Figure 38. Concentrations of 7-dehydrocholesterol (provitamin D₃) per unit area of human epidermis (●), stratum basale (∆) and dermis (○) obtained from surgical speciments from donors of various ages. A linear regression analysis revealed slopes of -0.05, -0.06, and -0.0005 for the epidermis (r = -0.89), stratum basale (r = -0.92), and dermis (r = -0.04), respectively. The slopes of the epidermis and stratum basale are significantly different from the slope of the dermis (p < 0.001). Reproduced with permission from.⁵⁸