Cellular and morphological characterization of blastoderms from freshly laid broiler eggs

The pioneering study of Eyal-Giladi and Kochav (EG&K; Eyal-Giladi and Kochav, 1976) on the early developmental stages-from fertilization, through oviposition, to the gastrulation process-set the standard for characterizing chicken embryos, and has been used in numerous studies over the years. During uterine development, the chicken embryo undergoes dramatic changes, extremely rapid cell cycles, massive cell death, and axial determination processes. However, once the egg is laid, the temperature drops and the embryo enters into a diapause-like state. This phenomenon is utilized to store fertile eggs prior to incubation. The ability to resume development to hatching, following storage, relies on several factors, including the number of living cells and the embryonic developmental stage. These factors are highly influenced by the storage conditions-mainly duration and temperature. Thus, to study the effects of storage conditions on embryonic viability, a comprehensive characterization of the starting point-shortly after oviposition-is needed. In this study, we characterized freshly laid broiler eggs from Ross 308 flocks for embryonic developmental stage, total cell count, and cell viability. Using the novel high-resolution episcopic microscopy (HREM) system, we show, for the first time, high-resolution 3D morphological models of blastoderms which allow for highly accurate embryonic staging. Staging was also done under a dissecting microscope thus allowing for a direct side-by-side comparison of the two methods. Analysis of freshly laid blastomeres showed that the total nucleus count increases with developmental stage from ∼60,000 at stage X EG&K to ∼130,000 at stage XIII EG&K, whereas the proportion of mitotic index and dying cells at oviposition are ∼2% and ∼5%, respectively. Moreover, staging embryos from young and old flocks revealed that the blastoderms of the old flocks are more developed. Specifically, the predominant embryonic stages were XI and XII EG&K in young and old flocks, respectively. Collectively, we characterized parameters that can serve to analyze the maladaptive effects of prolonged storage under various conditions on embryo survival.