Abstract
This study aimed to compare the effects of dietary methionine (Met) and 2-hydroxy-4-(methylthio)-butanoate (HMTBA) on the eggshell quality of broiler breeder hens and elucidate the mechanism of Met in improving eggshell quality from the perspectives of eggshell microstructure and shell gland physiological function. A total of 720 WOD188 broiler breeder hens at 40 weeks old were assigned to 3 groups, with 8 replicates per group and 30 birds per replicate. Over 7 weeks, birds were fed a basal diet or the same diet supplemented with 0.15% Met or 0.17% HMTBA. Our findings revealed significant improvements in the Met group for egg shape index, shell thickness, breaking strength, and fracture toughness (P < 0.05), whereas the HMTBA group showed no significant improvements (P > 0.05). Met supplementation increased calcium and phosphorus levels in both serum and shell gland tissue (P < 0.05), and enhanced Ca(2+) ATPase activity in shell gland tissue (P < 0.05). Histomorphological changes cluded enhanced mucosal fold dimensions and increased epithelial height in the shell gland (P < 0.05). Met also improved eggshell ultrastructure, resulting in a thicker effective layer and broader mammillae with fewer type B structures (P < 0.05). The mRNA levels for genes regulating eggshell ultrastructure, such as ovocleidin-116 (OC-116), calbindin 1 (CALB1), and integral membrane protein 2C (ITM2C), were significantly upregulated in the Met group (P < 0.05). Transcriptome analysis identified 248 differentially upregulated genes in the Met group, primarily linked to the non-canonical Wnt/Ca(2+) signaling pathway, crucial for calcium ion transport and cellular proliferation. This research highlights that Met supplementation improves eggshell quality by enhancing calcium transport and cellular proliferation in uterine function, particularly through the modulation of Wnt family member 11 (WNT11) and CALB1, influencing calcium deposition and ultrastructural development.