Signs of aging are apparent in the testis interstitium of Sprague Dawley rats at 6 months of age

Abstract

The present study investigated the effects of aging in the testis interstitium in Sprague Dawley rats. Rats of 3, 6 and 24 months of age were used. Testes of rats ( n = 5) were fixed by whole body perfusion using a fixative containing 2.5% glutaraldehyde in cacodylate buffer, processed and embedded in eponaraldite. Using 1 μm sections stained with methylene blue, qualitative and quantitative morphological studies were performed. Purified Leydig cell preparations, obtained by collagenase digestion followed by elutriation and density gradient centrifugation, were used to determine luteinizing hormone (LH; 100 ng/ml) stimulated testosterone secretory capacity per Leydig cell in vitro. Testosterone levels in the incubation medium, and testosterone and luteinizing hormone levels in serum of these three groups of rats were determined via radioimmunoassay. Morphological studies revealed that Leydig cells were more abundant in the testis interstitium at 6 and 24 months when compared to 3 months. Moreover, collagen fiber bundles were more frequently observed in the testis interstitium at older ages. Blood vessels of the testis interstitium in 24-month-old rats frequently showed partial and complete occlusion of their lumen and thickening of vessel walls. This feature was also present at 6 months, but less frequently. The results of the sterological studies revealed that the volumes of seminiferous tubules, interstitium and Leydig cells per testis was significantly higher ( P < 0.05), at 6 and 24 months of age than those at 3 months. Moreover, volume of macrophages per testis was observed to be significantly higher ( P < 0.05) at 6 months when compared to 3 and 24 months, and volume of connective tissue cells per testis was observed to be significantly lower ( P < 0.05) at 6 and 24 months when compared to 3 months of age. No significant difference ( P > 0.05) was observed for the volume of lymphatic space per testis in the three age groups studied. Volume of interstitial blood vessels per testis was not significantly different at 3 and 6 months of age, but a significantly greater ( P < 0.05) volume was observed at 24 months. However, at 6 and 24 months, only 71% and 31% of the total blood vessel volumes respectively had completely open lumen in them; the rest of the blood vessels were either partially (12.5% at 6 months and 17% at 24 months) or completely (16.5% at 6 months and 52% at 24 months) occluded. The number of Leydig cells per testis was doubled at 6 and 24 months of age compared to 3 months. The average volume of a Leydig cell was not significantly different between 3 and 6 months of age, however, at 24 months a significantly lower ( P < 0.05) value was observed. LH stimulated testosterone secretory capacity per Leydig cell in vitro was reduced by 50% at 6 months of age compared to 3 months; a further significant ( P < 0.05) reduction was observed at 24 months. Serum testosterone and LH levels were not significantly different between 3 and 6 months of age but at 24 months a significantly lower ( P < 0.05) value was observed for both of these hormones. In summary, the present study demonstrated many changes in the components of the testis interstitium in the aged Sprague Dawley rat. Modifications in the blood vessels and the occurrence of abundant collagen fibers in the interstitial space could possibly contribute to the reduced testosterone secretory capacity per Leydig cell with advancing in age. The observed Leydig cell hyperplasia could be suggested as a compensatory effort to maintain the normal androgen status of the aged rat, which is rather successful at 6 months but unsuccessful at 24 months. This investigation further revealed that these characteristic changes in the aged testis interstitium at 24 months are also present to some extent at 6 months of age in Sprague Dawley rats, suggesting that aging of the testis in this strain of rats commences early in life.