Abstract
Studies of calcium transfer across the placenta have been reviewed because of the physiological and nutritional importance of this mineral during pregnancy, especially in order to better understand its contribution to development of the fetal skeleton. The placental transfer of maternal calcium to the fetus represents a vital mechanism for fetal development and breast-milk production, yet little meaningful information is currently available regarding the biochemical mechanisms involved in this process. Once again, the use of different animal models as rodents, rabbit, sheep and bovine have demonstrate different mechanisms of calcium transport across the placenta and contribute to better understand its effects in both fetus and mother during the gestation. In relation to the transfer of calcium from the mother to fetus data suggest it occur via an active mechanism; thus calcium concentration is higher in fetus than in maternal blood. Despite conflicting reports, several investigators agreed that calcium concentration in the fetal blood is mainly regulated by fetal parathyroid hormone and plasma concentration of vitamin D3, a metabolite that plays a key role in calcium transport through the syncytial cells.
Highlights
A consider number of reports have demonstrated the importance of calcium for the embryo development and their physiological functions, including cellular division and differentiation for different cell types [1,2]
It is clear that chorioallantoic membrane (CAM) ectoderm cells quickly internalize large amounts of calcium, it has to be sequestered away from the cytosol [30]. Calcium transport across both maternal and fetal placental epithelium in the sheep is reportedly divided into three major steps: 1) passage through the basolateral membrane of the gland cell and the apical membrane of the trophoblast (mediated by one or more calcium channels which may include ionized Ca2+-release-activated channel (ICRMC), second-messenger-operated channels and even the G-protein-operated channel; 2) binding mechanisms that permit the maintenance of a low cytosolic concentration of free calcium whilst the bound calcium is transported across the cell and 3) extrusion mechanisms in the apical plasma of the gland cells, and at the basolateral plasma of the trophoblast, to deliver calcium to the fetal circulation [27]
The literature indicates that the concentration of calcium in the fetal blood serum is significantly higher than in the maternal serum
Summary
A consider number of reports have demonstrated the importance of calcium for the embryo development and their physiological functions, including cellular division and differentiation for different cell types [1,2]. The asymmetries in unidirectional influx into the trophoblast and rapid backflux indicated a mechanism by which the net transfer of calcium from maternal to the fetal circulation is maintained. In ruminants which have a well developed neonatal skeleton, bone accretion is maximized and reduced placental transport of calcium produces signs of rickets [9], whereas in fetal mice [7] if depletion of PTHrP occurs, the placental calcium transport to the maternal level is not accompanied by a reduction in total body calcium From these observations, it has been concluded that animals of small body size, like the mouse have a compensatory process of calcium diffusion as a result of the lower plasma ionized calcium concentration in the fetus. The transplacental diffusion of calcium can hardly contribute significantly to maintaining fetal calcium homeostasis in this species
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