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 that controls S versus 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.

The mechanisms underlying the specification of diverse neuronal subtypes within the human nervous system are largely unknown. The blue (shortwavelength/S), green (medium-wavelength/M) and red (long-wavelength/L) cone photoreceptors of the human retina enable high-acuity daytime vision and trichromatic color perception. Cone subtypes are specified in a poorly understood two-step process, with a first decision between S and L/M fates, followed by a decision between L and M fates. To determine the mechanism controlling S vs. L/M fates, we studied the differentiation of human retinal organoids. We found that human organoids and retinas have similar distributions, gene expression profiles, and morphologies of cone subtypes. We found that S cones are specified first, followed by L/M cones, and that thyroid hormone signaling is necessary and sufficient for this temporal switch. Temporally dynamic expression of thyroid hormone degrading and activating proteins supports a model in which the retina itself controls thyroid hormone levels, ensuring 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 the mechanisms of cell fate specification during human development.One sentence summaryCone specification in human organoids

The bioelectrical signatures associated with regeneration, wound healing, development, and cancer are changes in the polarization state of the cell that persist over long durations, and are mediated by ion channel activity. To identify physiologically relevant bioelectrical changes that occur during normal development of the sea urchin Lytechinus variegatus, we tested a range of ion channel inhibitors, and thereby identified SCH28080, a chemical inhibitor of the H(+)/K(+) ATPase (HKA), as an inhibitor of skeletogenesis. In sea urchin embryos, the primary mesodermal lineage, the PMCs, produce biomineral in response to signals from the ectoderm. However, in SCH28080-treated embryos, aside from randomization of the left-right axis, the ectoderm is normally specified and differentiated, indicating that the block to skeletogenesis observed in SCH28080-treated embryos is PMC-specific. HKA inhibition did not interfere with PMC specification, and was sufficient to block continuing biomineralization when embryos were treated with SCH28080 after the initiation of skeletogenesis, indicating that HKA activity is continuously required during biomineralization. Ion concentrations and voltage potential were abnormal in the PMCs in SCH28080-treated embryos, suggesting that these bioelectrical abnormalities prevent biomineralization. Our results indicate that this effect is due to the inhibition of amorphous calcium carbonate precipitation within PMC vesicles.

Humans rely on visual cues to navigate the world around them. Vision begins with the detection of light by photoreceptor cells in the retina, a light-sensitive tissue located at the back of the eye. Photoreceptor types are defined by morphology, gene expression, light sensitivity, and function. Rod photoreceptors function in low-light vision and motion detection, and cone photoreceptors are responsible for high-acuity daytime and trichromatic color vision. In this review, we discuss the generation, development, and patterning of photoreceptors in the human retina. We describe our current understanding of how photoreceptors are patterned in concentric regions. We conclude with insights into mechanisms of photoreceptor differentiation drawn from studies of model organisms and human retinal organoids.

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