Effect of ecdysterone on fibroin synthesis in the posterior division of the silk gland of the silkworm, Bombyx mori

An investigation of the effect of ecdysterone on fibroin synthesis in the posterior division of the silk gland of the fifth instar larvae of Bombyx mori is described. An inhibition of fibroin synthesis occurred in the 3-day-old larvae, whereas a significant stimulation was found in the 5-, 6-, and 7-day-old larvae receiving a critical concentration of ecdysterone. With the injection of 2 μg of ecdysterone, in vitro fibroin synthesis in the posterior division of the silk gland was inhibited in the 3-day-old larvae, but was stimulated in the 5-day-old larvae. In early stages of the fifth instar fibroin synthesis was much higher in the larvae which fed on an ecdysterone-containing artificial diet than in those which were reared on an ecdysterone-free diet.We conclude that ecdysterone is an important factor for the promotion and maintenance of fibroin synthesis in the posterior division of the silk gland, and that the effect of ecdysterone on such synthesis is dependent on the ecdysone pool of the larva.