It has long been assumed that normal tissues constantly lose some cells and that the materials enclosed in these cells are being used up or excreted by the tissue. With the advance of ultrastructural and electronic microscopy, it became clear that there is an active inherently programmed phenomenon in all tissues which can be initiated, stimulated, or inhibited by a variety of environmental, physiological, or pathological stimuli and which leads to controlled cell deletion by a similar process of cell destruction. Some cells undergo nuclear and cytoplasmic condensation followed by their breaking up into a number of membrane-bound fragments. Then these fragments, or apoptotic bodies, are shed from epithelial-lined surfaces or are taken up by other cells where they undergo autolysis within the phagocytes or are rapidly degraded by lysosomal enzymes derived from the ingesting cells. This process is called "apoptosis."
Apoptosis is found not only in normal healthy tissue;
Apoptosis: A Factor in Neoplastic Growth?. JAMA. 1973;223(4):434-435. doi:10.1001/jama.1973.03220040048014