The term "apoptosis" is introduced to describe a previously unrecognized mechanism of controlled cell deletion that plays a complementary role to mitosis in regulating animal cell populations. Apoptosis is characterized by two distinct stages: nuclear and cytoplasmic condensation, followed by the formation of membrane-bound, ultrastructurally well-preserved fragments. These apoptotic bodies are then shed or taken up by other cells, where they undergo autolysis within phagosomes and are rapidly degraded by lysosomal enzymes. Apoptosis is involved in cell turnover in healthy tissues, focal elimination during embryonic development, spontaneous cell loss in untreated malignant neoplasms, and atrophy and involution of various tissues and organs. It can be triggered by both physiological and pathological stimuli, including noxious agents. The authors suggest that apoptosis is an active, inherently programmed phenomenon and propose it as a vital biological phenomenon to explain its widespread implications in tissue kinetics.The term "apoptosis" is introduced to describe a previously unrecognized mechanism of controlled cell deletion that plays a complementary role to mitosis in regulating animal cell populations. Apoptosis is characterized by two distinct stages: nuclear and cytoplasmic condensation, followed by the formation of membrane-bound, ultrastructurally well-preserved fragments. These apoptotic bodies are then shed or taken up by other cells, where they undergo autolysis within phagosomes and are rapidly degraded by lysosomal enzymes. Apoptosis is involved in cell turnover in healthy tissues, focal elimination during embryonic development, spontaneous cell loss in untreated malignant neoplasms, and atrophy and involution of various tissues and organs. It can be triggered by both physiological and pathological stimuli, including noxious agents. The authors suggest that apoptosis is an active, inherently programmed phenomenon and propose it as a vital biological phenomenon to explain its widespread implications in tissue kinetics.