Fig. 6

Retinal sections from control and COVID-19 patients immunostained with calbindin (a–d) and recoverin (e–h). a, a’ Cones with a normal morphology are labeled. Outer and inner plexiform layer and some horizontal, cone bipolar, amacrine and ganglion cells are also marked with calbindin. b, b’ Cones showing a moderate response to damage: slight thinning of the axon which appears swollen at the HFL level is observed in some cells (b’ and white dotted area). c, d Severe degeneration of most cones with an excessively thin axon that swells at the HFL (c’, d’ and white dotted areas). Several alterations in the inner and outer segment were present (c’, d’, some indicated with green dotted areas). Degeneration between the cell body and the inner segment of cones and absence of pedicles were also found in some cases (d’). e, e’ Normal pattern of recoverin staining photoreceptors and OFF cone bipolar cells. (f–h) Less dense recoverin staining pattern in the ONL and mild swelling of the axon at the HFL (f’, h’, arrowheads) and ONL (g’, arrowheads) level were present in some retinas of COVID-19 patients. Alterations in the OS/IS (g–g’, green dotted areas) and in the axon terminals (h–h’, arrows) were also detected. i Proportion of the terminal area of cones in the retina relative to the number of cones between COVID-19 (n = 6) and control group (n = 4) (**P < 0.01). j Axon width values of the control (n = 5) and COVID-19 groups (n = 10) (P = 0.309). k Percentage of cones with an axon width inferior to 1.7 µm in control (n = 5) and COVID-19 (n = 10) patients (P = 0.252). OS, outer segment; IS, inner segment; ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer; HFL, Henle fiber layer. Scale bars: 50 µm