Effects of Microgravity on Bone Development

Bone and calcium physiology

Rattus norvegicus (Sprague-Dawley rat)
Flight: 10 dams, 24 pups Male/Female

Delayed synchronous: 10 dams, 24 pups; Vivarium: 10 nonlaparotomized dams, 10 laparotomized dams, 48 pups

Ambient Temperature Recorder, Animal Enclosure Module

As shown in previous studies, the development and mineralization of new bone decreases in microgravity environments. Other studies have shown that bone resorption is not effected by exposure to microgravity. Thus, the bone-forming cells, osteoblasts, have been implicated in the decrease of bone mass. Osteoblasts secrete the neural proteinases collagenase and tissue plasminogen activator (tPA) that are crucial to the process of bone mineralization. This study examines whether the expression of collagenase and tPA in rats of various ages is affected by prenatal exposure to microgravity.

Pups prenatally exposed to microgravity were delivered and prepared at various ages. Frozen blocks of bone from the pups were then sectioned for immunohistochemistry analysis. The antiserum used for collagenase was a monospecific polyclonal rabbit antiserum raised against rat osteoblastic procollagenase. The antiserum used for immunolocalization of tissue plasminogen activator was raised in rabbits against purified rat insulinoma tissue plasminogen activator. Stained sections were photographed and examined to determine distinct differences in the appearance or developmental pattern of the calvariae. The specificity of the tPA and collagenase antibodies was established with Western blot and Zymogram analyses. Thickness of the bone matrix was then analyzed with photoimagery.

Staining for collagenase was present at all ages and revealed little difference between the flight and the control animals. Tissue plasminogen staining also showed no significant differences between flight and control animals. Staining revealed that tPA is localized to blood vessels and that collagenase is localized to endocranial areas that are actively being modeled and in the matrix. This result suggests that tPA and collagenase are not produced by the same population of cells. Photoimagery analysis supported the results obtained with immunohistochemistry and showed that the calvariae thickness was similar for control and flight animals. The relatively brief exposure of the pups to microgravity might account for the lack of difference noted in this study.

Davis, B.A. et al.: Collagenase and Tissue Plasminogen Activator Production in Developing Rat Calvariae: Normal Progression Despite Fetal Exposure to Microgravity. Calcified Tissue International, vol. 63(5), Nov 1998, pp. 416–422.