Consequences of birth weight for postnatal growth performance and carcass quality in pigs as related to myogenesis.

In polytocous species such as the pig there is intralitter variation in birth weight and skeletal muscle fiber number. It is commonly recognized that low birth weight in piglets correlates with decreased survival and lower postnatal growth rates. In the majority of low birth weight piglets low numbers of muscle fibers differentiate during prenatal myogenesis, for genetic or maternal reasons, and those low birth weight piglets with reduced fiber numbers are unable to exhibit postnatal catch-up growth. Pigs of low birth weight show the lowest growth performance and the lowest lean percentage at slaughter. In addition, they tend to develop extremely large muscle fibers (giant fibers) and poor meat quality, which results in part from the inverse correlation between fiber number and fiber size. Prenatal growth and myogenesis are under the control of various genetic and environmental factors, which can be targeted for growth manipulation. Genetic selection is considered a suitable tool to improve fetal growth and myogenesis. Prenatal development is mainly dependent on a close interrelation between nutritional supply/use and regulation by hormones and growth factors. In particular, the maternal somatotropic axis plays a significant role in the control of myogenesis. Thus, treatment of sows with GH until mid-gestation was able to increase birth weight and the number of muscle fibers in the small littermates of the progeny that are disadvantaged by insufficient nutrient supply. Growth hormone treatment was associated with increased nutrient availability to the embryos and changes in regulatory proteins of the GH-IGF axis. Interactions between maternal nutrition and the somatotropic axis in determining prenatal growth and myogenesis are worthy of further investigation.