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Abstract
Modern laying hens have been selected for sustained production at older ages and produce an egg every 24 hours. Egg formation utilizes calcium and phosphorus from dietary and skeletal origins; however, as hens age, mineral imbalances occur, leading to poor-quality eggshells and osteoporosis. These challenges pose economic and welfare concerns, and a deeper understanding of mineral homeostasis in older hens is essential. Therefore, the objectives of this study were to (1) identify the intestinal distribution of transcripts regulating calcium and phosphorus uptake, (2) investigate the circadian regulation of calcium and phosphorus uptake and utilization, and (3) evaluate the physiological control of calcium and phosphorus homeostasis during the daily lay cycle through an extended production period. In the first study, expression of genes regulating mineral absorption in the small intestine and ceca was analyzed. The subsequent two studies investigated expression of similar genes in liver, kidney, ileum, and shell gland during the daily oviposition cycle (Studies 2 and 3) and across extended production up to 95 weeks (Study 3). Circulating vitamin D¬3 metabolites were measured in the second and third studies, dry matter digestibility and eggshell characteristics were evaluated in the third study only. The ileum and ceca were identified as potentially important sites for calcium and phosphorus uptake. Furthermore, the ileum, kidney, and shell gland may be capable of 1α-, 25- and 24-hydroxylation of vitamin D3 to regulate local mineral utilization within tissues. Aged hens likely transition into and out of eggshell calcification less efficiently than younger hens, reducing eggshell thickness and breaking strength. Termination of eggshell calcification appears to be mediated by reductions in bicarbonate availability coupled with elevated phosphorus delivery to the shell gland. Reductions in circulating 1,25(OH)2D3 with age likely reduced mineral transporter expression in the shell gland, ileum, and kidney, potentially impacting bone mineralization and eggshell calcification. Collectively, these data identified key hormonal signaling pathways and mineral transporters involved in eggshell calcification and bone mineralization during the daily lay cycle and how they become dysregulated with age. These data are essential for developing solutions to address poor eggshell quality and compromised skeletal structure in aged hens.