PARATHYROID-GLAND HORMONES IN THE SKELETAL DEVELOPMENT OF THE OVINE FETUS - THE EFFECT OF PARATHYROIDECTOMY WITH CALCIUM AND PHOSPHATE INFUSION

It has been confirmed that the foetal parathyroid glands are important in development and that thyroparathyroidectomy (TXPTX) of the ovine foetus with thyroxine (T4) replacement leads to hypocalcaemia, retarded skeletal development, depressed calcification and rickets, relative to thyroidectomy plus T4 replacement. Histomorphometric and biochemical (urinary excretion of deoxypyridinoline) indices of bone resorption are also reduced. However, skeletal calcification can be restored to normal by long-term infusion of the TXPTX foetuses with phosphate and calcium sufficient to normalise the plasma Ca2+ x Pi ion product. Nevertheless, depressed resorption, reduced osteoblast numbers and delayed development persisted. The evidence suggests that the abnormally low number of resorption cavities and osteoclasts may result from the reduction in circulatory parathyroid-hormone-related protein consequent upon the removal of the foetal parathyroid glands and that this hypercalcaemic factor has a direct effect upon the process of resorption and primary trabecular remodelling of the foetal skeleton.