[
Trends Cell Biol,
2001]
In vertebrates and higher eukaryotes such as Caenorhabditis elegans and Drosophila melanogaster, microtubules are in each cell primarily nucleated and organized by the centrosome, which has as its center a pair of centrioles. In recent years, it has become clear that bipolar spindle assembly is possible without centrosomes, and it appears that the centrosome might be required for proper spindle positioning rather than for spindle assembly.
[
Nature,
1996]
During the development of many, if not all, complex organisms, specific cells are marked out for elimination in a process known as programmed cell death, or apoptosis, a form of cell suicide. For example, during the development of the hermaphrodite nematode worm Caenorhabditis elegans, 131 of the 1,090 cells produced are genetically destined to die. Drosophila embryos without the necessary genes to execute this death programme do not survive. In vertebrates, failure to delete malformed or potentially autoreactive immune cells during development can eventually lead to autoimmunity or leukaemia. So too much or too little cell death threatens the whole organism.
[
Nature,
2001]
In all animals, the process of programmed cell suicide (apoptosis) is coordinated by enzymes known as caspases, which cut up key substrates in the cell. The dying cell is then neatly packaged, engulfed by neighbouring "phagocytic" cells, and cleared from the body without fanfare, leaving no evidence of the catastrophic events that preceded. It has always been assumed that there is a "point of no return" in this death cascade - at or shortly before the time at which caspases are activated - beyond which the process of cell execution proceeds inexorably. This view is challenged by Reddien et al. and Hoeppner et al. on pages 198 and 202 of this issue. It seems that cells in which caspases have been activated can in fact progress through a state of being "mostly dead", a stage that physically resembles the early phase of apoptosis but from