Retinoids, synthetic vitamin A analogs, stimulate mucous metaplasia and gap-junction
proliferation in embryonic and neoplastic epidermis. Such effects demonstrate that vitamin A
has potent effects on epidermal differentiation. Oddly, however, retinoid action in normal
postnatal tissues, where these drugs are used clinically, appears to be quite different. In
animals and humans, both topical and systemic retinoids produce acanthosis,
hypergranulosis, and a relative (but not absolute) decrease in the thickness of the stratum …