Female birds have a medullary bone (MB) on the innermost surface of a cortical bone (CB) during the egg-laying cycle. This bone tissue is known to store calcium (Ca) for eggshell formation. As a result of Ca isotopic analyses of MB and CB from the mature female chickens using a multiple collector-ICP-mass spectrometer (ICP-MS), the 43Ca/42Ca and 44Ca/ 42Ca ratios of MB were higher than those of CB in the bones of the same individual. Moreover, elemental mapping of the CB and MB using a laser ablation-ICP-MS revealed that Mn, Zn, and Cu contents in the MB which are used for activating bone morphogenic protein were higher than those in CB. According to the mechanism of Ca isotope fractionation reported in our previous study, the difference in Ca isotope ratio between MB and CB can be explained by the change in bone turno- ver rate, i.e., the balance between bone formation and resorption. Therefore, the difference in Ca isotope ratio between MB and CB reflects the different Ca balances during the bone tissue formation of each part.
