Cell numbers and cell sizes in organs of mice selected for large and small body size

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Abstract
Cell numbers in four organs of large, control and small mice were estimated by nuclear counts. Average cell mass was estimated from the cell number and the organ weight. The mice were from the selected Q-strain with six replicate lines in each size-group. The organs were lung, liver, spleen and kidney. At 6 weeks of age the large mice had more cells and larger cells than the controls in all organs; the small mice had fewer and smaller cells than the controls. The regression of log cell-number on log-organ weight provides a measure of how much, proportionately, cell number contributes to the differences in organ weight. In the lung and spleen, cell number contributed about 70% of the strain differences in organ weight, cell mass contributing about 30%; in the liver and kidney the relative contributions were about equal, at 50%.Cell counts at different ages from 3 to 15 weeks showed that cell number and cell mass contributed to the increases of organ weights during growth in roughly the same proportions as stated above. From this it is concluded that the main effect of selection for body weight has been to speed up or slow down the normal processes of cellular growth.