Single or group housing altered hormonal physiology and affected pituitary and interstitial cell kinetics

A significant negative correlation between testicular interstitial cell tumors and pituitary tumors in control male F344 rats has been reported associated with the number of animals per cage. Change in numbers of animals per cage may cause stress and increased serum corticosteroids that can impair testosterone synthesis by interstitial cells. Eventual atrophy of interstitial cells may result in pituitary hyperfunction and tumor development. For relevant risk assessments, understanding the effect husbandry has on cellular processes is necessary. Twenty-four 6-week-old male F344 rats were housed individually, as pairs, or as trios for 13 weeks. Measured parameters included feed consumption, body and organ weights, hemograms, hormonal levels, histopathology, and cellular kinetics in the pituitary and testicle. Several caging-related differences occurred, that, although not statistically different, could be biologically significant; these included increased serum levels of estradiol, progesterone, and corticosterone; increased spermatogonial proliferation; decreased apoptosis within seminiferous tubules; and increased BrdU immunoreactivity of the interstitial cells. The statistically significant decrease in lymphocyte numbers correlated with the associated increase in corticosterone levels. This study indicates that the number of animals in a cage is associated with hormonal and cellular kinetic changes in the pituitary and testes, which could influence the incidence of tumors in these organs.