An ultrastructural and histochemical developmental study of Drosophila auraria salivary gland cells during the third-instar period

Early in the third-instar stage of Drosophila auraria, the salivary gland cells produce small secretory granules of low electron density which empty their flocculent contents into the lumen of the gland. At that stage, the Golgi complexes consist of vesiculated, round cisternae which, as middle third instar is approached, change to their classical appearance. Cytoplasmic protrusions, intramitochondrial granules, and close contacts of mitochondria with lipid droplets are observed during the early developmental stages. At about the middle of the third instar the glue secretory granules start being produced. These secretory granules contain a granular material (where small amounts of vicinal glycol and sulfated groups of complex carbohydrates have been detected) of medium electron density. The glue secretory granules can be classified according to their electron density into two populations. Before exocytosis large amounts of glycogen particles are observed, providing some of the energy needed during the final step of exocytosis. Two different types of ribonucleoprotein particles in puffing sites of the chromosomes are observed. Of these, the larger ones consist of a core surrounded by smaller particles; we suspect that they may be produced in the Balbiani rings observed in the polytene chromosomes of this species. Nuclear blebs and oval bodies are common, particularly during the late developmental stages. The functional significance of these findings during development is discussed.