Chapter 2. Review of the literature
- Table of Contents
- 2.1. Apoptosis
- 2.2. Common mechanisms of apoptosis
- 2.3. Ovarian function
- 2.4. Endometrial function
2.1. Apoptosis
Multicellular animals often need to discard cells that are superfluous or potentially harmful. Apoptosis is an active dedicated programme that leads to regulated destruction of a cell. It counter-balances cell division and cell migration in the upkeep of homeostasis and ensures proper function in a variety of tissues, such as reproductive organs and the immune system.
The process starts with a death signal. This signal can be a physiological messenger molecule, e.g. a cytokine or hormone, but also radiation and some chemical and pharmacological agents can trigger the molecular cascade leading to apoptosis. After the signal is recognised by the cell, its strength is weighed against factors that promote cellular survival (reviewed in Hengartner 2000). Mitochondria represent not only the powerhouse of the cell, but also a central command centre that participates in the decision as in whether the cell should commit suicide through apoptosis, or continue its business (reviewed in Kroemer & Reed 2000). Mitochondria sequester a number of proteins that can initiate an automatic sequence leading to cell death when they are released to the cytoplasm. The Bcl-2 family is directly associated with actions on the mitochondrial membrane that can lead to an increase in mitochondrial permeability and outflow of pro-apoptotic proteins (Gross et al. 1999). The Bcl-2 family includes pro and anti-apoptotic members and execution of the apoptotic message is dependent on their relative proportions (Chao & Korsmeyer 1998).
In the actual execution stage of apoptosis, proteolytic enzymes, caspases are activated. They are responsible for the most visible stages of apoptotic cell death (Zimmermann & Green 2001). Caspases selectively cleave target proteins, which results in both deactivation and activation of the proteins (Dales et al. 2001). These substrates include structural proteins, other caspases and DNase (reviewed in Grutter 2000). The subsequent activation of other caspases functions as an enhancer of the process, creating a growing cascade of apoptotic proteinases (Grutter 2000). The eventual activation of caspase-activated DNase (CAD) leads to DNA laddering that is typical of apoptosis (Wyllie et al. 1984). In the end apoptosis reduces cells to small apoptotic bodies that are swiftly cleared up from the scene by phagocytosis (Platt et al. 1998).
2.1.1. History
Apoptosis has been discovered and rediscovered several times. With the development of the microscope and formulation of the cellular paradigm, the understanding of cells as the basic biological building blocks rapidly grew. Early studies depicted proliferation of cells taking place throughout development and in 1842 Vogt described for the first time that cell death was present in toad development (reviewed in Clarke & Clarke 1996). In 1885, nearly half a century later, Flemming discovered cell death in rabbit Graafian follicles (Flemming 1885). This was the first recognition of cell death as a part of physiological function. Later on Flemming further developed the theory and proposed that cell death involved chemical changes within the cells. Two years later he also observed that degeneration of testicular germ cell populations occurred through a similar mechanism (Flemming 1887). Flemming used the term chromatolysis to describe his new observations, and by the end of the 19th century, chromatolysis was widely accepted to depict a distinct form of cell death that is called apoptosis today. These studies were conducted over a century ago, and Flemming magnificently documented the morphological features of apoptosis almost nine decades before the concept was introduced. The importance of Flemming’s findings was not understood at the time and he himself strayed away from the study of cell death (reviewed in Clarke & Clarke 1996). The concept of physiological cell death was kept alive by a small number of scientists, mainly in the field of developmental biology. In 1914 the German anatomist Ludvig Gräper proposed a theorem that to compensate for mitosis there would also have to be mechanisms to keep the continuing proliferation in check, referring to Flemming’s chromatolysis as a possible candidate (reviewed in Majno & Joris 1995). While the logic behind Gräper’s idea seems obvious today, his paper on the issue was mainly ignored at the time.
Almost sixty years later, apoptosis was rediscovered one more time by the Australian pathologist, John Kerr. During his Ph.D. studies he observed a type of cell death in hepatocytes that was distinguishable from necrosis and he named it shrinkage necrosis. In 1972, Kerr, Wyllie and Currie proposed the term apoptosis to describe a relatively conserved set of morphological features observed in a wide variety of cell types during physiological episodes of cell death (Kerr et al. 1972). The term apoptosis was suggested by James Cormack, professor of classical Greek at the University of Aberdeen. It means “to fall away from” (apo = from, ptosis = a fall), previously used to describe the falling of leaves in the autumn. This paper, now considered as a landmark in the field of cell death, described how developmental and homeostatic cell deaths controlled by the body could be separated from classical, accidental cell death, necrosis. After these findings, another research group published their observations where they found that radiation caused the DNA in lymphocytes to break down into multiples of approximately 180 – 200 bp (Yamada et al. 1981). Wyllie incorporated these findings and realised that the DNA ladder formation was indeed a hallmark of apoptosis (Wyllie et al. 1984). The development of a molecular mechanism to study apoptosis led to a rapid expansion of apoptosis research. Since then a plethora of oncogenes and tumour suppressor genes have been found to have a role in regulation of apoptosis, emphasizing the role of apoptosis in control of homeostasis (reviewed in Cheng 1999). Furthermore, different transgenic and knockout animal models have enabled detailed studies of apoptosis in a variety of physiological functions.
2.1.2. Apoptosis vs. necrosis
Necrosis is a violent from of cell death that is caused by a range of noxious chemicals, biological agents, or physical damage (Wyllie 1997b). It is associated with rupturing of cellular membranes, swelling of the cells and random destruction of the cellular structures. The cytosolic proteins of the dying cell are released into the intercellular space, causing an inflammatory reaction (Wyllie 1997b). Necrosis typically involves large groups of cells that have become victims of the same pathological assault (Arends & Wyllie 1991) (Fig. 1).
Apoptosis occurs in single cells that separate themselves from neighbouring cells and/or the intracellular matrix (Kerr et al. 1972). Rather than swelling, an apoptotic cell loses volume and shrinks. At the same time the cellular matrix is actively dismantled by caspases. This is an energy-requiring, coordinated process opposite to necrosis, which does not require energy after the initial pathological attack that sets the process in motion (Wyllie 1997b). In the end, neighbouring cells or macrophages will take care of the apoptotic bodies by phagocytosis, averting an inflammatory response (reviewed in Savill & Fadok 2000) (Fig. 1).
However, as with most things in life, death also evades classification into two exact categories. Apoptosis and necrosis are often viewed as endpoints on a sliding scale. Some necrotic cell deaths display apoptotic properties such as chromosomal condensation or DNA fragmentation, and sometimes a process that would be defined as apoptosis possesses some qualities of necrosis (Majno & Joris 1995).