Dietary cadmium chloride impairs shell biomineralization by disrupting the metabolism of the eggshell gland in laying hens

Mingkun Zhu, Huaiyu Li, Liping Miao, Lanlan Li, Xinyang Dong, Xiaoting Zou

In this study, we identified cadmium (Cd) as a potential endocrine disruptor that impairs laying performance, egg quality, and eggshell deposition and induces oxidative stress and inflammation in the eggshell glands of laying hens. A total of 480 38-week-old laying hens were randomly assigned into five groups that were fed a basal diet (control) or a basal diet supplemented with Cd (provided as CdCl₂ ∙ 2.5 H₂O) at 7.5, 15, 30 and 60 mg Cd per kg feed for 9 weeks. The results showed that, when compared with the control group, a low dose of dietary Cd (7.5 mg/kg) had positive effects on egg quality by improving albumen height, Haugh unit, yolk color and shell thickness at the 3rd or 9th week. However, with the increase in the dose and duration of Cd exposure, the laying performance, egg quality, and activities of eggshell gland antioxidant enzymes (catalase (CAT), glutathione peroxide (GSH-Px)) and ATPase (Na+/K+-ATPase, Ca2+-ATPase and Mg2+-ATPase) deteriorated, and the activity of total nitric oxide synthase (T-NOS) and the level of malondialdehyde (MDA)increased significantly (P<0.05). The histopathology and real-time quantitative PCR results showed that Cd induced endometrial epithelial cell proliferation accompanied by upregulation of the mRNA levels of progesterone receptor (PgR) and epidermal growth factor receptor (EGFR), downregulation of the mRNA levels of estrogen receptor α (ERα) and interleukin 6 (IL6), and inflammation of the eggshell gland accompanied by significantly increased expression of complement C3 and pro-inflammatory cytokine tumor necrosis factor α (TNFα)(P<0.05). In addition, the ultrastructure of the eggshell showed that dietary supplementation with7.5 mg/kg Cd increased the palisade layer and total thickness of the shell, but with the increase in dietary Cd supplementation (30 and 60 mg/kg) the thickness of the palisade layer and mammillary layer decreased significantly (P<0.05), and the outer surface of the eggshell became rougher. Correspondingly, the expression of calbindin 1 (CALB1), ovocalyxin-32 (OC32), ovocalyxin-36 (OC36), osteopontin (SPP1), and ovocledidin-17 (OC17) decreased significantly (P<0.05) with increasing dietary Cd supplementation. Conclusively, the present study demonstrates that dietary supplementation with Cd negatively affects laying performance, egg quality and eggshell deposition by disturbing the metabolism of eggshell glands in laying hens but has a positive effect on egg quality at low doses.