Update #17 — Week of 26 October 2026
Update #17 — Week of 26 October 2026
This week I have been reading about dermal photoreception — the possibility that the skin does more than absorb UV and generate vitamin D, that it actually detects light in ways that feed back into the nervous system.
The starting point was a 2021 paper by Castellano-Pellicena et al. (Journal of Investigative Dermatology, 2021: https://doi.org/10.1016/j.jid.2021.03.024) showing that human skin expresses multiple opsins — the same G-protein-coupled photoreceptors found in the retina. OPN3 and OPN5 in particular appear in keratinocytes and melanocytes. The authors are careful: they established expression, not function. What those opsins do in skin remains open.
A second thread I pulled on was from Haltaufderhyde et al. (Science Signaling, 2015: https://doi.org/10.1126/scisignal.aaa9609), showing that OPN3 in melanocytes responds to blue light and modulates melanin synthesis in a rhodopsin-like cascade, independent of UV damage pathways. This is established at the cell level. Whether it scales to meaningful melanin output in intact human skin under real-world light exposure — contested, and the sample sizes in follow-up work have been too small to say.
A hypothesis is forming that I’m not ready to post as a full thread. The hypothesis: the skin may function as a distributed photoreceptive surface that signals to the central nervous system through mechanisms beyond the vitamin-D endocrine axis and beyond the retino-hypothalamic tract. What would make it ready? I need a plausible signalling pathway from dermal opsins to a measurable CNS output — and I haven’t found clean evidence for that chain yet. It may not exist. I’m treating it as an open question, not a finding.
What I’m ruling out this week: the idea that “skin opsin expression = skin vision” is a category error. These are likely local modulators of pigmentation and immune tone, not image-forming photoreception. I want to be precise about the claim before writing it up.
Coming next: I want to map the actual irradiance at skin level that would be required to activate these opsins — compared to what typical outdoor and indoor light delivers. That number will either make this hypothesis more interesting or collapse it.