EFFECTS OF SPACEFLIGHT ON MORPHOLOGY OF THE RAT ADENOHYPOPHYSIS

To determine the effects of spaceflight on the anterior pituitary glan d, the adenohypophyses of rats after a 7-day spaceflight aboard the sp ace shuttle Endeavor (STS-54) were investigated by histology, immunohi stochemistry, morphometry, electron microscopy, and in situ hybridizat ion and were compared with synchronous control rats. Morphometry revea led the corticotrophs of spaceflown rats to be significantly enlarged, demonstrating 46-48% increases in mean cell, nuclear, and cytoplasmic areas. These corticotrophs also exhibited striking ultrastructural si gns of heightened secretory activity. Furthermore, their expression of proopiomelanocortin mRNA, the transcript encoding the precursor prote in from which adrenocorticotropic hormone is posttranslationally cleav ed, was also significantly enhanced, a finding consistent with their h ypersecretory state. Gonadotrophs also exhibited significant increment s in mean nuclear, cell, and cytoplasmic areas of 22, 45, and 51%, res pectively; however, they were not accompanied by ultrastructural evide nce of increased secretory function. There were no morphological chang es in somatotrophs, lactotrophs, or thyrotrophs, nor were there any si gnificant 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 su ggest that spaceflight is accompanied by a selective morphological res ponse in the anterior pituitary, one characterized by hypertrophy of b oth corticotrophs and gonadotrophs and by enhanced endocrine activity of the former.