Mineral is removed from the maternal skeleton to provide calcium for milk by two processes: bone resorption by osteoclasts, and osteocytic osteolysis, when osteocytes remove mineral from the surrounding bone. The effects of lactation and recovery on material composition of the maternal skeleton have not been defined.

We analysed tibiae from 6 month old female mice after 21 days of lactation, during recovery (7 and 28 days post-weaning), and in unmated age-matched controls (n=5-11/group) by synchrotron-based Fourier-transform infrared microspectroscopy (sFTIRM). To account for difference due to bone maturity or mechanical load, spectra were collected in 15 x 15 micron regions spaced evenly from the periosteal to the endocortical surface on both medial and lateral cortices.

As expected, mineral accumulation, carbonate incorporation and collagen compression occurred with increasing bone maturity on medial and lateral cortices in all treatment groups. In controls, mineral:matrix ratio was 22% greater and amide I:II ratio 11% lower on lateral tibial cortices compared to medial (both p<0.0001).

In lactating mice, while neither carbonate:mineral nor amide I:II ratio were altered, mineral:matrix ratio was greater across the medial cortices compared to unmated controls (p<0.0003); this was not significant at days 7 and 21 of recovery. In contrast, in the lateral cortices, while there was no significant change in mineral: matrix, newly deposited bone had a low carbonate:mineral ratio and high amide I:II ratio compared to controls.

We conclude that mineral levels and collagen compression are greater in regions of high mechanical strain. We suggest the high mineral:matrix ratio in medial cortices of lactating mice is caused by retention of highly mineralised bone, and that during lactation and recovery, osteocytes preferentially remove and rapidly replace bone matrix with low mineral content in mature and immature bone at sites subject to less mechanical strain.