Visual acuity in medaka (Oryzias latipes) larvae after sub-lethal gamma irradiation during early embryogenesis

The vertebrate retina is a laminated tissue with a relatively simple structure compared with the brain, and its accessibility makes it an excellent model for studying damage and repair in the central nervous system. This study investigated the regenerative process of the photoreceptor layer in medaka (Oryzias latipes) larvae following embryonic exposure to sub-lethal gamma irradiation and examined whether transient damage influences visual function using the optomotor response (OMR) assay.

Medaka embryos at 3 days post fertilization (dpf) were irradiated with 7-10 Gy to determine the lethal threshold, from which 8 Gy was determined to be a sub-lethal dose. In 8 Gy-irradiated embryos, eye size was assessed by stereomicroscopy and photoreceptor regeneration was histologically evaluated by Zpr1 immunohistochemistry at 8, 14, and 21 dpf. Visual function was evaluated by optomotor response under standard and reduced-contrast conditions.

Irradiation at 10 Gy induced severe cone loss, resulting in mortality from 15 dpf. In contrast, larvae exposed to 8 Gy showed no significant alterations in central or dorsal cones compared with controls, whereas ventral cones were significantly shorter and fewer in number. These abnormalities, as well as eye size, gradually recovered to control levels by 21 dpf. Although transient reductions in eye size and ventral cones were observed, OMR testing revealed no impairment of visual performance at 8, 14, or 21 dpf, even under stringent low-contrast conditions.

Sub-lethal gamma irradiation transiently induced localized damage especially in the ventral retina and reduction in eye size, both of which were fully repaired within 21 dpf. Behavioral analysis demonstrated that such transient, repairable damage does not impair visual function in irradiated medaka larvae.