Processing of fluorescently labelled insulin and insulin-like growth factor-I by the rat visceral yolk sac

Insulin and the structurally related insulin-like growth factor I (IGF-I) are mitogenic peptides which have been implicated in the embryonic development of the rat. In addition to factors produced by the developing embryo itself, it is likely that maternally-derived growth factors play an important role also, with their postulated initial site of action being the extraembryonic membranes, which surround the embryo throughout gestation. We have examined the processing of these potential regulatory factors by the visceral yolk sac on the 17th day of gestation, using fluorescently-labelled ligands and fluorescence microscopy. Both insulin and IGF-I are rapidly internalized at the yolk sac surface, and appear in the tissue within discrete vesicular structures. Interestingly, in some cases when both labelled proteins are added simultaneously they do not appear to coexist within vesicles. Instead, insulin appears to remain within vesicles close to the apical surface of the yolk sac whereas IGF-I appears to penetrate the tissue more deeply, being readily transported to the internal face of the epithelium. It appears, therefore, that there is some difference in the sorting mechanisms of these related proteins, although the physiological significance of this observation is not clear.