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 morphologica lly examined in male Djungarian hamsters (Phodopus singorus) exposed to sho rt days and those kept under long days and receiving daily afternoon inject ions 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 inhibite d in short-day-exposed or melatonin-treated animals. It was suggested that a short photoperiod may suppress, via melatonin, the development of ACTH ce lls. 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 adreno cortical zones were depressed under short days. In addition, the volumes of the zona fasciculata and zona reticularis in long-day-housed animals becam e respectively larger and smaller subsequent to orchidectomy and melatonin administration, These results suggest that fasciculata cells in deeper leve ls become progressively differentiated into reticularis cells, that short p hotoperiod inhibits development of both zonae, and that such an inhibition is caused mainly by the decreased secretion of androgens.