Membrane glycoconjugate visualization and biosynthesis in normal and retinoid-treated epidermis

Cell-membrane glycoconjugates can be visualized for ultrastructural and fluorescence studies with certain surface markers, such as lectins and antisaccharide antibodies. When frozen sections of mammalian epidermis are treated with a battery of rhodamine-conjugated lectins, the cell membranes display a pattern of increased sugar complexity during keratinocyte maturation. Although this vectorial sequence is disrupted following retinoid treatment, these changes occur only at high doses and late in the course of treatment, suggesting that retinoid-induced alterations may be secondary. Seemingly as a result of specific glycosidase activity within the cytosol of both granular and cornified cells, lectin staining suddenly disappears from stratum corneum cell membranes. Although lectins stain membrane glycoconjugates of cultured human keratinocytes, quantitative techniques are required to recognize differences in proliferating versus postmitotic cells and in cultures supplemented with various growth factors. Whereas retinoids consistently depress glycoprotein synthesis in cultured keratinocytes, in organ culture they stimulate epidermal glycoprotein, and particularly glycolipid, biosynthesis. These studies suggest (1) that visualization of membrane glycoconjugates with lectins can reveal important variations in normal and pathologic epidermal differentiation, (2) that lectins may reveal subtle quantitative alterations in differentiation in vitro, (3) that retinoid stimulation of glycoconjugate biosynthesis either displays important species differences or requires a higher level of organization than occurs in cell culture, and (4) that the retinoid effect on glycoconjugate biosynthesis in organ culture may provide another useful bioassay for retinoid potency.