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Cellular Clock Theory: This theory suggests that biological aging is due to the fact that normal cells cannot divide indefinitely. This is known as the Hayflick limit, and is evidenced in cells studied in test tubes, which divide about 40-60 times before they stop (Bartlett, 2014).
2) Endocrine Theory. Biological clocks act through hormones to control the pace of aging. Recent studies confirm that aging is hormonally regulated and that the evolutionarily conserved insulin/IGF-1 signaling (IIS) pathway plays a key role in the hormonal regulation of aging.
Cellular theories of aging propose that human aging is the result of cellular aging, whereby an increasing proportion of cells reach senescence, a terminal stage at which cells will cease to divide. This will limit the body's ability to regenerate and to respond to injury or stress.
It is claimed that telomere shortening may be the molecular clock that triggers aging. Studies attempted to verify this hypothesis, by transfecting two telomerase-negative normal human cell types, retinal epithelial cells and foreskin fibroblasts, with vectors encoding the catalytic subunit of human telomerase (hTERT).
In recent years, breakthroughs in developing epigenetic clocks that track biological age, late-life health and mortality have not only yielded biomarkers of aging, but have also demonstrated a close correlation between certain methylation changes and aging (Bocklandt et al., 2011, Hannum et al., 2013, Horvath, 2013, Raj and Horvath, 2020).
Aging can be defined as a state of progressive functional decline accompanied by an increase in mortality. Time-dependent accumulation of cellular damage, namely lesions and mutations in the DNA and misfolded proteins, impair organellar and cellular function.
INTRODUCTION. To achieve lifespan extension in humans, we must understand which cellular programs are responsible for aging and how their dysregulation directs senescence and decline.
Collectively, these experiments demonstrate that epigenetic aging, as measured by the Skin&blood clock, is distinct from two of the best-characterized hallmarks of aging, cellular senescence...
To address this, we recommend that researchers describe the output of an aging clock based on the type of input data used or the name of the clock itself. Epigenetic aging clocks produce epigenetic age, transcriptomic aging clocks produce transcriptomic age, and so forth. If a clock has a unique name, such as our recently developed epigenetic ...
Biological clocks provide a means to evaluate whether a molecule, cell, tissue or even an entire organism is old or young. Here we summarize established and emerging molecular clocks as...