Programmed cell death, which is also called apoptosis involves a series of biochemical events making some morphological changes in the cells leading to cell death. In fact, this is a mechanism by which a cell, under certain conditions, commits a kind of suicide.
The cell shrinkage and nuclear/DNA fragmentation are among the most recognized morphological changes associated with apoptosis. The apoptotic cells can finally break into several sealed membrane vesicles called apoptotic bodies. Apototic cells/bodies are removed by the neighboring phagocytic cells such as macrophages (Picture from Genetics Home Reference shows engulfment of apoptotic bodies by a macrophage).
The process of apoptosis is under the control of different extrinsic and intrinsic factors, such as cytokines, hormones, toxins, radiation, which damage the DNA, viral infections and cell membrane/intracellular proteins. However, the executive elements involved in inducing this form of cell death are already inside the cell. It is in fact the cell itself, which utilizes those elements such as pro-apoptotic proteins or a group of proteases called caspases to kill itself. Mitochondria can play an important role in the regulation of apoptosis. Pro-apoptotic proteins such as those in the bcl-2 family, can induce the permeability transition pores in the mitochondrial membrane. This leads to mitochondrial swelling and release of factors such as cytochrome C, which activates caspases leading to programmed cell death or apoptosis.
Apoptosis plays an essential role in homeostasis to keep the number of cells relatively constant. It is also involved in the development of cells and tissues. Billions of cells die each day due to apoptosis. Defective apoptotic processes have been reported in variety of diseases such as cancer or autoimmunity. On the other hand, excessive apoptosis can cause tissue damage, immune deficiency or diseases such as Alzheimer. Billions of immune cells undergo apoptosis during thier development or after activation. In the immune system, apoptosis is a mechanism by which useless or auto-reactive immune cells such as T and B cells are removed. On the other hand, immune system can induce apoptosis in virally-infected cells. This is done via activation of cytotoxic lymphocytes, which make pores in the target cell's membrane by releasing chemicals such as perforin, which leads to activation of caspases and programmed cell death in the target cells. Sometimes viruses can prevent apoptosis and this can lead to formation of tumors.
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