Developmental transition of pectoralis muscle from atrophy in late-term duck embryos to hypertrophy in neonates

Unlike the mammalian fetus, whose growth is supported by the sustained provision of maternal nutrients, poultry embryos undergo development in a relatively closed space, and the yolk sac serves as the sole nutrient supply for embryonic development throughout the whole incubation period. To increase our understanding of the muscle developmental patterns in the final stage of incubation and early days posthatching, we used late-term duck embryos and newly hatched ducklings as animal models. Pectoralis muscle samples were collected at 22 days (22E) of incubation, 25 days (25E) of incubation, hatching and day 7 posthatching. The pectoralis muscle mass, muscle fibre bundles and myofibre cross-sectional area showed a marked reduction from 22E to hatching, but they increased dramatically by day 7 posthatching. The mRNA expression of Atrogin-1, a key mediator of the ubiquitin system responsible for protein degradation, increased dramatically with the age of late-term duck embryos, but it decreased by day 7 and reached a very low level. The extent of mRNA expression of FoxO1, one of the transcription factors of the Atrogin-1 gene, exhibited a transient increase at 25E and then decreased from hatching to day 7. The phosphorylated p70 ribosomal protein S6 kinase 1 (S6K1)/S6K1 ratio exhibited a dramatic reduction from 22E to hatching (P < 0.05) and then increased by day 7. The results of the present study indicated that there was a developmental transition of pectoralis muscle from atrophy in late-term duck embryos to hypertrophy in neonates.