G2 arrest following fractionated irradiation in the mouse jejunal crypt

A G2 block is a well-known response to irradiation, showing a delay time that depends on the given single dose. Our question was whether the division delay time is the same after multiple doses as after a single dose. We therefore studied mitosis in the mouse jejunal crypt using a quantitative approach. Three radiation schedules with a fraction size of 2 Gy were compared: a single dose and twice-daily irradiation at a 4-h interval, for 1 or 3 days. With the single dose, mitotic figures disappeared at 0.5 h and reappeared after 2 h. At the end of each multiple-dose regimen, significant numbers of mitoses could be observed at 0.5 and 1 h but a trough was seen at 2 h, with numbers then increasing again. Calculated average division delays after the single dose and repeated doses for 1 and 3 days were 1.5, 1.3 and 1.7 h respectively. In summary, most crypt cells treated with multiple fractions remain susceptible to G2 arrest, showing the same delay time as with the single dose.