34
L. Quadro and E. K. Spiegler
1996). When pregnant women with subadequate and adequate VA statuses were evaluated at term, maternal serum β-carotene levels correlated with cord serum retinol
levels only in women with a severely sub-adequate VA status. These studies indicated that the maternal VA status may influence the transfer and/or metabolism of
the various circulating VA forms in different ways.
Placental VA Levels Versus Fetal Retinoid Concentrations
Since the placenta is the site at which nutrients from the maternal circulation are
transferred to the fetal circulation, questions arose whether the placenta stores and/or
metabolizes retinoids and their precursors (e.g., β-carotene) before transferring them,
and whether placental retinoid levels determine fetal retinoid levels. In rats, the
placental VA concentration later in gestation was found to correlate with the total
litter VA concentration (Ismadi and Olson 1982). Barnes found that placental VA
levels in humans did not correlate with newborn cord blood VA or carotene levels,
and concluded that the placenta may not be a significant site of VA storage, and
that carotene is the main compound transferred to the fetus (Barnes 1951). When
pregnant women with sub-adequate and adequate VA statuses were evaluated at term,
placental retinol levels correlated with fetal retinol levels and maternal β-carotene
levels only in the sub-adequate group. Thus, it appeared that the placental conversion
of β-carotene to retinol depends on the maternal VA status (Dimenstein et al. 1996).
The placental transfer of various retinoids and carotenoids will be discussed in greater
detail in the section below.
Development of the Field
Maternal-Fetal Transfer of VA: Is There a Role
for the RBP-Retinol Complex?
The study of the maternal-fetal transfer of VA developed significantly as technological advances allowed the transfer mechanism to be studied with greater precision.
The transfer kinetics of exogenous VA from mother to fetus were explored by Okuda
(Okuda 1960), who histochemically analyzed the time course of placental VA transfer
in rats injected with “Chocola A” (a water-soluble VA preparation; 250000 units of
Chocola A/kg of body weight) in late gestation. Placental VA levels (determined by
fluoromicroscopy) were found to peak in the maternal blood vessels and syncytium
around 30 min after injection, but also increased gradually in the yolk sac and fetal
capillaries, and more rapidly in the decidua (Okuda 1960). Later, radioactive studies
revealed that the rate of placental VA transfer depended on the dose: after injecting
pregnant mice with teratogenic or non-teratogenic doses of
3 H-retinoic acid, Kochhar
L. Quadro and E. K. Spiegler
1996). When pregnant women with subadequate and adequate VA statuses were evaluated at term, maternal serum β-carotene levels correlated with cord serum retinol
levels only in women with a severely sub-adequate VA status. These studies indicated that the maternal VA status may influence the transfer and/or metabolism of
the various circulating VA forms in different ways.
Placental VA Levels Versus Fetal Retinoid Concentrations
Since the placenta is the site at which nutrients from the maternal circulation are
transferred to the fetal circulation, questions arose whether the placenta stores and/or
metabolizes retinoids and their precursors (e.g., β-carotene) before transferring them,
and whether placental retinoid levels determine fetal retinoid levels. In rats, the
placental VA concentration later in gestation was found to correlate with the total
litter VA concentration (Ismadi and Olson 1982). Barnes found that placental VA
levels in humans did not correlate with newborn cord blood VA or carotene levels,
and concluded that the placenta may not be a significant site of VA storage, and
that carotene is the main compound transferred to the fetus (Barnes 1951). When
pregnant women with sub-adequate and adequate VA statuses were evaluated at term,
placental retinol levels correlated with fetal retinol levels and maternal β-carotene
levels only in the sub-adequate group. Thus, it appeared that the placental conversion
of β-carotene to retinol depends on the maternal VA status (Dimenstein et al. 1996).
The placental transfer of various retinoids and carotenoids will be discussed in greater
detail in the section below.
Development of the Field
Maternal-Fetal Transfer of VA: Is There a Role
for the RBP-Retinol Complex?
The study of the maternal-fetal transfer of VA developed significantly as technological advances allowed the transfer mechanism to be studied with greater precision.
The transfer kinetics of exogenous VA from mother to fetus were explored by Okuda
(Okuda 1960), who histochemically analyzed the time course of placental VA transfer
in rats injected with “Chocola A” (a water-soluble VA preparation; 250000 units of
Chocola A/kg of body weight) in late gestation. Placental VA levels (determined by
fluoromicroscopy) were found to peak in the maternal blood vessels and syncytium
around 30 min after injection, but also increased gradually in the yolk sac and fetal
capillaries, and more rapidly in the decidua (Okuda 1960). Later, radioactive studies
revealed that the rate of placental VA transfer depended on the dose: after injecting
pregnant mice with teratogenic or non-teratogenic doses of
3 H-retinoic acid, Kochhar