Changes to desmosomal antigens and lectin-binding sites during differentiation in normal human epidermis: a quantitative ultrastructural study

Abstract

During epidermal differentiation, desmosomes undergo a series of changes in their abundance, structure and properties, which has previously been defined by conventional electron microscopy and the use of antibodies to desmosomal proteins at the light-microscope level. Such changes in a major adhesive organelle would be expected to have a significant role in the maintenance of epidermal organization, and therefore require more detailed characterization. In the present study, modifications to certain desmosomal components in normal human epidermis have been located and quantified by immunogold electron microscopy. Antibodies to desmosomal protein dp3 and glycoprotein dgl were used to label the cytoplasmic regions of the junctions and lectins concanavalin A (ConA) and wheat germ agglutinin (WGA) to probe the extracellular glycosylated material. Binding was measured at histologically defined levels and expressed as gold particles per μm of desmosome length (linear particle density: LPD). In addition, desmosome frequency, expressed as the percentage of the cell membrane length occupied by desmosomes, was measured. Highly significant changes in desmosome frequency, diameter and LPD were observed between epidermal strata and, in basal and upper horny cells, between different regions of the same cell surface. These parameters rose to a maximum in the spinous or granular layers: their subsequent decrease continued without interruption across the interface between the living and terminally differentiated homy layers. Remaining reactivity with antibodies, but not lectins, was almost completely abolished immediately before the final disintegration of the desmosome structure in the lower homy layer. In contrast, numerous large, highly immunoreactive desmosomes were retained up to the outer surface in the grossly thickened homy layer found in callus. Though the overall pattern of a rise followed by a fall was similar for all parameters measured, differences were observed between probes. Thus, the extent of the rise in available antigen between the lateral and apical surfaces of the basal cell was greater for dgl than for dp3; the subsequent decrease in dp3 antigens in upper epidermal layers was more rapid than that for dgl, and changes to both antigens preceded those to lectin-binding sites. These results show that differences in desmosome frequency and in the size and antibodybinding characteristics of individual junctions underlie the heterogeneous distribution of desmosomal components within epidermis that is found by light-microscope immunocytochemistry. They further suggest that the disintegration of desmosomes within normal homy layer, which is an essential preliminary to desquamation, is the culmination of a sequence of events that begins in the upper living tissue and initially involves cytoplasmic components. More generally, the study demonstrates the ability of quantitative immunoelectron microscopy to locate and measure modifications to specific desmosomal components, an approach that could be applied to other normal and pathological tissues and in vitro systems.