Presence of circadian rhythm in the mitotic index of the ruminal epithelium in sheep.

Mitotic indices of the ruminal epithelium of two castrated male sheep fed at 11:00 h were determined at 00:00, 04:00, 08:00, 12:00, 16:00 and 20:00 h. The schedules were repeated in four trials done in December 1975, February, March and June 1976 respectively. The indices ranged from 0.13 to 0.92 per cent, both animals showing a similar tendency in these fluctuations. The mean rate of cell division was low in the morning, high in the afternoon and in the evening, and declined at about midnight. Another pair of male castrated sheep were fed at 07:00, and rumen samples were biopsied six times in three days. The mean rate of epithelial cell division in the rumen was low at 08:00, but high at noon, suggesting that the rate possibly was affected by the food intake of the experimental animals.