Histograms of lymphocyte area-gradients and relative areas between birth and death or division.

<p>Area-gradients were computed as the difference between cell areas in two windows, each of duration 3 hours. The windows were fixed at cell birth and cell death or division. A median filter of size 13 was used to remove outliers. Relative area was computed similarly, using median-filtered windows of duration 3 hours at either end of the cell's lifetime. Relative area was computed as the ratio of the two windows' median values i.e. a relative area of 2 means that area doubled. “Area gradient” quantifies the rate of growth and “relative area” gives a measure of total growth achieved. Previous pedigree studies of lymphocytes have not tracked a large quantity of cells making distributions less informative. Although, as expected, dividing cells tend to grow and dying cells tend to shrink, approximately 8% of cells fated to die evidence some growth (defined as a relative area at death in excess of the mean plus one standard deviation). Measurement noise explains the majority of these; others showed some growth followed by a period of zero growth. 3.75% of cells were measured as growing substantially (relative area in excess of mean plus two S.D.), producing a second mode centred on a relative area of 1.9 (see D). Verification in the bright field revealed that these cells did not actually grow but instead fluoresced more intensely, resulting in a systematic segmentation bias.