Reduction in myotendinous junction surface area of rats subjected to 4-day spaceflight.

The surface area of myotendinous junctions (MTJs), expressed relative to the cross-sectional area of myofibrils attached to them, was determined using established morphometric techniques in which the digitlike processes of the cell at MTJs are modeled as circular paraboloids. The relative area, called the folding factor, was measured for six rats after a 4-day spaceflight and six control rats maintained in a vivarium under otherwise identical conditions. Spaceflight resulted in a significant reduction in relative MTJ surface area, from 19.7 +/- 2.3 (SD) in control animals to 13.3 +/- 2.5 for animals after spaceflight. Furthermore, space animals displayed increased numbers of fibroblasts enriched in rough endoplasmic reticulum near the MTJ, a greater number of ribosomes and mitochondria within muscle at the MTJ, and increased occurrence of lesions in the connective tissue near the MTJ. The results indicate that spaceflight, possibly through the removal of gravity-associated loading from muscle, causes a modification in MTJ structure and may result in injuries at MTJs after return to normal loading.