Secretory kinetics in the follicular cells of silkmoths during eggshell formation

Abstract

Procedures for quantitative autoradiography were used for studying the process of secretion of eggshell (chorion) proteins in the follicular epithelium of silkmoths. The method was based on photometric measurements of the reflectance of vertically illuminated autoradiographic silver grains. Results were analyzed and plotted by computer. Secretory kinetics were also determined by analysis of labeled proteins in physically separated epithelium and chorion. Rapid accumulation of radioactivity into "clumps" visualized by light microscope autoradiography and evidence from preliminary electron microscope autoradiography indicate that, within 2 min from the time of synthesis, labeled chorion proteins move to Golgi regions scattered throughout the cytoplasm. The proteins begin to accumulate in the apical area 10-20 min later and to be discharged from the cell. The time for half-secretion is 20-25 min, and discharge is essentially complete 30-50 min after labeling. At the developmental stages examined, the kinetics of secretion appear to be similar for all proteins. Within the chorion the proteins rapidly assume a characteristic distribution, which varies for different developmental stages. Two relatively slow steps have been identified in secretion, associated with residence in Golgi regions and in the cell apex, respectively. By contrast, translocation of proteins across the cell and deposition of discharged proteins in the chorion are rapid steps.