IN-VIVO ANALYSIS OF SERUM-BORNE DEVELOPING CO-TWINNED GROWTH-FACTORS IN FETUSES

Double-muscled fetuses develop more muscle fibers than normal-muscled fetuses. To examine whether serum growth factors modulate muscle devel opment in cattle, twin pregnancies were induced in eight Holstein heif ers using embryos from Belgian Blue and Holstein genetics representing heavy (HM) and light (LM) muscled cattle, respectively. Twin combinat ions were 1) two pairs of Belgian Blue fetuses that were designated as HM (HM), 2) two pairs of Holstein fetuses that were designated as LM (LM), and 3) four pairs of mixed fetuses; the four Holstein fetuses we re designated as LM (HM) and the four Belgian Blue fetuses were design ated as HM (LM). Pregnancies were terminated at 175 +/- 5 d after conc eption and fetuses, with evidence of vascular anastomosis, were dissec ted. Carcass weights were greatest (P < .05) for HM fetuses. Total bon e and individual femur weights were greatest (P < .05) for LM (LM) fet uses. Total skeletal muscle mass and mass of semitendinosus, quadricep s femoris, infraspinatus, and longissimus muscles were in the order of HM (HM) > HM (LM) > LM (HM) = LM (LM) (P < .05). Estimated apparent m uscle fiber number determined from a cross-section of semitendinosus m uscle was in the order of HM (LM) > HM (HM) > LM(HM) = LM (LM) (P < .0 5). These data show that the presence of a co-twinned fetus with a low er genetic propensity for muscle development reduces the capacity of h eavily muscled fetuses to develop muscle mass by 175 d after conceptio n and strongly support the idea that blood-borne factors regulate musc le hypertrophy in fetal cattle.