Involution patterns of the human thymus. I Size of the cortical area as a function of age.

Thymic tissue obtained at autopsy from 123 victims of sudden death, ranging from neonates to individuals older than 80 years and predominantly male, was studied with histometric techniques, i.e. a combination of test point analysis and planimetry on unit optical fields. The pattern of 'natural' age-dependent involution of the thymic cortex was examined using computerized mathematical models. The range of variations of results was greatest in children and young adults, followed by the very old, then the middle age group. In a first approximation, regression for thymic cortical volume in individuals older than 15 years corresponded better to a negative exponential than to a negative linear function of age. Best fits for the data suggest at least a two-component negative exponential function of age, with a steeper slope of the regression for individuals beyond the age of 30 years. Extrapolation on a log-normal plot of regression lines for thymic cortical involution points to near-zero values at an age range below the estimated maximum human life span, corresponding to the steepest slope of survival curves in Western Europe.