Quantitative analyses of the satellite cell content in the biceps brac
hii muscle of mice genetically selected for high (QL) and low (QS) bod
yweight showed that selection alters the total number of satellite cel
l nuclei rather than the relative proportions of nuclei in the myofibr
e and satellite cell populations. These findings are in accordance wit
h those previously published for other tissues of these mice and suppo
rt the hypothesis that regulatory mechanisms remain unaltered by selec
tion pressure. Size at birth, however, is a reflection of nutritional
status as well as genetic background, and comparisons between differen
tly sized littermates within each of the lines showed a significant in
crease in satellite cell density in larger compared with smaller indiv
iduals. These differences between littermates were not accompanied by
any alteration in myofibre nuclear density. It is therefore suggested
that whilst both genetic and nutritional factors exert their effects o
n muscle growth through an influence on satellite cell division, both
do so at different stages during the programme of satellite cell diffe
rentiation.