[Experimental morphological studies of stress-inducing effects of weightlessness in rats flown aboard Spacelab-2]

Adrenals and thymus of rats flown for 14 days on board the US space life sciences laboratory, Spacelab-2 (SLS-2), were analyzed histologically and histomorphometrically. Adrenals of the rats sacrificed on flight day 13 did not exhibit any morphological signs of elevated functional activity. Within 5 hours following the mission, the functioning of adrenals was growing, while thymus displayed sites of thymocyte histolysis and suppression of cell mitotic activity. Taken together, these observations infer the development of stress. Based on the morphological criteria, a conclusion has been drawn that the exposure on board SLS-2 did not exert any expressed stress-inducing effect on the rats and it was the transfer from microgravity to the normal gravity that produced acute gravitational stress of moderate severity.