Vahlquist , A. ., Stenström , E. ., & Törmä , H. . (1987). Vitamin A and β-Carotene Concentrations at Different Depths of the Epidermis: A Preliminary Study in the Cow Snout. Upsala Journal of Medical Sciences, 92(3), 253–257. https://doi.org/10.3109/03009738709178695