<Li> Psychological stress, through an increase in glucocorticoids, compromises the stratum corneum and thus the barrier function . </Li> <Li> Sudden and large shifts in humidity alter stratum corneum hydration in a way that could allow entry of pathogenic microorganisms . </Li> <P> The ability of the skin to hold water is primarily due to the stratum corneum and is critical for maintaining healthy skin . Lipids arranged through a gradient and in an organized manner between the cells of the stratum corneum form a barrier to transepidermal water loss . </P> <P> The amount and distribution of melanin pigment in the epidermis is the main reason for variation in skin color in Homo sapiens . Melanin is found in the small melanosomes, particles formed in melanocytes from where they are transferred to the surrounding keratinocytes . The size, number, and arrangement of the melanosomes varies between racial groups, but while the number of melanocytes can vary between different body regions, their numbers remain the same in individual body regions in all human beings . In white and Asian skin the melanosomes are packed in "aggregates", but in black skin they are larger and distributed more evenly . The number of melanosomes in the keratinocytes increases with UV radiation exposure, while their distribution remain largely unaffected . </P>

Which is not a layer of the epidermis