Cellular consequences in the brain and liver of age-specific selection forrate of development in mice

Changes in cell number (hyperplasia) and cell size (hypertrophy) in the bra in and liver are described for mice subjected to 24 generations of age-spec ific restricted index selection for rate of development in body weight. One selection treatment (E) altered rate of development between birth and 10 d ays of age, another treatment (L) involved changes in rate of development b etween 28 and 56 days of age, while a third control treatment (C) involved random selection. Each selection treatment was replicated three times. Thes e age-specific selection treatments focused on intervals during ontogeny wh en different developmental processes (hypertrophy or hyperplasia) were more predominant in the control of growth. Significant changes in brain and liv er weight occurred at both 28 and 70 days of age. Early selection (E) gener ated significant changes in the number of cells in the brain while later se lection (L) had no effect since the brain had stopped growth before selecti on was initiated. For the liver, early and late selection produced signific ant effects on both cell number and cell size. These results describe the d ynamic and multidimensional aspects of selection in terms of its ability to alter different cellular and developmental components of complex morpholog ical traits.