Mitochondria of rat lenses: distribution near and at the sutures

This study is part of an effort to clarify mitochondrial distribution in the lens in order to better understand lens metabolic function. This study of the rat lens involves: 1) Using confocal microscopy, Rhodamine-123 and Calcium Green fluorescent dyes, to characterise the distribution of mitochondria and calcium in whole rat lenses of different ages in epithelial and superficial cortical fibre cells approaching sutures and 2) Using a scanning laser system to measure the optical quality at the sutures.

Lenses of rats from age 1 week to 22 months were pre-incubated for 24 hrs in 1.5 ml medium 199 (M199). Those exhibiting damage, as evaluated by protein leakage or visual opacities, were discarded. Lenses were labelled with 50 microg/ml Calcium Green for 45 min and/or 14 microM Rhodamine-123 for 25 min and embedded in 1% agarose in M199 for inverted laser scanning confocal microscopy with a 40 x water immersion lens. The lens optical properties were determined with a scanning laser system.

Lens focal length variability significantly increased at the sutures of 13 month-old lenses, the only age investigated. An absence of both mitochondria and calcium was observed at the sutures in rat lenses of all ages. Elongated (up to 108 mm) mitochondria were present in superficial cortical fibre cells approaching the sutures of 16 month-old lenses. Calcium Green fluorescent staining was seen closer to the border of the suture, where mitochondria were absent. Along the axis, 1 week-old lenses showed a mitochondria free zone (MFZ) starting 177 microm below the lens surface, whereas in 22 month-old lenses the MFZ started only 29 microm below the surface. In the equatorial fibre cells, mitochondria were seen to a depth of 220 microm.

Optical quality near and at the suture decreased in 13 month-old lenses despite the reduction in light scattering that should be associated with absence of mitochondria at the sutures. This suggests that mitochondrial loss in superficial cortical fibre cells may originate at the sutures and may compensate for loss of optical quality at the sutures.