Growth and body composition of the mouse

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Abstract
Twenty groups of male mice composed of one or more litters of a definite age were subjected to whole-body analysis. The weight of mice, when plotted against age, demonstrated an S-shaped growth curve. The same curve was shown by fat and by water when plotted against age. Plotting fat-free dry solid on age revealed two linear relationships with time, separated by the period of growth spurt. The time of maximal growth rate was about 37 days—a time at which the first estrus occurs in the female. Total electrolyte and water revealed a linear relationship with fat-free dry solid from the preweaning period (4–21 days) and up to maturity (21–40 days) when regression equations were calculated for each group. Following maturity (40–150 days) a decrease in the slopes of regression lines was demonstrated. The over-all relationship was curvilinear. The findings predict that during the main period of growth total K (and growth of cell mass) is probably linearly related to body protein or fat-free dry solid (protein plus mineral ash). During the weaning period, mice show loss of electrolyte and water per unit of fat-free dry solid. Reduction in K per unit of fat-free dry solid is possibly associated with an increased deposition of protein and a delay in increase of cell volume. The phenomenon may be related to prematurity.