[On the mechanism of evagination ofDrosophila imaginai discs culturedin vitro: Effects of substances affecting the cell membrane].

Abstract

The mechanism of evagination of isolated imaginai discs has been studiedin vitro. Pro-, meso-, or metathoracic leg discs were obtained from late 3rd instarDrosophila larvae and cultured in the presence or absence of α-ecdysone and of various substances (cytochalasin B, concanavalin A, neuraminidase, trypsin) known to affect the cell membrane and morphogenetic movements in vertebrates.In the presence of cytochalasin B, evagination was reversibly inhibited. Cytochalasin B apparently does not act on intracellular microfilaments, which could not be detected in the disc cells. It does not prevent ecdysone from being fixed in the cells. It probably modifies the physico-chemical properties of the plasma membrane, precluding the change in cell shape which is required for evagination.In the presence of concanavalin A, which binds specifically to hydroxyl groups of D-mannopyranose or D-glucopyranose, evagination was irreversibly inhibited. The inhibitory effect could however be neutralized by the addition of α-methyl-D-glucopyranose in the medium or prevented by pre-treating the discs in a 0.1% trypsin solution for 2 min.In the presence of neuraminidase, discs evaginated normally under the influence of α-ecdysone; in a few cases, neuraminidase caused partial evagination in the absence of moulting hormone.After treatment by a 0.1% trypsin solution for 2 min, discs evaginated normally under the influence of the moulting hormone; whereas in the absence of ecdysone, evagination was never observed. In the latter case, evagination could however be obtained by a mechanical pull.When normal evagination was inhibited by one of the tested substances, cells did not secrete either a pupal or an imaginai cuticle and did not form any integumentary differentiations.It is concluded that change in cell shape during evagination is related to changes of the cell membrane. The alterations of the physico-chemical properties of the cell membrane, which are required for evagination, are probably caused, during normal development, by the moulting hormone.