Thyroid hormone signaling specifies cone subtypes in human retinal organoids

The mechanisms underlying specification of neuronal subtypes within the human nervous system are largely unknown. The blue/S, green/M and red/L cones of the retina enable high-acuity daytime and color vision. To determine the mechanism controlling S vs. L/M fates, we studied the differentiation of human retinal organoids. Organoids and retinas have similar distributions, expression profiles, and morphologies of cone subtypes. S cones are specified first, followed by L/M cones, and thyroid hormone signaling controls this temporal switch. Dynamic expression of thyroid hormone-degrading and activating proteins within the retina ensures low signaling early to specify S cones and high signaling late to produce L/M cones. This work establishes organoids as a model for determining mechanisms of human development with promising utility for therapeutics and vision repair.