The term "programmed cell death” is used for developmentally occurring cell death, where cells are "programmed" to die during normal development. As cell death seems to follow an intracellular "program" programmed cell death is sometimes also used as a synonym for apoptosis.
Apoptosis is a cell autonomous genetically defined program, in which cells respond to internal or external signals by actively participating in their suicide and organization of their disposal. Originally, the term "apoptosis" was defined by a characteristic pattern of morphological changes and is now increasingly used to describe the underlying molecular mechanisms.
Any cell death lacking the features indicative for active cell death is referred to as necrosis. Necrosis presents a passive form of cell death with relatively slow disintegration of the cells.
The morphological changes, by which apoptotic cell death can be characterized and identified, occur in a consecutive fashion. Dying cells start to detach from neighboring cells and extra cellular matrix and round up. The cells start to show pertrusions from the plasma membrane, referred to as blebs. Many dying cells show nuclear condensation and disintegration of the nucleus into several fragments. Organelles are generally intact, but may be affected at later stages. Mitochondria have been described to either swell or condense; there is dilatation of the ER, release and aggregation of ribosomes and the occurrence of cytoplasm vacuoles. Whole cells condense and reorganize into so-called apoptotic bodies, which are membrane bound vesicles containing cytosolic elements, organelles and parts of the condensed nuclei in various combinations. Apoptotic cells are rapidly engulfed and digested by neighboring cells, which makes it quite difficult to study morphological changes in vivo.