[Ultrastructure on imaginalDrosophila leg discs culturedin vitro].

Ultrastructural observations were made on imaginal leg discs ofDrosophila cultured in vitro in Mandaron's medium M in the presence of α- or β-ecdysone.During evagination of discs in vitro the tight epidermal folds of the disc flatten out progressively and the cells change their shape.As the discs evaginate, the pupal cuticle is secreted at the apex of the cells: it comprises a three-layered cuticulin, an epicuticle and a thick endocuticle. Secretion of pupal cuticle always starts by the appearance of dense deposits at the apex of microvilli. Only the latter apparently participate in the construction of the three successive layers of the pupal cuticle.During disc development an abundant rough endoplasmic reticulum is formed. Nuclei and nucleoli become progressively enlarged. At the same time, the nucleus, initially located near the base of the cell, moves closer to the apical cell surface. Microtubules become fragmented as soon as evagination begins and are reorientated parallel to the cell surface (i.e. parallel to the new long axis of the cell) at the end of evagination.