Deteriorations in eggshell and bone quality are major challenges in aged laying hens. This study compared the differences of eggshell quality, bone parameters and their correlations as well as uterine physiological characteristics and the bone remodeling processes of hens laying eggs of different eggshell breaking strength to explore the mechanism of eggshell and bone quality reduction and their interaction. A total of 240 74-week-old Hy-line Brown laying hens were selected and allocated to a high (HBS, 44.83 ± 1.31 N) or low (LBS, 24.43 ± 0.57 N) eggshell breaking strength group. A decreased thickness, weight and weight ratio of eggshells were observed in the LBS, accompanied with ultrastructural deterioration and total Ca reduction. Bone quality was negatively correlated with eggshell quality, marked with enhanced structures and increased components in the LBS. In the LBS, the mammillary knobs and effective layer grew slowly. At the initiation stage of eggshell calcification, a total of 130 differentially expressed genes (DEGs, 122 upregulated and 8 downregulated) were identified in the uterus of hens in the LBS relative to those in the HBS. These DEGs were relevant to apoptosis due to the cellular Ca overload. Higher values of p62 protein level, caspase-8 activity, Bax protein expression and lower values of Bcl protein expression and Bcl/Bax ratio were seen in the LBS. TUNEL assay and hematoxylin-eosin staining showed a significant increase in TUNEL-positive cells and tissue damages in the uterus of the LBS. Although few DEGs were identified at the growth stage, similar uterine tissue damages were also observed in the LBS. The expressions of runt-related transcription factor 2 and osteocalcin were upregulated in humeri of the LBS. Enlarged diameter and more structural damages of endocortical bones and decreased ash were observed in femurs of the HBS. The lower eggshell breaking strength may be attributed to a declined Ca transport due to uterine tissue damages, which could affect eggshell calcification and lead to a weak ultrastructure. Impaired uterine Ca transport may result in reduced femoral bone resorption and increased humeral bone formation to maintain a higher mineral and bone quality in the LBS.

中文翻译：

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子宫钙运输受损导致的蛋壳强度下降与老年蛋鸡骨矿物质和质量的提高相一致

蛋壳和骨骼质量的恶化是老年蛋鸡面临的主要挑战。本研究比较了不同蛋壳断裂强度产蛋蛋鸡的蛋壳质量、骨参数及其相关性的差异以及子宫生理特征和骨重塑过程，探讨蛋壳和骨质量降低的机制及其相互作用。选取 240 只 74 周龄海兰褐蛋鸡，分为高蛋壳断裂强度组（HBS，44.83 ± 1.31 N）或低蛋壳断裂强度组（LBS，24.43 ± 0.57 N）。LBS 中观察到蛋壳厚度、重量和重量比下降，并伴有超微结构恶化和总 Ca 减少。骨质量与蛋壳质量呈负相关，表现为结构增强和 LBS 成分增加。在LBS中，乳头结和有效层生长缓慢。在蛋壳钙化起始阶段，LBS 母鸡子宫中相对于 HBS 共鉴定出 130 个差异表达基因（DEG，122 个上调，8 个下调）。这些差异基因与细胞 Ca 超载导致的细胞凋亡有关。LBS 中 p62 蛋白水平、caspase-8 活性、Bax 蛋白表达较高，而 Bcl 蛋白表达和 Bcl/Bax 比值较低。TUNEL 测定和苏木精-伊红染色显示 LBS 子宫中 TUNEL 阳性细胞和组织损伤显着增加。尽管在生长阶段发现的DEG很少，但在LBS中也观察到了类似的子宫组织损伤。LBS 肱骨中 runt 相关转录因子 2 和骨钙素的表达上调。在 HBS 股骨中观察到内皮质骨直径增大、结构损伤更多，灰分减少。蛋壳断裂强度较低可能是由于子宫组织损伤导致钙转运下降，这可能会影响蛋壳钙化并导致超微结构薄弱。子宫钙转运受损可能会导致股骨骨吸收减少和肱骨骨形成增加，以维持 LBS 中较高的矿物质和骨质量。