Resting on a basement membrane is the stratum basale, consisting of a single layer of cuboidal to columnar cells with large, round to oval nuclei and 1 or 2 prominent nucleoli.
These are (from outer to inner): the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Where vitamin D is concerned, however, it's the stratum basale and stratum spinosum that are the most important.
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