Stem cell competition and tissue-sparing effect. Panel A: Stem cell competition for residence in the niche is shown. Panel B: Nonirradiated spermatogonial stem cell pool after 50% micro-slit irradiation and the complete tissue-sparing effect are shown. Panel C: Nonirradiated spermatogonial stem cell pool after 87.5% micro-slit irradiation and the incomplete tissue-sparing effect are shown. Red arrows indicate exposure to spatially fractionated radiation.