Abstract
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 and described.
Modern commercial lay ing 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 tlirough 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 fow l by tamoxifen. Oestradiol-trealed
males had significantly lower structural bone volumes than control males, w hile tamo.xifentreated
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.
