Direct measurements of basal bone resorption in microphthalmic mice in vivo

Microphthalmic mice and their normal littermates were prelabeled extensively in utero with 3H-tetracycline and 3H-proline, or with 45Ca alone. The loss of 3H-tetracycline and 3H-collagen from whole femur was measured from birth to 24 days of age at weekly intervals to determine bone resorption of mineral and matrix in vivo. The ratio of blood 45Ca specific activity to that of bone (blood/bone ratio) was used to reflect the interrelationship between bone, blood, and dietary calcium, as well as bone resorption. There was little, if any, loss of 3H-tetracycline and 3H-collagen from bones of the microphthalmic mutants whereas there was a marked and continuous loss (67 and 51%, respectively) from bones of the normal littermates due to normal bone modeling. Blood/bone ratio was much lower in the mutants (42-70%) compared with the normals, suggesting that blood calcium is maintained by diet. These data provide direct evidence that basal bone resorption in growing microphthalmic mice was almost completely inhibited.