Muscle and adipose cell development in mice selected for postweaning growth rate.

Genetic selection for postweaning growth rate between 3 and 6 weeks of age resulted in significant changes in body weight, semimembranosus muscle weight, and epididymal fat pad weights. Body weights at 15 weeks of age ranged from 24.9 gm for the Low line (slow growth) mice and 59.4 gm for the High line mice (rapid growth). Total muscle DNA content was decreased in Low line mice and increased in High line mice when compared to Control mice (random bred). RNA to DNA and protein to DNA ratios were not significantly affected by growth rate selection pressure. Selection for high or low growth rate appeared to affect the development of muscle by altering the amount of DNA synthesized, suggesting that growth hormone status was altered by this type of selection. Adipose cell number was reduced in the Low line and increased in the High line when compared to Control mice. Adipose cell size was increased in High line mice when compared to Low and Control lines. The role of adipose cell development in supporting growth selection success is suggested.