Printed in Great Britain FUNCTIONAL AND STRUCTURAL CHANGES IN DROSOPHILA SALIVARY GLAND CELLS TRIGGERED BY ECDYSTERONE

In normal development a complex pattern of changes in the submicroscopic structure of Drosophila hydei salivary gland cells occurs in relation to the onset of secretory activity of these cells prior to puparium formation. The major changes involved in this structural reorganization of cytoplasmic components are as follows. The mucopolysaccharide secretory product present in the form of globules which are randomly distributed over the cytoplasm in the course of the third instar migrate to, and accumulate at, the cell apex. The arrangement and number of microvilli undergo drastic changes involving elongation and increase in number of the villi. The RER, which is originally distributed randomly, is reorganized into whorls and vesicles. These major changes can be experimentally reproduced by administration of the steroid moulting hormone derivative ecdysterone to larvae in the middle of the third instar. Administration of ecdysterone to early third instar larvae, in which salivary glands are still inactive with respect to mucopolysaccharide synthesis, results in only a partial reproduction of the features displayed by middle third instar glands. The changes in the arrangement and number of microvilli do not occur in the early third instar glands. The changes observed in the early third instar salivary gland cells indicate that the onset of mucopolysaccharide synthesis is inhibited by the steroid treatment. Support for this assumption is provided by studies on the features of the RER and Golgi systems in middle third instar salivary gland cells. On the basis of this investigation it may be suggested that the steroid ecdysterone is an effective trigger of mechanisms active in cytodifferentiation. It seems that not only secretion of mucopolysaccharides is initiated by the steroid, but also the synthesis of this substance inhibited. INTRODUCTION DURING larval development the salivary glands of Drosophila grow by increase in cell size, and the cell number remains constant. During the first, the second, and the very early third instar the salivary glands are small narrow tubes. All cells are of the same type and probably produce digestive enzymes (DEMEREC, 1965). During the third larval instar, a substance of mucopolysaccharide nature is syn­ thesized and stored in the form of granules or globules in the cytoplasm (BEHENDES, 1965). Synthesis of this product is first observed in the distal cells of the gland, * Present address: Faculty of Agricultural Sciences, Ghent, Belgium.