14.5 Cell Death and Replacement [in progress]
Key Concepts
By the end of this section, you will be able to do the following:
- Explain how different types of cell stress can lead to different types of cell death
- Describe the process of apoptosis (programmed cell death) in animal cells
- Distinguish between intrinsic and extrinsic signalling pathways that activate apoptosis
Cell death can occur in response to environmental stressors, severe cell damage, or as a part of normal cell functioning. In multicellular organisms, cell death does not have to be lethal to the organism itself, as long as those cells can be replaced (e.g., via mitosis). This chapter focuses on types of cell death in animal cells (e.g., Figure 14.15), and the mechanisms and signalling pathways involved.
Figure 14.15 Cell death through necrosis (cell death due to damage) and apoptosis (programmed cell death). [source? Probably need to replace (if copyright issues) or credit]
Types of cell death
Cell death may be a controlled process initiated by a multicellular organism, or can occur in an uncontrolled or unregulated manner. Necrosis is a type of cell death that is uncontrolled and unplanned when cells are exposed to extreme stress or damage (Figure 14.15). During necrosis, cells swell and burst, releasing their contents into the surrounding tissue. This can cause inflammation and damage to nearby cells and can also activate the immune system in animals. Apoptosis, also known as programmed cell death, is a regulated process that eliminates unnecessary or damaged cells in multicellular organisms (Figure 14.15). In animal cells, apoptosis is a tightly controlled process that involves a sequence of molecular events. Apoptosis can be triggered by internal signals such as DNA damage, or external signals such as the presence of a pathogen or toxin. Autophagy, the physiological process of a cell degrading its own components, may also play a role in cell death. It is still unclear whether the role of autophagy in cell death are simply a consequence of attempting to prevent death or an important cause in the cell death process.
Apoptosis: programmed cell death in animal cells
During apoptosis, the cell undergoes several distinct morphological changes in five major steps (Figure 14.16). Once a cell has been programmed for cell death, the chromosomes condense, and the cytoplasm shrinks. Subsequently, DNA (“laddering”), the nucleus and the cell undergo fragmentation. These apoptotic bodies are then engulfed via phagocytosis and digested by neighbouring cells or specialized immune cells. Apoptosis makes it possible to remove cells that are abnormal and those that are no longer necessary for our development. As humans, programmed cell death not only allows for the formation of complex structures such as our hands, but it also helps eliminate probable cancer cells while helping to maintain homeostasis within our body.
Figure 14.16 The five steps of apoptosis in animal cells, beginning with c