Temporal regulation of green and red cone specification in human retinas and retinal organoids

Trichromacy is unique to primates among mammals, enabled by specification of blue (short/S), green (medium/M), and red (long/L) cones. During retinal development in humans and Old World monkeys, cones make a poorly understood choice between M and L cone subtype fates. Two nonexclusive models have been proposed: a stochastic model, in which a regulatory DNA element randomly loops to either the M- or L-opsin promoter to drive expression, and a temporal model, in which cone subtypes are generated in a developmental progression. Here we present data that support a temporal mechanism for M and L cone specification. M-opsin is expressed prior to L-opsin during fetal retinal development. Natural variation in the ratios of M and L cone subtypes is associated with a polymorphism in the NR2F2 gene, a mediator of retinoic acid (RA) signaling. RA-synthesizing enzymes are highly expressed in early development and then decrease, suggesting high RA signaling early. Human retinal organoids grown in standard culture conditions were enriched for L cones, whereas early addition of RA yielded M cone-rich organoids. Our data suggest that M cones are generated before L cones and that RA signaling induces M cones and suppresses L cones early during development. These studies advance human retinal organoids as a model to study developmental processes unique to humans and primates. One sentence summary Temporal specification of human cones ### Competing Interest Statement The authors have declared no competing interest.