Cell death occurs by two distinct processes:
Apoptosis is defined as the natural or programmed death of the cell under genetic control. Originally, apoptosis refers to the process by which the leaves fall from trees in autumn (In Greek, apoptosis means ‘falling leaves’). It is also called ‘cell suicide’ since the genes of the cell play a major role in the death.
This type of programmed cell death is a normal phenomenon and it is essential for normal development of the body. In contrast to necrosis, apoptosis usually does not produce inflammatory reactions in the neighboring tissues.
The purpose of apoptosis is to remove unwanted cells without causing any stress or damage to the neighboring cells. The functional significance of apoptosis:
- Plays a vital role in cellular homeostasis. About 10 million cells are produced every day in the human body by mitosis. An equal number of cells die by apoptosis. This helps in cellular homeostasis
- Useful for removal of a cell that is damaged beyond repair by a virus or a toxin
An essential event during the development and in the adult stage.
- A large number of neurons are produced during the development of the central nervous system. But up to 50% of the neurons are removed by apoptosis during the formation of synapses between neurons
- Apoptosis is responsible for the removal of tissues of webs between fingers and toes during developmental stage in fetus
- It is necessary for regression and disappearance of duct systems during sex differentiation in the fetus.
- The cell that loses the contact with neighboring cells or basal lamina in the epithelial tissue dies by apoptosis. This is essential for the death of old enterocytes that shed into the lumen of intestinal glands.
- It plays an important role in the cyclic sloughing of the inner layer of the endometrium, resulting in menstruation.
Apoptosis removes the autoaggressive T cells and prevents autoimmune diseases.
Apoptosis is activated by either withdrawal of positive signals (survival factors) or the arrival of negative signals.
Withdrawal of positive signals
Positive signals are the signals which are necessary for the long-time survival of most of the cells. The positive signals are continuously produced by other cells or some chemical stimulants. Best examples of chemical stimulants are:
- Nerve growth factors (for neurons)
Interleukin-2 (for cells like lymphocytes).
The absence or withdrawal of the positive signals activates apoptosis.
Arrival of negative signals
Negative signals are the external or internal stimuli which initiate apoptosis. The negative signals are produced during various events like:
- Normal developmental procedures
- Cellular stress
- Increase in the concentration of intracellular oxidants
- Viral infection
- Damage to DNA
Exposure to agents like chemotherapeutic drugs, X-rays, ultraviolet rays and the death receptor ligands.
Death receptor ligands and death receptors
Pages: 1 2