Stimulation of fibroin synthesis elicits ultrastructural modifications in spider silk secretory cells

Electron microscopic studies of unstimulated and stimulated spider fibroin glands show that the fibroin synthesis stimulus evokes visible changes in both the endoplasmic reticulum and Golgi apparatus of their secretory epithelium. Gradual increase in distension of the reticulum accompanies the increase of evoked fibroin synthetic activity. The flattened translucent Golgi vesicles, seen in inactive cells, display a gradual increase in size and number, also with time. The stimulation also elicits a gradual transition in the gland's luminal membrane, during which the microvilli on the lining gradually disappear acquiring an electron dense appearance. Correlations of the observed transitions to the gland's increase in rate of elicited synthetic activity are discussed. The parallelisms between the ultrastructural modifications observed in the spider secretory cells with those described in the silkworm glands during their progression through the fifth instar have been stressed.