Figure 7: hiPSC-derived rod and cone photoreceptors achieved an advanced level of differentiation including outer-segment disc formation.

(a) During normal development photoreceptor precursors (PRPs) differentiate into rods, L/M- and S-cones (in green, red and blue colour, respectively). (b) A W22 CB-iPSC6.2-derived RC that has been exposed to 1-μM RA in W10–W14. (c–j) Under this condition, photoreceptors expressing high levels of rod opsin in the entire cell body were first observed by W17 (c), significantly increasing in number and forming large patches by W21 (e,f). S-opsin expression could be observed in some rod opsin-negative photoreceptors (d,f). High-magnification images of W21 RCs showing rods (g), S- (h) and L/M- (i) cones with a morphology and a topological organization similar to those of the in vivo retina⁴⁴, including structures reminiscent of short, nascent outer segments (arrowheads). By W25, elongated structures resembling more developed outer segments were rarely observed (j, arrow). (k–n), Transmission electron microscopic analysis revealed the presence of an outer limiting membrane (*), inner segments (arrows), basal bodies (BB), connecting cilia (CC) and stacks of outer-segment discs (demarcated by arrowheads). BB and CC presented the photoreceptor-specific microtubule arrangement consisting of 9 × 3+0 and 9 × 2+0, respectively (inserts in m). C, centriole. Scale bars, 50 μm (b,c,e); 10 μm (d,f and g–j); 0.5 μm (k–n); 0.05 μm (m inserts).