Ageing

Introduction to Aging

  • The lecture serves to explore aging as an inevitable part of the human life cycle following the previous lecture focused on growth.

  • Learning Outcomes:

    • Understand why aging is an inevitable aspect of life.

    • Explore some of the cellular mechanisms that lead to aging.

The Aging Process

  • Aging starts from conception and progresses through different life stages including birth, puberty, and maturity until senescence occurs.

  • Senescence is defined as the physiological deterioration that occurs after maturity, leading to decreased efficiency in our physiological systems and ultimately death.

  • Senescence manifests through:

    • Changes in physical appearance (skin, hair, posture).

    • Loss of elastin fibers and muscle structure.

    • Significant changes in physical capacity (cardiovascular, respiratory, reproductive).

Variability in Senescence

  • There is significant variability in how individuals experience senescence.

    • Example of postmenopausal women running marathons while older arthritic men can still reproduce.

  • Aging affects different individuals in varying degrees at different rates.

  • Senescence ultimately affects all individuals, leading to mortality, as shown through survival and mortality data curves.

Survival Curves

  • Survival curves show the percentage of individuals surviving with respect to age:

    • Comparison of survival rates from different birth years (1850, 1870, etc.).

    • Predictive survival curves for individuals born in the 2030s and beyond.

  • *Interesting Observations:

    • In the late 1800s to early 1900s, postnatal survival rates were significantly lower; about 70% of individuals survived past 10 years.

    • Advancements in neonatal care have improved survival rates dramatically.

Mortality Curves

  • Mortality curves depict the chances of dying over time.

  • Mortality rate shows:

    • A sharp increase during the first decade of life indicative of physiological demands.

    • Another increase during adolescence attributed to risky behaviors, particularly among males.

    • A log-linear phase where mortality rates double approximately every eight years.

    • As senior age increases, the rate of mortality may slow down due to factors such as genetics and lifestyle leading to better health maintenance.

Proximate Reasons for Senescence

  • Identified mechanisms contributing to senescence include:

    1. Hayflick's Limit:

      • Cells have a finite number of divisions (40 to 60 divisions) before they lose their ability to replicate.

      • Example: HeLa cells are an exception as they can replicate indefinitely.

    2. Free Radical Damage:

      • Byproducts of metabolism can lead to oxidative damage in cells, affecting lipids, proteins, and DNA.

      • Antioxidants like superoxide dismutase (SOD) offer some protection from this damage.

    3. Genetic Mutations:

      • Mutations accumulated over time through cellular divisions and environmental factors may contribute to senescence.

Environmental Contributors to Senescence

  • Caloric Restriction Studies: Experimentation in rodents shows caloric restriction (50% of normal intake) enhances longevity:

    • Linked to reduced metabolic rate and oxidative damage.

    • Slower development and growth patterns resulting from fewer calories.

Characteristics of Senescence

  • Aging leads to:

    • Reduced protein synthesis and muscle mass, leading to weakness and frailty.

    • Compromised immune response.

    • Decreased cardiovascular and respiratory efficiency.

    • Changes in reproductive hormone production and functionality.

Reproductive Hormones and Aging

  • Menopause (average age 51):

    • Loss of ovarian follicles leads to decreased estrogen production, affecting bone density, body fat, muscle mass, and overall quality of life.

    • Hormone Replacement Therapy (HRT) can mitigate symptoms but comes with potential risks, including breast cancer.

  • Andropause: Male equivalent with a gradual decline in testosterone and sperm count.

    • Variability in testosterone levels makes this less straightforward than menopause.

Societal Implications of Aging

  • Increasing aging population leads to increased healthcare burdens:

    • 80% of 80-year-olds are considered chronically ill.

    • 45% of 85-year-olds may require daily care.

  • Care responsibilities will require societal attention and planning as the population ages.

Conclusion

  • Aging and senescence are complex processes influenced by various biological, environmental, and societal factors.