Cell Aging and Death

Section 3: Cell Aging and Death

Characteristics of Cell Aging

Growth and aging, birth and death, are normal phenomena in the biological world, applicable not only to individual organisms but also to cells, the fundamental units of life systems.

Features of Cell Aging

Cell aging is a process where the physiological state and chemical reactions of cells undergo complex changes, ultimately manifesting as changes in cell morphology, structure, and function.

Aged cells exhibit several characteristics:

Aging cells affect the appearance of individuals, and the aging of individuals is prominently visible externally. For instance, due to the aging of melanocytes in hair follicles, there is a decrease in tyrosinase enzyme activity and melanin synthesis, leading to hair turning white in elderly people. Aging skin develops "age spots," a result of pigment accumulation within cells.

Causes of Cell Aging

Since the 1990s, significant progress has been made in understanding the mechanisms of cell aging. Scientists have proposed several hypotheses, with the most widely accepted being the Free Radical Theory and the Telomere Theory.

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Free Radical Theory: Typically, highly reactive charged molecules or groups are termed free radicals. Free radicals possess unpaired electrons, exhibiting high reactivity. During metabolic processes, cells constantly engage in various oxidation reactions, which easily generate free radicals. Additionally, radiation and harmful substances can also stimulate the production of free radicals. For instance, water under ionizing radiation produces free radicals. Once formed, free radicals attack and damage various biomolecules essential for normal cell function. Particularly severe is the attack on phospholipid molecules, constituents of biological membranes, which in turn generate more free radicals, triggering a cascade reaction that significantly damages the cell membrane. Furthermore, free radicals attack DNA, potentially causing genetic mutations, and attack proteins, reducing protein activity, thereby contributing to cell aging.

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Telomere Theory: Each end of every chromosome possesses a special sequence of DNA and protein complex called a telomere (Figure 6-9). After each cell division, the telomere DNA sequence shortens. With an increase in the number of cell divisions, the shortened portion gradually extends inward. As the telomere DNA sequence is "cut" short, the inner normal genes of the telomere suffer damage, causing abnormal cell activity.

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Relationship Between Cell Aging and Individual Aging

For unicellular organisms, cell aging or death equates to the aging or death of the individual; however, for multicellular organisms, cell aging and death are distinct from individual aging and death. Cells within multicellular organisms are constantly renewing, with some cells always in a state of aging or approaching death. Nevertheless, overall, the process of individual aging is also the universal aging process of cells composing the individual.

Experiments discussed in the next page explore the relationship between age factors and cell aging.

Generally, human cells cultured in vitro can divide up to about 50 times before ceasing division and losing normal function. With increasing age, the number of cell divisions gradually decreases, indicating that cells age with increasing division. Elderly individuals heal slowly from bone fractures, which is related to the aging of osteoblasts.

The cell nucleus serves as the control center for cell life activities. With an increase in the number of cell divisions or entry into a senescent state, the genetic material in the cell nucleus shrinks, and the activity of some enzymes decreases, thereby affecting cell life.

Cell aging is a normal biological phenomenon occurring within the human body. Normal cell aging facilitates better self-renewal of the organism. For example, skin basal layer cells constantly divide to produce new cells to replace aging cells; rapid turnover of red blood cells in the blood ensures oxygen supply. However, aging of many cells and tissues within the organism leads to human aging, resulting in decreased immunity and reduced ability to adapt to the environment.

Cell Death

Cell death includes apoptosis and necrosis, among other methods, with apoptosis being a primary mode of cell death.

The term apoptosis in English, derived from ancient Greek, means the shedding or falling off of petals or leaves. Choosing this term emphasizes that apoptosis is a natural physiological process.

During the embryonic stage, humans undergo a tailed phase, after which tail cells undergo automatic death, causing the tail to disappear (Figure 6-10). The disappearance of tadpole tails is also achieved through cell autophagy. Observation of Figure 6-11 shows that during fetal hand development, initially five fingers are connected like a spatula, later developing into formed fingers as the cells between the fingers undergo automatic death. Thus, the process of genetically determined cell automatic death is called apoptosis. Since apoptosis is rigorously regulated by genetic mechanisms, it is a programmed cell death (programmed cell death).

In mature organisms, natural cell renewal and clearance of some cells infected by pathogens are also accomplished through apoptosis. Apoptosis plays a critical role in completing normal development, maintaining internal environmental stability, and resisting interference from various external factors.

Necrosis refers to cell damage and death caused by impaired or interrupted cell metabolism due to various adverse factors, such as extreme physical or chemical factors or severe pathological stimuli.

Autophagy is colloquially referred to as cells eating their own structures and substances. Under certain conditions, cells degrade damaged or functionally degraded cell structures through lysosomal degradation and reuse, known as autophagy. Under conditions of nutrient deficiency, cells obtain the substances and energy needed to maintain survival through autophagy; when cells are damaged, invaded by microorganisms, or undergoing cell aging, autophagy can eliminate damaged or aging organelles, as well as infected microorganisms and toxins, thereby maintaining stability within the cell environment. Intense autophagy may induce apoptosis. Research indicates that many human diseases may be related to impaired autophagy mechanisms; therefore, the study of autophagy mechanisms is of significant importance for preventing and treating many diseases.