Dynamics of Kidney Calcium and Phosphorus Mineral Handling in Laying Hens During the Transition to Egg Production

The transition from immature pullets to sexually mature female chickens is accompanied by pronounced structural and functional modifications. This is accompanied by coordinated changes in hormonal and metabolic pathways that primarily regulate reproduction, nutrient utilization, and mineral homeostasis to compensate for temporary imbalances. In kidney, specific regulation of reabsorption processes is required which depend on dietary calcium (Ca) and phosphorus (P) intake and physiological needs. This study aims to elucidate the physiological processes underlying kidney adaptation along maturation in female laying hens. Lohmann Selected Leghorn (LSL) and Lohmann Brown (LB) laying hens were sampled for plasma (n = 140) and kidneys (n = 84) at immature pullet stages (weeks 16-18) along transition and onset of lay (weeks 20-23). Hens were fed a developer diet (weeks 14-16), a pre-layer diet (weeks 16-17), and a layer diet (weeks 17-23). Genes involved in active renal mineral transport (e.g., SLC34A1, TRPV6) exhibited time-dependent expression with reduced abundance in immature birds until approximately 20 weeks of age and upregulation toward week 23. Genes involved in tight junction formation (e.g., CLDN10, OCLN) showed likewise reduced mRNA abundances in immature pullets, with lowest values around week 20. Genes affiliated with regulatory endocrine pathways (e.g., PTH2R, VDR) showed altered expression indicating an adaptation toward a decrease in sensitivity of the kidney to PTH and active vitamin D3 as an immediate response to the high Ca supply prior egg laying activity. Plasma and gene expression data indicate that hypercalciuria and phosphaturia are actively promoted in laying hens fed high Ca diets prior to sexual maturity. Between weeks 20-21 of life, the kidneys of high-yielding laying hens undergo a physiological shift enhancing mineral efficiency that sustains hypercalcemia supporting eggshell calcification.