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Effects of the environmental light conditions on the ultrastructure of the harderian gland in the newborn rat. biochemical correlation with the pineal gland.

Morphological changes at the levels of both light and electron microscopy were studied regarding the secretory elements of the Harderian glands of neonatal rats, induced by environmental lighting conditions. The results were correlated with biochemical determinations of serotonin levels and hydroxyindole-O-methyl transferase (HIOMT) activity in the pineal gland. Neonatal male albino rats were divided into three groups: 12 were exposed to a light period for seven days; 12 to a dark period, and 12 to diurnal lighting conditions (light, 12 hours; darkness, 12 hours), as controls. In each group some animals were killed at 1.00 pm. The Harderian gland of each animal was immediately fixed and processed for light and electron microscopy while the pineal gland was homogenized in a fixative for biochemical determination of serotonin levels and HIOMT activity. On the light period the acinar lumen was observed to be dilated with increased secretions from the acini and excretory ducts while on the dark period the acinar lumen narrowed with hardly any secretions. On the dark period there were no changes in the cyclic variations of serotonin levels and HIOMT activity in the pineal gland; while on the light period, these cycles disappeared. It is concluded that secretory activity of the Harderian gland is modified by environmental lighting conditions, and that secretions produced by this gland appear to be directly related to the control mechanism of melatonin production by the pineal gland in neonatal rats.