Figure 3.

Pathological SICD vs physiological apoptosis. Left-panel: SICD occurs to useful cells and is often caused by an external stress directly or indirectly via causing an internal stress (such as a genomic DNA damage). The external or internal stressor acts on a death receptor at the cell membrane, the endoplasmic reticulum (ER), the mitochondria, or the lysosomes, executing a suicidal procedure via a caspase-dependent (CASP-SICD) or -independent (Non-CASP SICD) pathway or via a lysosome (Ly) mediated pathway. Right-panel: Apoptosis occurs as an order from the animal's body to eliminate no-longer useful (thus redundant) cells. Because few in vivo studies on authentic apoptosis without involving a stressor as the death inducer have been documented, we really know little about apoptosis, but it cannot be ruled out, or ruled in, that the apoptotic mechanism is the same as, or similar to, that used in SD and/or SICD.