Vitamin A and its active metabolites are important for growth and differentiation of a variety of cells, mainly in mucosa-associated epithelia, where they exhibit a wide spectrum of activities.
Vitamin A, stored as
retinyl esters (REs), is delivered from liver stores into the bloodstream as
retinol bound to
retinol binding protein. This process is regulated homeostatically, ending up in a more or less constant plasma
retinol level. In situations of a high
vitamin A demand (e.g.,
inflammation, diseases, prenatal period), this supply can be insufficient because of delayed production of
retinol binding protein, leading to local deficiencies and impairment of structure and function in the respective tissues. This delay may be overcome by cellular RE stores. Several cell types, including buccal mucosa cells, can take up RE.
Retinyl palmitate is taken up when it is applied topically to either metaplastically mutated rat vaginal epithelium (as a gel) or to human meta- and dysplastic bronchial epithelia (via inhalation) that have a
vitamin A deficiency. In rats and humans, the modified epithelia can be normalized, at least in part. In conclusion, topically applied
retinyl esters may be a promising
therapy for local
retinol deficiencies and may reverse the morphological alterations of the epithelium in tissues that are
vitamin A deficient.