Published online by Cambridge University Press: 05 August 2012
INTRODUCTION
From a life history perspective, growth and development are processes that enhance the functional capacities of an organism through attainments of competencies and physiological signaling across somatic systems, the “growth span” (Arking,2006). The period following growth and development is the mature state. This may usefully be described as the “health span” during which the soma is most able to maintain itself, while also reproducing and investing in reproductive effort. Senescence follows the health span. It is a later occurring age-independent, event-driven process measured in biological time as cascades of physical and chemical events that increase risks of death in the next time interval, the “senescent span” (Crews, 2003; Arking, 2006).
Senescence is a biological process that only the living experience; it is the final period of life history (LH)1 for most organisms whether unicellular or possessing both a soma and a germline.
Senescent changes tend to be cumulative (increasing vulnerability to challenge over time), progressive (losses are gradual), intrinsic (not resulting from modifiable environmental factors), and deleterious (reduced function increases mortality risk in the next time interval) (Arking, 2006). Senescence may be measured in biological time using physiological biomarkers, cascades of inter-related metabolic events (that contrary to developmental integration, lead toward disintegration), cell and transcription cycles (the fundamental units of biological time), gene expression patterns, changes in the proteome, epigenetic effects, and other factors (Arking, 2006). Measures of senescence do not depend upon the passage of absolute time.
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