Histomorphometric Study of SLS-2 Rat Tibia

Bone and calcium physiology

Rattus norvegicus (Sprague-Dawley rat)
Flight: 15 Male

Basal: 5, Synchronous: 15

Research Animal Holding Facility

Previous flight experiments demonstrated reduction of trabecular bone caused by inhibited neoformation and enhanced resorption. However, it was difficult to determine what changes in spongy bone were caused by space flight and what were caused by the stress of returning to the Earth's gravity. The objective of this experiment was to differentiate bone changes caused by microgravity from those caused by re-entry factors.

Flight rats were divided into three groups, with five dissected on flight day 13, 5 dissected 6 hours after flight (F+0), and five dissected 14 days after flight (F+14); ground control rats were dissected simultaneously with corresponding flight groups. Proximal metaphyses of tibiae were fixed in 4% paraformaldehyde, dehydrated, and embedded in a mixture of methylmetacrylate and JB-4. Sections (5 mm thickness) were cut with a Polycut S microtome. Sections for histomorphometric study of bone mass parameters and osteoid surface were stained using a trichrome method of Gomori as modified by Villanueva. Osteoclasts were identified using tartrate-resistant acid phosphatase activity. Histomorphometric measurements of spongy bone of tibiae were performed using image analyzer MOP-VIDEOPLAN. The following parameters were determined: growth plate height, primary thickness, trabecular bone volume, trabecular number, trabecular thickness, trabecular separations in the primary and secondary spongiosa, osteoid surface, and osteoclast surface.

No changes were seen in the growth plate and primary spongiosa parameters of IF rats; however, F+0 rats sacrificed 6 hours after flight had significantly lower values of these parameters. In the secondary spongiosa of tibiae, bone volume was decreased by 22% and by 35% in IF and F+0 rats; this was associated with a reduction of trabecular number. Osteoid surface in IF and F+0 rats was significantly lower, and osteoclast resorption surface showed greater values than ground control rats. In F+0 rats the signs of tibial osteopenia were more evident than in IF rats; this could be due to increased resorption under the influence of an acute gravitational stress. The flight-induced changes in tibiae were reversible, and after 14 days of recovery, most of the parameters returned to control levels.