Light and electron microscopical localization of filagrin-like immunoreactivity in normal and regenerating epidermis of the lizard Podarcis muralis

Summary stratum granulosum of mammalian epidermis, keratohyalin contains the histidine-rich protein filagrin which determines the aggregation of keratin bundles in terminally differentiating keratinocytes. Among reptiles, the lizard epidermis possesses keratohyalin-like granules during a period preceding molting. Tritiated histidine is taken up at 3-22h post-injection mainly in keratohyalin-like granules of the clear layer, and less intensely in oberhautchen cells, in pre-keratinizing cells of wound epidermis and in the alpha-layer. No labelling was observed in beta-cells and mesos cells. Single and double immunogold localization of filagrin and keratin, using mammalian antibodies, show that some filagrin-like immunoreactivity is localized in 0.15-0.40µm diameter roundish keratohyalin-like granules in cells of wound epidermis, and lacunar and clear layers (soft alpha-layers). Pro-filagrin was not immuno-localized in these layers with the mammalian antibodies. The small granules merge with keratin bundles into mature keratinocytes where immunopositive keratin filaments predominate and the filagrin-like immunolabelling rapidly disappears. Little or no labelling is observed in the large keratohyaline-like granules of the clear layer. This may be due to lack of filagrin-like immunoreactivity but may also be due to epitope-masking or chemical degradation of filagrin-like molecules. No immunoreactivity is present in the beta-layer and mesos-layer but the immunolabelling reappears in the maturing alpha-layer and lacunar layer. This study suggests that histidin-rich protein with some filagrin-like immunoreactivity is initially present in those alpha-layers of lizard epidermis where keratohyalin-like granules are present, such as lacunar and clear cells, and that a filagrin-like molecule is degraded or altered in mature keratinocytes.