Broiler chicken myofiber morphometrics and myogenic stem cell population heterogeneity1,2

Myogenic stem cells (MSC), also known as satellite cells, play a key role in mediating post-hatch skeletal muscle growth in broiler chickens. Information regarding the heterogeneity of MSC populations and myofiber size distributions in different muscles of modern broiler strains of both sexes is scarce. Thus, myofiber cross-sectional area (CSA) and myogenic regulatory factor expression by MSC populations in 2 functionally different muscles (Biceps femoris, BF and Pectoralis major, PM) from both sexes of broilers from 2 strains, Ross 708 x Ross 708 (ROSS; Aviagen Group, Huntsville, AL) and Red Ranger (RR; S&G Poultry, Clanton, AL) was evaluated. Female and male broilers from both strains (n = 80 birds per strain) were reared for 43 d on a common corn and soybean meal-based diet. At 43 d of age, 8 birds per sex per strain (n = 32 total) were euthanized and PM and BF muscle samples from each bird were analyzed using cryohistology, immunofluorescence (IF) staining, and fluorescence microscopy. Cryosections from each muscle from each bird were IF stained to detect Myf-5, MyoD, and Pax7-expressing MSC. The CSA of every fiber in each digital image was measured and MSC populations (Myf-5+, MyoD+, Pax7+, Myf-5+: MyoD+, Myf-5+: Pax7+, MyoD+: Pax7+, and Myf-5+: MyoD+: Pax7+) were enumerated. Myofiber CSA was similar among the 2 strains and 2 sexes (P > 0.05). As expected, mean myofiber CSA was greater in the predominantly fast-twitch PM than the predominantly slow-twitch BF muscle (P = 0.006). At 43 d of age, ROSS broilers had a 32% larger population of Myf-5+: MyoD+: Pax7+ MSC compared with RR (P = 0.04). The heterogeneity of MSC was similar among the sexes. While the total MSC population was larger in the BF muscle compared with the PM (P < 0.05), the Myf-5+ and Pax7+ MSC densities were similar among muscles (P > 0.05). The major differences in myofiber morphometrics and MSC heterogeneity were not among the different broiler strains or sexes but were instead, between the 2 functionally different muscles.