| Summary: | Medullary bone is a type of woven bone birds produce in response to oestrogen. It acts as a mineral reservoir for the calcium demands of egg shell formation. The morphology and distribution of medullary bone in the modern laying hen at different stages in its life was investigated are described. Modern commercial laying hens suffer from osteoporosis (structural bone loss), leading to bone fractures. The association between structural bone loss and medullary bone modelling and remodelling was investigated in three further studies. Bone samples were processed for examination with light microscopy and ultrastructurally. Histomorphometric techniques were used to quantify cancellous, cortical, and medullary bone volumes in undecalcified sections of samples collected from three studies. In the first of these studies, female fowl were killed either during ovarian follicular development, after laying a single egg, or half way through the laying cycle. Structural bone volume decreased significantly during both medullary bone modelling and subsequent remodelling. Medullary bone volume increased significantly during the same period. In the second study, medullary bone modelling was induced in male fowl by the administration of oestrogen, and prevented in female fowl by tamoxifen. Oestradiol-treated males had significantly lower structural bone volumes than control males, while tamoxifen-treated females had significantly higher structural bone volumes than control females. The final study determined the effects of the bisphosphonate alendronate on the structural bone loss associated with medullary bone modelling and remodelling. Alendronate administered before follicular development resulted in significantly greater structural bone volumes both at the onset of lay and at mid-lay than in vehicle-treated controls.
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