Diurnal mitochondrial changes in avian retinal pigment epithelium: a search for correlation with thyroid state.

The avian retinal pigment epithelium (RPE) is well endowed with mitochondria, 10-15% of which appear to be "donut-shaped". The number of such mitochondria presenting ring profiles varies with the photoperiod, with a peak early in the dark period and a low early in the light period. This cycle is inversely correlated with the diurnal rhythm of plasma thyroxine (T4), but is in phase with the T3 cycle. We could detect no effect on the numbers of ring mitochondria when exogenous TSH, T4 or T3 was injected. Numbers and appearance of ring mitochondria were likewise unaltered in chicks breathing pure oxygen or spent air for 15 min. before sacrifice. We speculate that this aberrant mitochondrial form may have some relationship to the processing of shed outer segment discs, also a photoperiod-driven phenomenon. It has been suggested that the daily manufacture of hydrolytic enzymes for breakdown of disc proteins is probably highly energy-expensive. We also note that the peroxidation of the lipid components of these membranes must be another aspect of disc disposal. Ascorbic acid, whose photooxidation in the eye generates hydrogen peroxide, may play a role in this process.