Transcriptional and endocrine orchestration of medullary bone formation and mineral turn-over in female chickens

Medullary bone is deposited in the cavities of avian long bones and serves as a calcium reservoir for successful eggshell mineralization in sexually mature female chicken. Osteoblasts and osteoclasts located in medullary bone respond to endocrine changes, including high estrogen levels at sexual maturation, and represent important targets for improving mineral turn-over in laying hens. In this study, weekly samples of blood and femur to extract medullary bone material were taken from Lohmann Brown (LB, n = 54) and Lohmann Selected Leghorn (LSL, n = 54) hens from pullet stage (week 16 of age) until onset of oviposition (week 24). Blood beta CTX-1 (C-terminal telopeptides of type I collagen) levels increased in week 17 (LSL) and week 18 (LB) until week 21, indicating organic matrix breakdown for initial bone remodelling. Estradiol increased not before week 20 (LSL) and week 21 (LB). Subsequently, medullary bone calcium content increased in week 23 (LB, LSL), whereas the phosphorus content increased in week 22 (LSL) and week 23 (LB). The longitudinal gene expression patterns of medullary bone material across the maturation period showed pronounced synergistic activities to ensure vascularisation, energy metabolism, and ossification. These analyses identified key genes and pathways involved in successful medullary bone formation in both LB and LSL strains, offering potential targets for genetic or nutritional interventions aimed at maintaining efficient mineral turn-over throughout the laying period. Taken together, the cascade-like sequence of bone remodelling, which controls the differentiation and activity of osteoblasts and osteoclasts and ultimately drives the transition from haematopoietic bone marrow in pullets to medullary bone in mature female birds, is initiated earlier in LSL than in LB hens.