Estimated volume increase of mammary rudiments over 24 hours due to ectodermal influx.

Explanation of calculations: Unlabeled ectodermal cells — linearly correlating with pixel-number — have an equal chance of entering the MR as labeled ectodermal cells, their fraction correlating inversely to the labeled fraction of the ectoderm. Hence, the fold increase in BrdU+ve pixels (), divided by the BrdU+ve ectodermal fraction, yields the total number of influxing ectodermal pixels into MRs. This calculation is slightly overestimated, as the small fraction of BrdU+ve cells within the MRs (column 7) is expected to undergo mitosis within 24 h, contributing to the increase in BrdU+ve pixels at t = 24 h. The number of BrdU+ve pixels (or cells) in column 5 constitutes the fraction of all cells in MR (i.e fraction of MR volume) in column 7. Thus, the fold increase in pixels (columns 3–6), multiplied by that fraction yields the expected volume increase, VExp, if all additional pixels came from ectodermal influx. VObs is the observed MR volume, based on measurements of all pixels within the MR contours of consecutive sections throughout MRs. Abbreviation: MR: mammary rudiment.1VObsExp due to proliferation and apoptosis in MR and apoptosis in overlying periderm.2VObs>VExp due to hypertrophy of peripheral cells within all MRs.3VObs>VExp due to hypertrophy.