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Effects of photoperiod and melatonin on the development of growth hormone cells and the pituitary-adrenal axis in the Djungarian hamster, Phodopus sungorus.

The development of GH cells and the pituitary-adrenal axis was morphologically examined in male Djungarian hamsters (Phodopus sungorus) exposed to short days and those kept under long days and receiving daily afternoon injections of melatonin, from the time of weaning (20 days) until 100 days of age. The postnatal increase in area of ACTH cells under long days was inhibited in short-day-exposed or melatonin-treated animals. It was suggested that a short photoperiod may suppress, via melatonin, the development of ACTH cells. GH cells were not affected by age, photoperiod or exogenous melatonin. Under long days, the zona fasciculata decreased in volume with age, while the zona reticularis increased. Such changes in the volumes of these adrenocortical zones were depressed under short days. In addition, the volumes of the zona fasciculata and zona reticularis in long-day-housed animals became respectively larger and smaller subsequent to orchidectomy and melatonin administration. These results suggest that fasciculata cells in deeper levels become progressively differentiated into reticularis cells, that short photoperiod inhibits development of both zonae, and that such an inhibition is caused mainly by the decreased secretion of androgens.