Together, these results suggest that effects on testicular weight are due to simulated hypogravity rather than stress. Atrophy of testes in HLS rats was accompanied by significant reductions in plasma T compared with nonsuspended controls and an 85% loss of spermatogenic cells, indicating that both spermatogenesis and steroidogenesis were impaired in HLS rats.
The results of the HLS studies of T levels have generally been difficult to interpret because of the lack of standardization across studies of animals and methods for blood collection. Testosterone levels vary dramatically during testicular development in maturing rats and within normal diurnal cycles. In addition, studies of male reproduction using HLS require certain modifications because of the unique anatomical characteristics of rats. Unlike humans, the inguinal canal between the scrotum and abdominal cavity does not close in rats. Therefore, the 30° head-down angle used in the HLS model to unload the hindquarters can translocate the testes and epididymis into the abdomen. Exposing the testes to body cavity temperature results in infertility due to destruction of the germinal epithelium and death of spermatogonia. Histological examination of the testes of HLS rats revealed cellular damage, whereas preventing the testes from translocating into the abdomen by inguinal ligation resulted in no aberrant histological features in the testes and normal spermatogenesis. Despite inguinal ligation, T levels remained reduced in HLS rats, and despite reduced T levels, there were no significant effects on the androgen-dependent accessory sex glands of HLS rats. However, the study duration (7 days) may not have provided sufficient time for changes in the accessory sex glands to be observed.
HLS is an extremely useful model of simulated hypo-gravity in studies of male reproductive physiology in rats provided that care is taken to prevent translocation of the testes into the abdomen and maturational factors are taken into account. Studies using the HLS model are providing evidence that the hypogravity component of spaceflight exerts pronounced effects on male reproductive processes.