Purpose

The retina of the largest adult mammal in the world, the whale, was analysed morphologically by immunohistochemistry. The eye of these aquatic mammals have been poorly studied, thus, the aim of this study was to examine the different neurons and glial cells in the whale retina using a range of molecular markers.

Methods

The eyes of beached whales (n = 2, Balaenoptera physalus and Balaenoptera borealis) were obtained, and after dissection and fixation of the retinas, whole-mount preparations and cryostat sections were immunostained. The neurons and glial cells in these tissues were analysed using different antibodies to label RGCs, photoreceptors, bipolar cells, amacrine cells, microglia, astrocytes and Müller cells. Thioflavin S was also used to label misfolded proteins.

Results

Most of the molecular markers used labelled their specific structures in the whale retinas as in terrestrial mammalian retinas. However, whale cones do not express cone markers (M/L and S opsin, and cone arrestin). It is important to highlight the large size of whale RGCs and there is a heterogeneity in NFs expression. It is also noteworthy that intrinsically photosensitive RGCs labelled with melanopsin form an extraordinary network in the whale retina, where these cells are more abundant in the centre, and different subtypes of melanopsin positive cells were identified. Thioflavin S is weakly labelled of some RGCs in a punctuate pattern and it could easily represent an early sign of neurodegeneration. In addition, degenerative neuritic beading has been observed in RGCs when the retina was analysed after 48 hr postmortem.

Conclusions

In conclusion, there are some notable differences in the retina of the whales when compared with that of terrestrial mammals. Their rod-monochromatic vision due to an evolutionary loss of cone photoreception and the well-developed melanopsin-positive RGCs network could in part be responsible for their perception in the deep sea.

Supported by ELKARTEK (KK-2019/00086), MINECO-Retos (PID2019-111139RB-I00) Grupos UPV/EHU (GIU2018/50).