A form of programmed cell death characterized by rounding-up of the cell, retraction of pseudopodes, reduction of cellular volume (pyknosis), chromatin condensation, nuclear fragmentation (karyorrhexis), classically little or no ultrastructural modifications of cytoplasmic organelles, plasma membrane blebbing (but maintenance of its integrity until the final stages of the process) and engulfment by resident phagocytes. Apoptosis is usually induced by external or internal signals that trigger the activity of proteolytic caspases, whose actions dismantle the cell and result in cell death.