Daily Rhythms in Rectal Temperature and in Epithelial Mitoses of Hamster Pinna and Pouch

¹ From the Division of Cancer Biology, Department of Pathology, University of Minnesota Medical School, Minneapolis, and Cambridge State School and Hospital for Epileptics, Cambridge, Minnesota

Temperature measurements were made at 4-hour intervals during a 24-hour period on 23 male and 32 female hamsters, kept under conditions standardized for the evaluation of 24-hour periodicity. Rectal temperatures were significantly higher during the daily dark period (18:00 to 06:00) than during the light period (06:00 to 18:00) and significantly higher in females as compared to males. At the times of the daily high and low in rectal temperature (at about 12:30 and 20:30, respectively), specimens of pinna and pouch were then removed from two groups, each composed of 10 male hamsters; histologic sections were prepared and mitotic counts were made. Highly significant day-night differences were found, the daily high of mitoses in pouch and pinnal epithelium occurring in each tissue at about the middle of the daily light period, roughly at the time of the daily low in rectal temperature.