A seasonal study of pinealocytes in the 13-lined ground squirrel, Spermophilus tridecemlineatus.

A quantitative histological study of pinealocytes in the 13-lined ground squirrel, Spermophilus tridecemlineatus, was conducted over a period of one year. Consistent and statistically significant seasonal differences were found in nuclear and nucleolar dimensions suggesting a circannual rhythm in the metabolism of these cells. Annual peaks occurred during hibernation, whereas the nadirs occurred during the period of sexual activity. Differences between sexes in any season were not statistically significant. An inverse relationship between nuclear and nucleolar size and the normal sexual cycle suggests that reproduction in this species may be mediated by the pineal gland. The question of what role, if any, the pineal gland plays in hibernation awaits further experimental evidence.