Physiological responses of broiler embryos to egg storage duration: a transcriptomics study

After oviposition, hatching eggs are generally stored at the breeder farm and hatchery before incubation is initiated. In certain cases, egg storage duration is extended beyond 7 days, which is referred to as 'prolonged storage'. Prolonged storage is associated with lower hatchability, delayed hatching, and worse chicken quality at hatch, but the underlying embryonic cellular processes that occur during egg storage are not yet fully understood. In contrast to previous targeted studies on single pathways, the current study employs a whole-transcriptome analysis to investigate physiological responses of broiler embryos to egg storage duration. Gene expression levels from broiler embryos isolated from fresh eggs (on the day of oviposition; n = 10) and from eggs that had been stored at 18 °C for either 4 days (n = 5) or 14 days (n = 5) were compared. Data were analysed by using the CLC Genomics Workbench platform. Results showed that genes common between broiler embryos from 4-day- and 14-day-stored eggs were associated with the positive regulation of the p38 mitogen-activated protein kinase cascade, controlling the balance between cell survival and death under cellular stress. Broiler embryos from 4-day-stored eggs showed gene expression levels indicative of cell cycle arrest at the Gap 1 to Synthesis transition in the cell cycle and mild endoplasmic reticulum stress compared to broiler embryos from fresh eggs. Broiler embryos from 4-day-stored eggs showed gene expression levels indicative of active cell proliferation and repair of minor DNA lesions compared to broiler embryos from 14-day-stored eggs. Broiler embryos from 14-day-stored eggs also showed gene expression levels indicative of cell cycle arrest at the Gap 1 to Synthesis transition, and additionally gene expression levels indicative of oxidative stress, apoptosis, repair of severe DNA damage, and severe endoplasmic reticulum stress compared to broiler embryos from fresh eggs. Current findings suggest that egg storage duration induced a shift from adaptive stress responses in embryos from 4-day-stored eggs to maladaptive stress responses in embryos from 14-day-stored eggs, characterised by oxidative stress, apoptosis, and severe cellular damage. This study reveals a transcriptome-wide transition underlying the decline in poststorage embryo viability with increasing egg storage durations.