Induction of myoblast proliferation in L6 myoblast cultures by fetal serum of double-muscled and normal cattle.

Double-muscled (DM) cattle possess more muscle fibers than do normal-muscled (NM) beef or dairy cattle. Serum-borne growth factors have been shown to modulate myogenesis. Media containing serum from 12 DM and 60 NM fetuses grouped by crown-rump lengths (CRL) of < or = 25, 26 to 50, 51 to 75, or > 75 cm were used to test the effect of fetal serum on L6 myoblast proliferation. Because DM and NM fetuses were similar in CRL at ages corresponding to 20- to 60-cm CRL, CRL was used to determine fetal age. Normal-muscled fetal serum-induced thymidine incorporation in L6 myoblasts was greater (P < .05) at CRL > 50- than at < or = 50-cm CRL. Mean incorporation tended to increase with CRL. Thymidine incorporation was 56, 41, and 41% greater (P < .05) with serum from DM fetuses than with that from NM fetuses at CRL of < or = 25, 26 to 50, and 51 to 75 cm, respectively. Morphological examination of cross-sections of the semitendinosus muscle showed that apparent muscle fiber number was greater (P < .05) for DM fetuses than for NM fetuses. These results confirm greater apparent muscle fiber number in DM cattle and show the existence of greater growth factor activity in serum of DM fetuses during early fetal development. This greater growth factor activity may play a role in bovine muscle fiber hyperplasia.