Rates of cell entrance into different periods of the cell cycle sustain that negative exponential cell age distribution occurs in Allium cepa L. meristems

Abstract The rates of cell entrance into three cell cycle periods, G 1 , S and metaphase, have been measured in meristems of Allium cepa L. roots. The objective was to study, in a direct way, the question of cell age distribution in a tissue formed by proliferating cells with different average generation times. Entrance rates significantly changed from one period to another: the number of cells entering into G 1 per unit time almost doubled the entry rate into metaphase. These data rules out a linear cell age distribution. The ratio of entry rates into different pairs of cell cycle periods was calculated, by assuming negative exponential cell age distribution, from the frequency of cell number between birth and the initiation of each period. Comparison of expected values with experimental ones strongly supports a negative exponential age distribution similar to that found in cell cultures with a single cell cycle average time.