamic images, we calculated the motion speed of the BRS," Dr Kroisamer said. "So, in our study we found an OS growth rate that was in good agreement with animal studies and the indirect adaptive optics ophthalmoscope measurements."⁵

Conclusion

"So in conclusion we were able to observe in vivo the 3D structure of the human cone photoreceptor mosaic over an extended period of time," affirmed Dr Kroisamer. "The measured motion speed of the BRS corresponded well with the estimated cone renewal rate, so this would be the first direct in vivo investigation of the OS renewal process. But of course, further investigations are needed to get a closer insight on this topic."

Special contributorDr Julia-Sophie Kroisamer is a resident at the Department of Ophthalmology and Optometry at the Medical University of Vienna, Austria. She may be reached by Email: julia-sophie.kroisamer@meduniwien.ac.at

Dr Kroisamer has indicated no financial interests in the subject matter.

References

1. Young et al., J. Cell. Biol., 1967;33(1):61–72.

2. Pircher et al., Opt. Express, 2010;18(13):13935–13944.

3. Pircher et al., Biomed. Opt. Express, 2010;2(1):100–112.

4. Pircher et al., Opt. Express, 2007;15(25):16922–16932.

5. Jonnal et al., Opt. Express, 2010;18(5):5257–5270.