Impact of aging on the morphology and function of the somatotroph cell population in rats

There is little information regarding the impact of aging on pituitary somatotroph (STH) cell population in rats. We therefore undertook a quantitative immunocytochemical assessment of this cell type in young (3 months), old (20 months) and senescent (29 months) male rats. An attempt was also made to correlate morphological parameters with serum levels of growth hormone (GH). Since thyroid status is highly influential on somatotropic function, serum levels of thyroxine (T4) and triiodothyronine (T3) were also measured in the three age groups. We found a marked age-related reduction in STH cell number, volume density, and surface density, as well as a milder but significant decline in STH cell area and perimeter. Basal serum levels of GH remained unchanged with age, whereas the estimated number and amplitude of GH pulses declined from young to old animals. Thyroxine but not T3 levels also declined with age. We conclude that in rats, aging causes a marked reduction in somatotroph number and, to a lesser extent, cell size. These alterations do not affect trough levels of circulating GH. Our data also suggest that the progressive hypothyroidism associated with aging in this species may contribute to the promotion of the above changes. The present study emphasizes the convenience of combining hormone measurements with quantitative morphological analysis of cell populations for the study of pituitary function during aging.