Human sebocytes maintained in medium containing delipidized serum were studied for ultrastructural characteristics, cell proliferation, lipid synthesis, immunophenotype, and keratin expression before and after the addition of the synthetic retinoids isotretinoin and acitretin (10(-8)-10(-5) M). Compared to the properties of sebocytes cultured in normal sebocyte medium (1-2 x 10(-7) M vitamin A), the use of delipidized serum (undetectable amounts of vitamin A) resulted in prominent decrease of i) proliferation; ii) number of intracellular lipid droplets and synthesis of total lipids, especially triglycerides, squalene, and wax esters; and iii) labeling with monoclonal antibodies identifying progressive and late-stage sebocyte differentiation. Intercellular spaces narrowed and cell-to-cell contacts were established by abundant desmosomes. Lanosterol was induced. Keratins 14, 16, 17, and 18 were upregulated and the keratin 16: keratin 4 ratio, negatively correlating with sebocyte differentiation, increased. Addition of isotretinoin and acitretin exerted a biphasic effect. At concentrations < or = 10(-7) M, both compounds enhanced sebocyte proliferation and synthesis of total lipids, especially triglycerides and cholesterol, and decreased lanosterol, keratin 16, and the keratin 16:keratin 4 ratio. In contrast, retinoid concentrations > 10(-7) M inhibited sebocyte proliferation in a dose-dependent manner. Our findings indicate that vitamin A is essential for proliferation, synthetic activity, and differentiation of human sebocytes in vitro. Synthetic retinoids partially reinstate the altered functions of sebocytes maintained in medium containing delipidized serum. In contrast to the previously shown isotretinoin-specific response of cultured sebocytes in the presence of vitamin A, similar effects of isotretinoin and acitretin were obtained in its absence. This suggests different interactions of synthetic retinoids with vitamin A, possibly influencing their efficacy on the sebaceous gland.