In vitro imaginal disc development and moulting hormone.

Imaginal leg and wing discs obtained from late-third-instar Drosophila larvae were cultured in vitro in various concentrations of ecdysterone ranging from 10(-10) to 10(-5) M in order to test the effect of hormone concentration on evagination and cell differentiation. At the optimal concentration of 8 x 10(-8) M discs evaginated normally, secreted the pupal cuticle, underwent apolysis, differentiated imaginal structures and secreted the imaginal cuticle. At suboptimal concentrations (10(-8) M and less), evagination was incomplete in a variable proportion of appendages. Morphogenetic movements were limited to the earlier ones; so that appendages did not emerge from the peripodial sac. Subsequent development, whenever it occurred, took place inside the peripodial sac. This particular type of 'endoevagination' was only obtained with sub-optimal hormone concentration. At supra-optimal concentrations (10(-6) M and more), evagination was always complete but further differentiation was inhibited. These results show that endoevagination is strictly related to insufficient supply of hormone and that morphogenesis and cell differentiation in imaginal discs are two independent phenomena, which respond to different levels of hormone stimulation.