Sex differences in secretion pattern of neonatal rat Harderian gland under various environmental lighting conditions

1. Secretion pattern of Harderian gland of neonatal rats maintained under (a) diurnal lighting conditions; (b) continuous light or (c) continuous darkness was studied at light microscopy level. 2. All animals were placed in especially designed cages at 13:00 hr on day 1 and studied on day 7 at 13:00 and 23:00 hr, respectively. 3. Acini with intraluminal secretion were counted in glands from each animal and the results were separately grouped for male and female animals. 4. A diurnal rhythm in secretion pattern of rat Harderian gland in neonatal period was demonstrated. 5. A statistically significant difference was observed in the gland secretion pattern between males and females at both, 13:00 and 23:00 hr when the animals were kept under diurnal lighting conditions. 6. Under continuous light or continuous darkness, the diurnal rhythm in secretion pattern was lost and no significant differences were seen when data from males were compared to those from female neonates. 7. Results are discussed in terms of the possible function of Harderian gland as element of an extraretinal photoreceptor system involved in the regulation of pineal function in neonatal rats.