Cell death: current difficulties in discriminating apoptosis from necrosis in the context of pathological processes in vivo.

The term apoptosis was proposed to define a type of cell death morphologically, biochemically, and molecularly distinct from necrosis, which plays a fundamental regulatory function in the control of the overall size of cell populations, being complementary but opposite to cell proliferation [Kerr et al. (1972): Br J Cancer 26:239-257]. This view has led to the appreciation that apoptosis is an integral part of normal biological processes and may impact on disease states. Introduction of the concept of apoptosis has raised great interest and many studies have been aimed to the identification of genes responsible for the induction of cell death. Indeed, over the past few years, many genes whose expression is associated with cell death have been described, and the molecular mechanisms underlying cell death have been, in some circumstances, clearly established. However, it is now evident that extension of the conclusions achieved by studies performed with highly selected in vitro systems (simple systems), to in vivo conditions (complex systems), has generated a certain degree of confusion. This is in part due to the indiscriminate use of the term apoptosis and to the uncertainty whether apoptosis is always different from necrosis, and, if this is the case, to the lack of well established criteria to discriminate the two processes; in addition, it still remains to be established whether both types of cell death, although different, could be induced simultaneously by the same agent, depending on the cell type and the experimental condition used. The distinction between apoptosis and necrosis, is not simply a problem of terminology; if necrosis and apoptosis are different from a mechanistic point of view, and if necrosis is merely the passive result of cellular injury (still to be shown), it becomes critical to discriminate between the two processes, in order to understand how to modulate apoptosis in view of its potential therapeutic use. This review will summarize existing informations and discuss some of the conflicting issues related to cell death in the liver.