Developmental changes in fine structure associated with secretion in larval salivary glands of Chironomus

Salivary glands from fourth-instar larvae, prepupae, and early pupae of Chironomus thummi were examined with light- and electron microscopes to determine whether developmental changes in the structure of the large polytenic gland cells might be correlated with the gland's secretory activity. It was found that the Golgi complexes, previously implicated in the production of dense “secretory” granules [13], are most extensively developed during mid-prepupal stages, when the glands are the largest, swollen with secretion and apparently at a peak of saliva production. In recently molted pupae, which appear inactive in the production of salivary secretion, the glands are smaller and display Golgi bodies that lack the dense secretory material found associated with Golgi complexes at earlier stages. Those dense secretory granules remaining in the cytoplasm of pupal glands are found no longer accumulated just beneath the secretory brush border, as in earlier stages, but rather aggregated in clusters within the endoplasm, apparently in preparation for autophagic digestion. Electron microscope radioautographs prepared from glands labeled with radioactive amino acids in culture have provided further evidence that the dense secretory granules are indeed composed, at least in part, of proteins synthesized de novo by the large gland cells.