Studies on the Posterior Silk Gland

Cytolytic processes in posterior silk gland cells of the silkworm, Bombyx mori, during metamorphosis from larva to pupa have been studied. During this stage, the wet weight and the amounts of RNA and protein of the gland decrease rapidly and markedly, while the amount of DNA decreases slowly and slightly. The ultrastructural changes observed at the beginning of the prepupal stage consist of the appearance or the increase in the number of autophagosomes containing endoplasmic reticulum (ER), or "early autophagosomes" as we have called them, which seem to be gradually transformed to autolysosomes. A number of usual lysosomes, which frequently contain myelin figures, also appear in the cytoplasm. Sometimes they fuse with each other to form large conglomerates. In the middle of the prepupal stage, a number of smooth membrane-bounded vacuoles appear in cytoplasm. Towards the end of the prepupal stage the partition or sequestration of cytoplasm was observed. Thus large autophagosomes containing cytoplasmic organelles such as rough ER and/or mitochondria are formed. The nucleus is partitioned in a similar way by smooth membranes, and then autophagosomes containing condensed chromatin blocks are formed. These various kinds of autophagosomes, or "late autophagosomes" as we have generally called them, are continuously released into the hemolymph until the gland is completely disintegrated.