Studies on the transfer of calcium across the ovine placenta and incorporation into the foetal skeleton

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Abstract
1. Radioactive calcium has been used to study the placental transfer of Ca in ewes at three stages of gestation.2. At each stage virtually all the radioactive Ca injected into a foetus was still present 5–6 h later and little or none was found in the mother or in a twin foetus if present.3. It was concluded that there is neither exchange of Ca between foetal and maternal pools nor transfer from foetus to mother. Passage of Ca across the placenta of the ewe is therefore a one-way process.4. The rapidly exchangeable Ca pool of the foetus tended to increase in size with the stage of gestation and was found to contain only about half of the total Ca of the foetal soft tissues.5. Two methods for calculating the rate of accretion of Ca into the foetal skeleton have been applied. The results obtained from these calculations suggested that rates of foetal bone accretion also increased with the stage of gestation. Accretion per unit foetal weight, however, remained fairly constant irrespective of age or number of foetuses present.6. Bone accretion rates calculated from the results of the present work were very similar to the rates obtained previously for transfer of Ca across the sheep placenta, indicating that resorption of Ca from bone is negligible in the foetus.