Myogenic Regulatory Factors Expressed During Postnatal Hy- perplastic Growth in Porcine Muscles

Muscle progenitors cells migrate from the somite to develop into primary and secondary myotubes due to the activation of a number of regulatory factors, which are classified as myogenic determination and differentiation factors (MRFs). The presence of a third generation of myotubes has been described in the sheep, human and bovine during foetal life whereas in pig they appear shortly after birth and massively from the 3rd/4th weeks of post-natal life. To explore the relative contribution of different molecules (MRFs, Myosta- tin and Pax7) to the establishment of postnatal hyperplastic mechanisms, we have used standard RT-PCR and immunocytochemistry. RT-PCR indicated a descending trend from 1 day towards 20 and 50 days after birth for MyoD, Myf5 and myogenin but not for Pax7 and myostatin. This study suggests that tertiary myotubes of the pig might be committed during the end of pregnancy and, presumably, satellite cells are the cellular elements responsible for novel fibres development.