Ultrastructural changes of the posterior silk gland cells in the early larval instars of the silkworm, Bombyx mori

The posterior silk gland cells in the first three larval instars show characteristic changes during growth that are essentially similar to those undergone in the fourth larval instar. In the feeding stage, when the cells grow rapidly, vesico-tubular rough endoplasmic reticulum and a number of Golgi vacuoles occur in the cytoplasm and the glandular lumen is filled with fibrous materials, probably fibroin. In the moulting cycle when the cells stop growing, a series of degenerative changes occur such as the appearance of autophagosomes, autolysosomes, and large vacuoles. Fibrous materials disappear from the glandular lumen. These cyclic changes are discussed in relation to hormonal changes. Intercellular junctions and the tracheal system of the silk gland are described.