what is the upper most layer of skin
Epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and hypodermis.[1] The epidermis layer provides a barrier to infection from environmental pathogens[2] and regulates the amount of water released from the body into the atmosphere through transepidermal water loss.[3] The epidermis is composed of multiple layers of flattened cells[4] that overlie a base layer (stratum basale) composed of columnar cells arranged perpendicularly.
Vitamin D The skin consists of two primary layers: the inner layer called the dermis, composed largely of connective tissue, and the outer, thinner epidermis.[162] Thick epidermis in the soles and palms consists of five strata; from outer to inner, they are: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Vitamin D is produced in the keratinocytes[163] of two innermost strata, the stratum basale and stratum spinosum.[160]