Osteoclast cell‐surface specializations and nuclear kinetics during egg‐laying in Japanese quail

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Abstract
Medullary bone deposits serve as a reservoir of labile calcium for egg‐shell calcification in birds. Quantitative transmission‐electron‐microscope methods and light‐microscope autoradiographic cell‐population‐kinetic analyses were used to determine changes in cell‐surface specializations and population dynamics of medullary bone osteoclasts during egg‐laying in Japanese quail. Prior to egg‐shell formation, from 0 to about 8 hours after the previous oviposition, very few osteoclast profiles had ruffled borders. The appearance of ruffled borders coincided with the beginning of egg‐shell calcification, about 9‐10 hours after the previous oviposition. During egg‐shell calcification, from about 10‐21 hours after the previous oviposition, most osteoclast profiles had ruffled borders. Ruffled borders disappeared at the completion of egg‐shell calcification and commencement of egg‐shell pigmentation. Thus, functional activities of medullary bone osteoclasts appear to be closely synchronized with egg‐shell calcification during egg‐laying. From 1 to 48 hours after a single injection of 3H‐thymidine (3H‐TdR), very few labeled osteoclast nuclei were seen during egg‐laying. Following multiple injections of 3H‐TdR, the percentage of labeled nuclei reached a peak at about 170 hours after the first injection. At this peak‐labeling time, relatively few of the osteoclast profiles that had labeled nuclei had two or more; although the average number of nuclei per osteoclast profile was about 3.6. These kinetic data suggest that the medullary bone osteoclast population has a prolonged rate of turnover compared to rapid changes in cell activities associated with each 24‐hour egg‐laying cycle; and collectively they would suggest that rapid changes in osteoclast functions occur independently of changes in cell‐population dynamics.