The differences of gonadal hormones and uterine transcriptome during shell calcification of hens laying hard or weak-shelled eggs

Background Eggshell breaking strength is critical to reduce egg breaking rate and avoid economic loss. The process of eggshell calcification initiates with the egg entering the uterus and lasts about 18 h. It follows a temporal sequence corresponding to the initiation, growth and termination periods of shell calcification. During each period of shell calcification, our study investigated the differences of gonadal hormones and uterine transcriptome in laying hens producing a high or low breaking strength shell. Results 60 Hy-line Brown laying hens were selected and divided into two groups according to eggshell breaking strength. Eggshell breaking strength of 44.57 ± 0.91 N and 26.68 ± 0.38 N were considered to be the high strength group (HS) and low strength group (LS), respectively. The results showed that mammillary thickness and mammillary knob width of eggshells were significantly lower in the HS. Serum progesterone (P 4 ) and 1,25-dihydroxy vitamin D 3 [1,25-(OH) 2 D 3 ] were significantly higher in the HS compared to the LS during the initiation period of calcification. Serum estradiol (E 2 ) and calcium did not change significantly. All factors mentioned above had no significant differences in the growth and termination periods of calcification. The relative expression of CaBP-D 28k and PMCA 1b were not significantly different between HS and LS. The relative expression of NCX1 was significantly higher in HS compared to LS. Moreover, 1777 differentially expressed genes (DEGs) were obtained in the initiation period of calcification. However, few DEGs were identified in the growth or termination periods of calcification. 30 DEGs were selected as candidate genes involved in eggshell calcification during the initiation period of calcification by the analysis of GO terms and KEGG pathways. Conclusions Our study concluded that mammillary thickness and mammillary knob width of the HS were significantly lower than LS. P 4 and 1,25-(OH) 2 D 3 were significantly higher in the initiation period of HS. They may impact initial calcification when the mammillary layer is formed. The initiation period of calcification determined eggshell strength rather than the growth or termination periods. We inferred P 4 or 1,25-(OH) 2 D 3 may effect the ultrastructure of the mammillary layer by regulating the expression of uterine genes.