What are apoptotic signals?

Apoptotic signals refer to any type of stimuli that trigger the apoptotic pathway, which is a type of programmed cell death pathway. Apoptotic signals can be categorized as two main types – intrinsic or extrinsic - based on whether the signals originate from within the cell itself or from outside the cell. 

In the intrinsic apoptotic pathway, the apoptotic stimuli originate internally from within the target cell. Severe, irreparable DNA damage is the most significant internal signal that induces the intrinsic apoptotic pathway. 

In the extrinsic apoptotic pathway, the apoptotic stimuli originate from external sources outside of the cell. Cytokines, such as TNF (Tumor Necrosis Factor) is the most significant external signal that induces the extrinsic apoptotic pathway.