Effects of spaceflight on morphology of the rat adenohypophysis

To determine the effects of spaceflight on the anterior pituitary gland, the adenohypophyses of rats after a 7-day spaceflight aboard the space shuttle Endeavor (STS-54) were investigated by histology, immunohistochemistry, morphometry, electron microscopy, and in situ hybridization and were compared with synchronous control rats. Morphometry revealed the corticotrophs of space-flown rats to be significantly enlarged, demonstrating 46-48% increases in mean cell, nuclear, and cytoplasmic areas. These corticotrophs also exhibited striking ultrastructural signs of heightened secretory activity. Furthermore, their expression of proopiomelanocortin mRNA, the transcript encoding the precursor protein from which adrenocorticotropic hormone is posttranslationally cleaved, was also significantly enhanced, a finding consistent with their hypersecretory state. Gonadotrophs also exhibited significant increments in mean nuclear, cell, and cytoplasmic areas of 22, 45, and 51%, respectively; however, they were not accompanied by ultrastructural evidence of increased secretory function. There were no morphological changes in somatotrophs, lactotrophs, or thyrotrophs, nor were there any significant changes in the overall frequency of any one adenohypophyseal cell type in comparison with control. The structural integrity of all adenohypophyseal secretory and vascular elements was preserved after spaceflight, as there was neither evidence of necrosis nor other forms of cellular injury in spaceflown specimens. Capillaries were patent, and neither endothelial damage nor thrombosis was noted. These data suggest that spaceflight is accompanied by a selective morphological response in the anterior pituitary, one characterized by hypertrophy of both corticotrophs and gonadotrophs and by enhanced endocrine activity of the former.