Sexual dimorphism in broiler chick embryos and embryonic muscle development in late incubation

Studies were conducted to compare the weights and the characteristics of the Pectoralis superficialis, Semimembranosus, and Gastrocnemius of male and female chicken embryos at 16 and 20 d of incubation. Male embryos were significantly heavier than females at 16 d of incubation, but not at 20 d. The cross-sectional area of myofibers in the P. superficialis of 20-d-old female embryos was greater than that of males and the area of Semimembranosus myofibers was greater at 16 and 20 d. The Semimembranosus apparent myofiber number of 16-d-old male embryos was greater than of females (P = 0.002). There was a significant increase in apparent myofiber number in this muscle between 16 and 20 d of incubation but there was no sex difference at 20 d. The number of Type I myofibers per 30,000 microm2 area of the Gastrocnemius of 20-d-old male embryos was greater than that in females (P = 0.06). There were no significant sex differences in the protein concentration, DNA concentration, or protein:DNA ratio in the muscles at either age when data were pooled across the whole population. There were significant differences in these characteristics between sire families and in some sire families significant sex differences were noted. The protein:DNA ratio of the muscles increased between 16 and 20 d of incubation and the mitotic index decreased. These data suggest that the muscles of male embryos have more but smaller, myofibers than females, which may be responsible for the sex difference in embryo weight and provide the framework for the greater posthatching muscle growth.