Muscle development - could environmental manipulations during embryogenesis of broilers change it?

Faculty of Agriculture, Dept. of Animal Sciences, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot, Israel, Institute of Animal Sciences, ARO The Volcani Center, Bet-Dagan, P.O. Box 6, Israel, Corresponding author: halevyo@agri.huji. ac.il __________________________________________________________________________________ Growth rate and meat yield of commercial broilers and turkeys have improved each year, with greater input efficiency and an earlier market age. As a result, the period of embryonic development and early posthatch now makes up a greater proportion of the bird’s life. Therefore, any technology that could enhance muscle growth during this period would help maintaining the progress of growth performance in the future. Skeletal muscle fibers are formed during embryogenesis and continue to enlarge postnatally until the mature size has been reached. This postnatal myofiber growth entails an increase in myofiber protein accretion and in the number of myofiber nuclei. The primary source of these additional myofiber nuclei are the satellite cells, myogenic stem cells situated on the surface of the myofiber between the myofiber plasmalemma and its covering basement membrane. In chicks, satellite cells (also termed adult myoblasts) are first detected in their sublaminar position between embryonic day E13 and E16, when the basement membrane surrounding the myofibers is developed. The number of satellite cells peaks on the second and third day of age and rapidly declines at around 8 days of age, after which they become quiescent. Previously, we have shown that nutritional treatments, i.e., providing feed immediately posthatch, or environmental treatments, such as heat conditioning or monochromatic green-light illumination during the first days posthatch, increase muscle growth and breast muscle weight at marketing day. In all cases, the increase in muscle growth was due to changes at the cellular and molecular levels leading to increased satellite cell proliferation and differentiation. The significant effects on muscle growth resulting from the treatments in the first days posthatch raised the hypothesis that muscle growth could be affected during the embryonic development. In experiments in which fertile eggs were illuminated under monochromatic green light from E5, there was a positive effect on embryo development and posthatch muscle growth. Further studies revealed that this enhanced muscle weight was due to increased satellite cell numbers and enhanced fiber synchronization during early days posthatch. In another study, a temperature manipulation (at 38.5C or 39.5C and 65% humidity) from E16 to E18 for 3 h/day augmented muscle cell proliferation even at later days such as day 13 of age. These data suggest that under the conditions of this study, the temperature manipulation may alter muscle development by delaying satellite cell differentiation and allowing these cells to remain in the cell cycle longer. This results in more cell proliferation and subsequently, in enhanced muscle growth and meat production. Collectively, these data highlight the notion that environmental manipulations in broilers during embryonic development or early on posthatch can serve as a powerful tool in affecting muscle development and enhancing muscle growth and meat production.