Abstract
BackgroundEggshell 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.Results60 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 (P4) and 1,25-dihydroxy vitamin D3 [1,25-(OH)2D3] were significantly higher in the HS compared to the LS during the initiation period of calcification. Serum estradiol (E2) 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-D28k 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.ConclusionsOur study concluded that mammillary thickness and mammillary knob width of the HS were significantly lower than LS. P4 and 1,25-(OH)2D3 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 P4 or 1,25-(OH)2D3 may effect the ultrastructure of the mammillary layer by regulating the expression of uterine genes.
Highlights
The eggshells of laying hens are a composite bioceramic material containing 95% calcium carbonate as polymorphic calcite and 3.5% organic matrix proteins [1]
Our study concluded that mammillary thickness and mammillary knob width of the high strength group (HS) were significantly lower than low strength group (LS)
Compared with the LS, mammillary thickness, mammillary knob width, and mammillary layer ratio were significantly lower in the HS (P < 0.05)
Summary
The eggshells of laying hens are a composite bioceramic material containing 95% calcium carbonate as polymorphic calcite and 3.5% organic matrix proteins [1]. The ultrastructure of eggshell is related to matrix proteins that regulate morphology, growth kinetics, and crystallographic orientation of calcite crystals [8, 9], such as ovocleidin-116 [10], ovocleidin-17 [11], ovocalyxin-32 [12], and ovocalyxin36 [13]. These proteins have been shown to be involved in the biomineralization of the eggshell [14]. 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
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