Understanding placental nutrient transfer--why bother? New biomarkers of fetal growth

Abstract

The placenta, in general and the physiology of maternofetal nutrient transfer is under-researched compared to other organs with epithelial transport function, as evidenced, for example, by publication numbers. This report provides reasons why more researchers should become involved in this topic. First, the syncytiotrophoblast, the transporting epithelium of the placenta, though having many basic cell physiology properties similar to those of other transporting epithelia, has several properties which are markedly different. Better information on these might help fundamental understanding of how epithelia in general function as well as improving knowledge of how the syncytiotrophoblast operates. Second, the synctiotrophoblast has a key role in controlling fetal growth, not only by transporting nutrients and waste products of metabolism but also because it increasingly appears to be one site, perhaps even the dominant site, in which integration of, sometimes conflicting, signals between mother and fetus takes place. Finally, better understanding of placental nutrient transfer and especially of how it is regulated by maternal and fetal signals could provide better information on the placental phenotype in fetal growth disorders--information which might contribute to providing better biomarkers which the obstetrician could use to improve early diagnosis of these disorders.

Figure 1. Electron micrograph showing the key features of the exchange barrier in the human placenta

Image kindly provided by Dr Carolyn Jones.

Figure 2. Permeability–surface area product for placentas as measured in vivo in the mouse, human and rat for three inert hydrophilic tracers plotted against their diffusion coefficient in water (D_w)

Data taken from Robinson et al. (1988); Bain et al. (1990) and Sibley et al. (2004).

Figure 3. Immunocytochemical staining of alphafetoprotein in a fibin-containing fibrin deposit within a denuded area of syncytiotrophoblast

Micrograph kindly provided by Dr D. Michael Nelson.

Figure 4. Diagram illustrating the polarity of the syncytiotrophoblast in regard to localization of Ca²⁺-ATPase, P-glycoprotein (P-gp) and Na⁺/K⁺-ATPase

Also shown is the microvillous membrane potential difference (Memb. PD) and trans-syncytiotrophoblast (trans-syncyt.) PD. See text for references.

Figure 5. Magnetic resonance image of a placenta

Image was acquired at 1.5 T from a woman at 36 weeks gestation having a normal pregnancy. Image kindly provided by Dr Caroline Wright and Dr David Morris.