In the epidermis of the thick skin of the soles and palms, there are 5 strata; from the outer to the inner they are: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. It is in the two innermost strata, stratum basale and stratum spinosum, that vitamin D is produced. Vitamin D is activated by the liver and kidneys.