Ossification of the femur in thyroxine-treated tadpoles of Rana pipiens

Abstract

Tadpoles of Rana pipiens at Taylor-Kollros stage IX were treated by immersion in a thyroxine solution at a concentration of 6.25 x 10-8 M. Hindlimbs developed precociously, and alizarin-stained specimens showed that treatment with thyroxine induced accelerated ossification of limb bones.Light microscopy of thick Epon sections showed that cartilage and perichondrium were beginning to organize in the femur of normal animals at stage IX and had become separated by a narrow zone of osteoid matrix by stage XI. After 4 days of exposure to thyroxine, inner perichondrial cells had enlarged to become osteoblasts bordering a prominent zone of osteoid matrix. By 9 days of treatment some inner osteoblasts were entirely surrounded by bone matrix and thus had become osteocytes.Electron microscopy revealed that rough endoplasmic reticulum began to accumulate in osteoblasts by the second day of treatment. Fusion of collagen fibrils was observed in the osteoid matrix of a specimen treated for 5 days. Deposition of hydroxyapatite crystals along collagen protofibrils in scattered mineralization sites began after 6 days of treatment. Mineralized sites grew and became confluent, so that by 9 days the lower two-thirds of the bone matrix was almost completely mineralized. Enlarged mineralization sites at 9 days usually were organized with collagen protofibrils in the interior and hydroxyapatite crystals clustered around the periphery of the mineralizing mass.Thyroxine appears to stimulate differentiation of osteoblasts from perichondrial cells. The hormone may stimulate osteoblasts to secrete depolymerases which prepare osteoid matrix for mineralization along collagen fibrils.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32873/1/0000251.pd