STRUCTURAL ALTERATIONS OF THE MITOTIC APPARATUS INDUCED BY THE HEAT-SHOCK RESPONSE IN DROSOPHILA CELLS

The general architecture of the mitotic apparatus was studied at the u ltrastructural level in Drosophila cultured cells. Its two main charac teristics are a very polarized spindle and a strong compartmentalizati on, ensured by large remnants of the nuclear envelope. Such compartmen talization has previously been reported for the rapid syncytial divisi ons of the early embryo; a similar finding in these cells with a long cycle strongly suggests that this organization constitutes a general m echanism for mitosis in Drosophila. We followed the modifications of t hese structures after a heat shock of 20, 50 or 120 min at 37 degrees C. Contrary to interphase cells, mitotic cells appear very sensitive t o hyperthermia. This stress treatment induced a disruption of the mito tic spindle, a reappearance and an extension of the Golgi apparatus, a n inactivation of microtubule nucleation and a disorganization of the centrosome. This organelle seems the first to be affected by the heat shock response. The centrosome is not only inactivated, but also is st ructurally affected. During the recovery phase after heat stress, the mitotic cells presented a remarkable ring-shaped accumulation of elect ron-dense material around the centrioles. We conclude that in Drosophi la cells the mitotic phase, and more specifically the centrosome, are targets of the stress response.