الفهرس 
MelatoninOnly 14 pages are availabe for public view
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Abstract
This work was done to study some histological and histochemical changes occurring in the thyroid glands and gonads of the albino rat under the effect of melatonin in different photoperiods. Two hundred albino rats of different ages and sexes were used in this study. They were divided into 4 equal groups each ofthem was composed of 50 animals. Each group was further subdivided into 5 equal subgroups according to their ages as follows: 15,45,90,210 days and 1.5 years. The control group: This group was kept under a 14 hours Light110 hours Dark photoperiod (1 4 L/10 D). The group treated by melatonin: This group was treated by melatonin given from the first day of life and the rats were kept under 14 L/10 D photoperiod. Melatonin was administrated subcutaneously in the first 2 weeks of life then orally after that. The group exposed to short photoperiods: This group was kept under short photoperiods (1 Ll23 D) since birth. The group exposed to short photoperiods and treated by melatonin: The animals in this group received melatonin and exposed to short photoperiod (1 L/23 D) since birth. The thyroid, testes and ovaries of the rats of all groups were dissected and prepared for staining by the following techniques: -Haematoxylin and Eosin to study the general histological features. -Periodic acid Schiff’s (PAS) technique to detect the PAS- positive material. -Mallory7s trichrome stain for studying the collagen fibers. -Feulgen reaction for stain deoxyribonucleic acid (DNA) content of the nuclei. The following histological changes were found: THYROID GLAND The group treated by melatonin: The thyroid glands of rats aged 15,45,90,210 days showed follicles lined by cuboidal epithelium and contained vacuolated colloid. Moreover, apparent increase in the PAS-positive reaction was detected, which indicate formation and absorption of thyroglobulin. This nearly normal activity may be related to the mode of melatonin administration used in this study that is different from previous studies, which reported inhibition of the gland. The thyroid follicles of 1.5 years old rats showed scalloped colloid with lightly stained nuclei in the follicular cells. The degenerative change demonstrated in the control group could not be detected in melatonin treated rats indicating its protective effect secondary to its antioxidant properties. The group exposed to short photoperiods: It was observed that some thyroid follicles of 15 and 45 days old rats contained more eosinophilic colloid with no vacuoles. Some of their follicular cells contained deeply stained condensed chromatin with apparent increase in the connective tissue stroma. Also, these inhibitory effects were noticed in the 90 days old rats. These signs of inhibition may be due to direct action of short photoperiods on the thyroid with further pineal melatonin effect, which leads to decrease in the thyroid stimulating hormone (TSH) levels. In the age of 210 days and 1.5 years, the thyroid glands exhibit the normal picture seen in the control groups of the same age. This could be related to an insensitivity (photorefractory) state after chronic exposure to short photoperiods. The group exposed to short photoperiods and treated by melatonin: The thyroid glands of 15,45,90,210 days showed follicles contained large vacuolated colloid with their lining follicular cells ranged from cuboidal to columnar which showed an apparent increase in the PASpositive reaction. This normal structure might occurred secondary to melatonin administration that has a counter-inhibitory effect (counterantithyroid) when it is administered via the drinking water in prolonged darkness. In the thyroid follicles of 1.5 years old rats, the majority ofthe follicular cells contained lightly stained dispersed chromatin with an apparent increase in the PAS-positive reaction in the colloid and the follicular cells. This increased activity might be interpreted by the combated action of melatonin against DNA damage. TESTIS The group treated by melaton in: The testes of 15 days old rats showed the normal structure in addition to the presence of more lightly stained chromatin in the seminiferous cells that contained larger nuclei. This finding may be secondary to melatonin inspite of some previous studies, which showed no effect of melatonin-in the testes of rats at this age. On the other hand, the testes of 45 days old rats showed inhibition, which was manifested by shrunken seminiferous tubules and presence of few numbers of primary spermatocytes with more hyperchromic nuclei of seminiferous epithelium. This period was said to be sensitive for the inhibitory action of exogenous melatonin secondary to a decrease in the pituitary gonadotropins releasing hormone receptor contents. This finding could indicate that melatonin delays puberty. However, the testes of 90 and 210 days old rats showed an apparent increase in the size of the tubules with appearance of large spermatocytes and presence . of spermatozoa in almost all tubules. Also, the seminiferous cells showed more lightly stained nuclei. This activity could be explained by an increase in testosterone level secondary to melatonin administration. Also, the apparent decrease in the cellular component of some seminiferous tubules and the irregularity of their basement membrane that were observed in the control group of 1.5 years could not be seen in rats of the same age group after administration of melatonin. Moreover, more dispersed chromatin in the seminiferous epithelium and an apparent increase in the number of Leydig cells could be demonstrated. These results might be caused by melatonin that prevents age-related decline in testosterone production. The group exposed to short photoperiods: It was noticed that the testes of 15 days showed an increase in the interstitium with apparent decrease in the diameter of the seminiferous tubules. By the age of 45 days, an apparent decrease in the number of spermatogenic cells and Leydig cells was detected. Also, the testes of 90 days old rats showed the same inhibitory changes. The seminiferous epithelium showed more PAS-positive reaction with increased thickness of the basement membrane. Moreover, condensed chromatin was seen in the seminiferous cell nuclei. These inhibitory findings in the previous three ages could be explained by a probable depression of LH secondary to the exposure to short photoperiods on the pineal gland. On the other hand, the testes of 210 days and 1.5 years old rats showed regression of the inhibitory effect of short photoperiod. The insensitivity state reported after chronic exposure could explain this regression. The group exposed to short photoperiods and treated by melatonin: The testes of 15 days old rat showed the normal histological picture, like that of the control in addition to presence of more lightly stained, dispersed chromatin in the nuclei of the seminiferous epithelium. These results may be due to melatonin administration. On the other hand, in the 45 days old rats, small seminiferous tubules were seen with an apparent decrease in their cellular component similar to what occurred in the testes of melatonin and short photoperiod exposed groups of the same age. The testes of 90 days old rats were different from that of rats exposed to short photoperiod only. The seminiferous tubules showed smooth contour and were apparently comparable to the size of the control group. They surrounded by thin basement membrane, which had mild PASpositive reaction. These results could be explained by that the pituitary gonadal axis become refractory to the inhibitory influence of darkness after melatonin administration. Moreover, in 210 days and 1.5 years old rats, more dispersed chromatin in the sperrnatogenic nuclei with apparent increase in the interstitial Leydig cells were detected. These stimulatory results were hypothesized to occur in response to a probable increase in the LH and testosterone blood level secondary to melatonin administration.