40 Texel Genetics Increase Muscle Growth by Altering Satellite Cell Characteristics

Abstract

Abstract The Texel breed is a double muscled breed of sheep that is commonly used as a terminal sire in meat-production. The increase in muscle mass is related to a single nucleotide polymorphism in the myostatin gene, a myokine that inhibits muscle growth. There is limited information on how postnatal skeletal muscle fiber hypertrophy is altered in Texel lambs. Satellite cells or muscle stem cells have a major role in postnatal hypertrophy of muscle fibers. Satellite cells are generally found in a state of quiescence and must activate, proliferate, and differentiate to increase DNA content in myofibers during periods of postnatal muscle growth. The objective of this study was to evaluate muscle fiber hypertrophy and satellite cell proliferation and differentiation in lambs of two sire breeds, Texel or Suffolk. Male lambs of each sire breed (n = 4/sire breed/time) were harvested at d 2, 14, and 203 of age. Muscle samples were collected from the longissimus and snap frozen for examination of myofiber cross-sectional area by fluorescent immunohistochemistry. Another portion of the longissimus was used to isolate satellite cells for in vitro experiments to examine population number, proliferation, and differentiation capacity. Isolated satellite cells were counted, seeded for activation, and grown until colonies reached 80% confluence. Activated cells were seeded for a proliferation assay measuring DNA content daily for 6 days. Following proliferation, cells were differentiated for 4 days, and the number of differentiated nuclei were counted with fluorescent immunohistochemistry. Sire breed did not impact (P < 0.05) the weight of the longissimus; however, muscle ribeye area tended (P < 0.10) to be increased for Texel lambs compared with Suffolk lambs. In the early postnatal period, from d 2 to 14, longissimus mass increased (P < 0.05) by 160%, and ribeye area increased by 100%. Between d 4 and 203 longissimus mass increased (P < 0.05) by 350%, while longissimus area only increased by 100%. Muscle fiber cross sectional area expanded (P < 0.05) from d 14 to 203 of age, and cross-sectional area was greater (P < 0.05) for Texel than Suffolk lambs at d 203. Satellite cell populations per gram of tissue increased (P < 0.05) between d 2 and 14 and decreased from d 14 to 203. The number of satellite cells that activated from quiescence was reduced (P < 0.05) as animal age increased but was not impacted (P > 0.05) by sire breed. Following activation, satellite cell proliferation capacity was greater (P < 0.05) for Texel compared with Suffolk satellite cells. Differentiation capacity was not altered (P > 0.05) by sire breed. Texel-sired lambs had larger longissimus cross-sectional muscle fiber area at d 203 of age than Suffolk. These changes appear to be related to greater proliferation of satellite cells, the stem cells of muscle, to enhance muscle fiber hypertrophy during postnatal growth.