Measurement of on-orbit and ground DNA synthesis in rat gastric epithelium is the subject of this study. Is will not be known at the conclusion of the experiment what elements of the space environment might be responsible for any changes in DNA synthesis that are observed. Regardless of the cause, demonstration of such changes would be an important finding. Ultimately, electromagnetic etiologies could rather easily be confirmed or ruled out by subsequent on-orbit experiments in which magnetic shielding materials are employed.
As part of the Biological Sharing Program during the STS-54 mission, stomachs from six flight and six control rats from the PARE.02 experiment were obtained for DNA analysis. The tissues were placed in fixative within 20 min postmortem and sent to our laboratory for analysis. The material was processed by initially embedding the tissue in paraffin blocks, cutting histology sections for microscopic viewing, and then cutting thick (50 micron) sections for deparaffinization and flow cytometric analysis for DNA content. The flight rat gastric epithelium demonstrated a 15% reduction in DNA synthesis compared to the controls, using the S-phase and G2-phase as a quantitative parameter. The relative number of stimulated cells which had completed DNA synthesis (G2/S + G2) was unchanged for the two groups. The effect of spaceflight on DNA gastric synthesis must be viewed as a complex summation of many factors, such as hormonal influences (EGF, somatostatin, gastrin, thyroid hormone), local factors, and external factors unique to spaceflight including but not limited to microgravity.
The data thusfar should not be interpreted in any fashion due to the statistically low number of observations. It would be tempting to state that a lesser number of gastric epithelium cells in the flight rats successfully received and executed the "prepare to divide" signal, as the numbers indicate approximately a 15% decrement in (S+G2) for these rats versus controls. This may be due to factors external to the epithelial cells, such as a decrease in a particular division-stimulating hormone, or an increase in an inhibiting hormone. Many more data points are needed.