A 3D micro-computed tomography study comparing embryonic skeletal development in layer versus broiler strains of the domestic chicken

Our objective was to analyze the effect of selection for divergent traits in the domestic chicken on embryonic skeletal development, which could affect postnatal bird welfare. Development was compared between the Ross 308 broiler line (fast growth and muscle mass accrual) and Novoponte layers (high laying rate and egg quality). In Study 1 (Initial Conditions), we characterized egg composition prior to incubation and identified the onset of embryonic skeletal mineralization in the 2 strains. In Study 2 (Developmental Dynamics), we used 3D X-ray tomographic imaging (µCT) on incubation days ED11, ED13, ED15 and ED17 to track skeletal maturation trajectories as a pseudo-time series. Results showed that Ross 308 embryos, which are heavier than Novoponte embryos, possess higher levels of yolk nutrients including phosphorus and calcium, but lower eggshell mineral content, than Novoponte embryos. Skeletal mineralization started synchronously in both strains, on ED11. The higher mineral ion content in the larger yolk of Ross 308 eggs compared to Novoponte eggs may partly explain why skeletal mineralization in Ross 308 embryos advances faster: using µCT, we show that the mandible and tibiotarsi in Ross 308 embryos are larger at ED11 and ED13 compared with Novoponte embryos. However, Novoponte embryos catch up from this initial lag in mineralization by ED15. The timing of the Novoponte acceleration coincides with the functional activation of the chorioallantoic membrane in releasing calcium from the inner eggshell. This correlates with a decrease in eggshell thickness from ED11 to ED17 in Novoponte eggs, which was not observed during Ross 308 incubation. To conclude, while some temporal discrepancies exist in early skeletal development between the embryos of Ross 308 and Novoponte strains, overall prenatal skeletal maturation seems to be robustly regulated. Despite selection for antagonist production traits, layer and broiler prehatch skeletal morphology ultimately synchronizes. Practically, since the extent of skeletal maturity equalizes between strains towards the end of incubation, refinements of farming practices, postnatal environment, and diet should be considered for improving domestic fowl welfare.