Effects of photoperiodism on the histomorphometry of the thyroid gland of the African giant rat (<i>Cricetomys gambianus</i>)

Abstract

The effects of photoperiodism on the histomorphometry of the thyroid gland of African giant rats (AGR) waa investigated. A total of 30 captive AGR of both sexes were randomly divided into 3 different photoperiodic groups of 10 rats each (5 males and 5 females). Group 1 rats had 12 hours darkness and 12 hours lighting for 8 weeks, Group 2 rats had total darkness (24 hours) for 8 weeks while, Group 3 rats had total lighting (24 hours) for 8 weeks. Each rat was euthanized using sodium pentobarbital at 86mg/kg BW intraperitoneally. There was no significant increase (P>0.05) in the body weight of the rats in exposed to total darkness (Group 2). However, there was a significant decrease in the body weight of rats exposed to light for 8 weeks (P<0.05) when compared to rats in control group. Continuous lighting decreases the adipose tissue of thyroid glands with increase number of follicular cells, while, continuous darkness decreases follicular cells with increased adipose tissue. The thyroid weights increased significantly (P<0.05) in Group 3 and Group 2 rats when compared to Group 1. Exposure of AGR to continuous light significantly decreased the body weight and increased the weight and width of the thyroid gland. It is therefore recommended to house AGR under controlled conditions with at least 12 hours each of light and darkness.