Radioautographic localization of epidermal growth factor receptors in human fetal gut

The present investigation was undertaken to study the localization and accessibility of epidermal growth factor binding sites in the human fetal gut (15-19 weeks of gestation) using light-microscopic and quantitative autoradiography. Exposure of colonic explants to 5 nmol/L 125I-labeled epidermal growth factor for 60 minutes at 22 degrees C revealed extensive accumulation at binding sites in undifferentiated cells of the crypts and at the base of the villus, as well as in the inner circular layer of the muscularis externa bordering the submucosa. Some labeling was also present in the mesenchymal and vascular elements of the lamina propria. Labeling was virtually absent on the brush border at all levels of the epithelium. Quantitative analysis revealed a distinct gradient in grain density along the various compartments of the crypt-villus axis. Epithelial cells in the deep portions of the crypts showed the highest grain density (9.2 grains/microns 2) with values gradually decreasing to 6.5 in upper crypt and 3.9 in lower villus cells. The upper third of the villus showed very little labeling (0.4 grains/microns 2). Cellular distribution of silver grains in lower villous cells revealed a polarization of labeling in the basolateral infranuclear region. Experiments performed at 4 degrees C and at various incubation times showed similar results. Using isolated loops of intact colon and jejunum, segments in which labeled epidermal growth factor was only accessible on the serosal side showed extensive labeling and distribution similar to that found in explanted tissue. On the other hand, labeled epidermal growth factor could not access these same receptor sites when infused into the lumen, either at 22 degrees C or 4 degrees C. These results show that in the human fetal gut (a) the greatest concentration of epidermal growth factor binding sites is found in regions of high proliferative activity and (b) binding sites are absent from the brush border, suggesting that, under normal circumstances, systemic but not luminal epidermal growth factor has free access to its specific receptor.