**6. Apoptosis**

The word apoptosis has its etymological origin in the Greek apó, which means "from" and ptosis, which means "falling off." The term encompasses a genetically regulated process of cell death through which a cell destroys itself [66] with the key participation of cysteine-dependent proteases called caspases that are specific for aspartic acid [67]. Caspases are functionally divided in initiator (caspases 8, 9, and 10) and executioner (caspases 3, 6, and 7) [68]. In metazoans, apoptosis is an essential step in a great variety of physiological events such as embryogenesis, tissue remodeling, and the elimination of damaged or non-functional cells [69].

The sequence of events that lead to apoptosis can be unchained by two main routes: the extrinsic and the intrinsic pathways [66]. The intrinsic pathway initiates with the binding of death ligands (TNF-α, Fas-L, among others), present in soluble form or in the surface of effector cells, to their respective death receptors localized in the membrane of target cells. This binding results in the recruitment of cytosolic factors into the cytoplasmic domains of the death receptor, forming the death-inducing signaling complex or DISC. Initiator caspases such as procaspase 8, 10, or both are recruited to the DISC where they are activated. Caspases 8 and 10 in turn activate the following caspases in the pathway [70].

On the other hand, the intrinsic pathway, also known as the mitochondrial pathway of apoptosis, can be induced by various factors such as environmental stress or absence of growth factors. In this pathway, the formation of pores in the outer mitochondrial membrane allows the release of cytochrome-c into the cytosol, as well as other molecules such as endonuclease-G and Bcl-2 proteins. The interaction between cytochrome c, procaspase 9, and protease activating factor 1 or APAF 1 stimulates the formation of a heptameric complex known as apoptosome, which recruits procaspase 9 molecules, activating them. Caspase 9 molecules proceed to activate the following procaspases that execute the pathway, inducing apoptosis [66, 70].

Cells that die by apoptosis undergo characteristic morphological and biochemical changes that include a reduction in size, the collapse of the cytoskeleton, the disassembly of the nuclear envelope, and the fragmentation and condensation of chromatin. There are changes in the cell membrane composition and structure. Phosphatidylserine (PS) is translocated to the outer face of the membrane and protrusions are formed that finally break into membrane-enclosed fragments called apoptotic bodies. The mitochondrial membrane also changes with the formation of pores in the outer sheath that provokes the loss of the membrane potential of this organelle. These traits are used to quantitatively assess apoptosis [71, 72]. In metazoans, apoptotic cells and cell fragments are rapidly recognized and phagocytosed by cells of the immune system such as macrophages, which efficiently recognize PS expressed on the membrane of apoptotic cells. This early removal of cellular debris prevents the inflammatory response [73].
