Inhibitory action of the pineal gland on the nuclear volume of thyroid follicular cells in male gerbils (Meriones unguiculatus)

: Karyometric investigations of thyroid follicular cells were performed in four groups of adult male gerbils [I. sham-operated (Sham PX) + short photoperiod (SPP)-exposed; II. Sham PX + long photoperiod (LPP)-exposed; III. pinealectomized (PX) + SPP-exposed; IV. PX + LPP-exposed]. The mean nuclear volume (MNV) of thyroid follicular cells was significantly higher in both PX groups (III and IV) when compared to Sham PX controls (I and II). In Sham PX gerbils the maintenance of animals in SPP (10 h light: 14 h darkness) for 13 weeks resulted in significant decrease of MNV of thyroid follicular cells in comparison to that in Sham PX animals kept in LPP (14 h light: 10 h darkness). Lighting conditions did not significantly affect the MNV of thyroid follicular cells in PX gerbils. These data suggest that the pineal gland itself and not light deprivation is the most important factor depressing the activity of the thyroid in male gerbils. Lighting conditions seem to affect the function of thyroid follicular cells inasmuch as they activate or block the action of the pineal gland.