Pars distalis vasculature: Discovery Shuttle STS-29 rats compared to ground-based antiorthostatic rats

The anterior pituitary glands of male, adult Long Evans rats carried 5 days in the Space Shuttle Discovery (STS-29) have been compared with two groups of ground-based controls. All of the animals were part of a study (SE82-08) into the effects of gravity versus a microgravity environment on fracture healing. All had sustained a right, mid-shaft fibular osteotomy. The duration of the study was 10 days, and animals in all groups were weight bearing for the 5 days prior to shuttle lift off. The three experimental groups consisted of four rats each: flight (F) and two ground-based control groups, weight bearing (WB) and suspended (S). The suspension group was in a Holton/Sweeney head-down suspension apparatus (antiorthostatic) for the final 5 days of the study. The anterior pituitary glands of F and WB rats were essentially identical. The vasculature and parenchymal cells appeared unaffected in both instances. However, the anterior pituitary glands of S rats were dramatically altered. The vasculature was widely expanded with proteinaceous deposition covering the lumenal endothelial surfaces, and entrapping numerous platelets and aggregates of red blood cells. Parenchymal cells were highly vacuolated, occasionally with membranous vacuoles, but most often revealing large, clear cytoplasmic zones unlined by any membranes. Whereas profiles of exocytosis were numerous in F rats, and present in WB rats, they were essentially absent in S rats. These results indicate that weightlessness over a 5-day flight period does not influence the structural integrity of the anterior pituitary gland and may in fact promote secretory granule release. However, the head-down tilt model, frequently used to study fracture repair under conditions that mimic weightlessness, has a profound impact on the vasculature of the anterior pituitary gland which then affects the structural and functional characteristics of the parenchymal cells.