The secretion of tissue-specific proteins during mouse skin development, was investigated by incubating day 16 fetal skin explants in the presence of [35S]methionine and analyzing the medium electrophoretically. The medium was found to contain five proteins, which could be classified into two groups according to molecular weight. The kinetics of release of these proteins indicated that they were specifically secreted and not released by cytolysis. Mapping of the proteins by partial proteolytic digestion revealed that although the digestion patterns between the two molecular weight groups were different, within each group similar patterns were seen, suggesting that they were structurally related. Incubation in the presence of tunicamycin resulted in the decrease in molecular weight of the secreted proteins, indicating that the proteins were glycosylated. The results suggest that the two groups of structurally related glycoproteins were secreted by the peridermal layer of the fetal skin.
The periderm of the early fetal skin is believed to be a secretory tissue on the basis of the presence of secretory vesicles in the periderm cells (Hayward and Kent, '82). In the mouse, the periderm is a single layer of cells that surrounds the fetus from day 11 to day 17 of gestation (Bonneville, '68; Weiss and Zelickson, '75a,b,c). During this period, the periderm does not change in morphology while the underlying ectoderm and mesoderm differentiate into the epidermis and dermis, respectively, with the accompanying changes in morphology and protein synthesis (Balmain et al., '77, '79, Van Blerkom et al., '82). While electron microscopic evidence suggests that the periderm may be secretory, no biochemical evidence has been presented. In this study, we present electrophoretic analysis of proteins found in the medium of day 16 mouse fetal skin explants. The analysis revealed the presence of a group of glycoproteins that may be secreted by the periderm.