In-vivo analysis of serum-borne growth factors in developing co-twinned fetuses

Double-muscled fetuses develop more muscle fibers than normal-muscled fetuses. To examine whether serum growth factors modulate muscle development in cattle, twin pregnancies were induced in eight Holstein heifers using embryos from Belgian Blue and Holstein genetics representing heavy (HM) and light (LM) muscled cattle, respectively. Twin combinations 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 were designated as LM (HM) and the four Belgian Blue fetuses were designated as HM (LM). Pregnancies were terminated at 175 +/- 5 d after conception and fetuses, with evidence of vascular anastomosis, were dissected. Carcass weights were greatest (P < .05) for HM fetuses. Total bone and individual femur weights were greatest (P < .05) for LM (LM) fetuses. Total skeletal muscle mass and mass of semitendinosus, quadriceps femoris, infraspinatus, and longissimus muscles were in the order of HM (HM) > HM (LM) > LM (HM) = LM (LM) (P < .05). Estimated apparent muscle fiber number determined from a cross-section of semitendinosus muscle was in the order of HM (LM) > HM (HM) > LM (HM) = LM (LM) (P < .05). These data show that the presence of a co-twinned fetus with a lower genetic propensity for muscle development reduces the capacity of heavily muscled fetuses to develop muscle mass by 175 d after conception and strongly support the idea that blood-borne factors regulate muscle hypertrophy in fetal cattle.