Dissecting primary and secondary senescence to enable new senotherapeutic strategies

•There is no universal gene expression signature for senescent cells or the SASP.•Primary and secondary senescent cells seem to be distinct from each other in some respects.•Secondary senescence can spread via secreted molecules, extracellular vesicles, or cell-to-cell contact.•Secondary senescence contributes to the accumulation of senescent cells and is associated with the diseases of aging.