The apoptosis cascade - morphological and immunohistochemical methods for its visualization

Apoptosis is involved in morphogenesis of embryonic tissues as well as in homeostasis of adult organs and tissues. It is the main process by which organs maintain cell mass and at the same time eliminate excess and aged cells that have lost their functional importance. The typical morphological signs of apoptosis (cellular shrinkage, membrane blebbing, nuclear condensation and fragmentation) are the final results of a complex biochemical cascade of events, some of which are inextricably linked to the process of differentiation. Studies that analyze all stages of this cascade, rather than the final morphological stages of apoptotic death, are essential in order that specific link(s) between differentiation and apoptosis are appreciated. This review outlines the main stages of the apoptosis cascade together with current methods for their morphological visualization. Starting with (a) receptors and ligands known to induce apoptosis, we continue with (b) early initiator stages of apoptosis and (c) proteins regulating and potentially inhibiting further progression of the cascade, into (d) irreversible execution stages of the cascade, and finally (d) the morphological events of apoptotic death. For each stage we present those aspects of the biochemical background that are morphologically relevant, together with proven methods for their visualization. We offer technical advice at each stage based upon our experience of studying differentiation and apoptosis in human placental trophoblast.