Fetal calcium homeostasis.

The mammalian fetus is maintained hypercalcaemic relative to its mother primarily by the action of a placental calcium pump located in the basal plasma membrane of the trophoblast. It is suggested that the activity of this pump is stimulated by a mid-molecular fragment of parathyroid hormone-related protein [PTHrP(38-94NH2)], produced in the placenta (and also in the parathyroid glands of fetal lambs and calves) as a result of post translational processing. In the sheep, calcitriol is an important determinant of fetal calcium homeostasis and it, too, stimulates the transport of calcium across the placenta. Fetal bone resorption is under the control of calcitriol, parathyroid hormone (PTH) and PTHrP. This resorption is modulated by the inhibitory effect of calcitonin. PTHrP also plays an important role in the regulation of endochondral ossification and chondrocyte maturation.