Water and nitrogen content of the body of young human embryos

Gestation-Specific Changes in the Anatomy and Physiology of Healthy Pregnant Women: An Extended Repository of Model Parameters for Physiologically Based Pharmacokinetic Modeling in Pregnancy

BACKGROUND

In the past years, several repositories for anatomical and physiological parameters required for physiologically based pharmacokinetic modeling in pregnant women have been published. While providing a good basis, some important aspects can be further detailed. For example, they did not account for the variability associated with parameters or were lacking key parameters necessary for developing more detailed mechanistic pregnancy physiologically based pharmacokinetic models, such as the composition of pregnancy-specific tissues.

OBJECTIVES

The aim of this meta-analysis was to provide an updated and extended database of anatomical and physiological parameters in healthy pregnant women that also accounts for changes in the variability of a parameter throughout gestation and for the composition of pregnancy-specific tissues.

METHODS

A systematic literature search was carried out to collect study data on pregnancy-related changes of anatomical and physiological parameters. For each parameter, a set of mathematical functions was fitted to the data and to the standard deviation observed among the data. The best performing functions were selected based on numerical and visual diagnostics as well as based on physiological plausibility.

RESULTS

The literature search yielded 473 studies, 302 of which met the criteria to be further analyzed and compiled in a database. In total, the database encompassed 7729 data. Although the availability of quantitative data for some parameters remained limited, mathematical functions could be generated for many important parameters. Gaps were filled based on qualitative knowledge and based on physiologically plausible assumptions.

CONCLUSION

The presented results facilitate the integration of pregnancy-dependent changes in anatomy and physiology into mechanistic population physiologically based pharmacokinetic models. Such models can ultimately provide a valuable tool to investigate the pharmacokinetics during pregnancy in silico and support informed decision making regarding optimal dosing regimens in this vulnerable special population.

Three-dimensional fetal ultrasound

Three-dimensional (3D) imaging by ultrasound was proposed in the 1950s. Since the end of the 1980s, 3D ultrasound has become a major field of research in obstetrics and gynaecology. The technique of acquiring 3D data involves making a set of consecutive 2D ultrasound slices by moving the transducer and continuously storing the images. These ultrasound data must be converted into a regular cubic representation before presentation in different 3D visualization modes. The creation of new ultrasound sections from the 3D block and also the surface shading of a structure of interest promise improvement in the diagnosis of congenital anomalies and dysmorphology. In addition, the possibility of volume calculation by 3D ultrasound has to be considered as a clear innovation. At present, almost all of the diagnoses illustrated by 3D ultrasound can be made by 2D ultrasound, and this will continue to be so in the foreseeable future.

In-vivo three-dimensional ultrasound reconstructions of embryos and early fetuses

BACKGROUND

Three-dimensional (3D) imaging of the living human embryo has become possible in the monitoring of embryological development, as described by classic human embryology. We aimed to create 3D images of organs in embryos on early pregnancy.

METHODS

We used a specially developed 7.5 MHz annular array 3D transvaginal probe to examine embryos. We included 34 women at 7-10 weeks' gestation. We measured the crown-rump length (CRL) of the embryos and fetuses and transferred the 3D ultrasound data to an external computer for further processing to calculate volume.

FINDINGS

The CRLs ranged from 9.3 mm to 39.0 mm. The quality of the images of the embryos and fetuses made it possible to outline in detail the outer contours and the contours of the brain cavities, and the calculated volumes corresponded well to the descriptions from classic human embryology.

INTERPRETATION

Our 3D imaging system allowed visualisation of structures of less than 10 mm. Therefore, development and abnormal development of fetuses can be monitored.

Physiological "constants" for PBPK models for pregnancy

Physiologically based pharmacokinetic (PBPK) models for pregnancy are inherently more complex than conventional PBPK models due to the growth of the maternal and embryo/fetal tissues. Physiological parameters such as compartmental volumes or flow rates are relatively constant at any particular time during gestation when an acute experiment might be conducted, but vary greatly throughout the course of gestation (e.g., contrast relative fetal weight during the first month of gestation with the ninth month). Maternal physiological parameters change during gestation, depending upon the particular system; for example, cardiac output increases by approximately 50% during human gestation; plasma protein concentration decreases during pregnancy; overall metabolism remains fairly constant. Maternal compartmental volumes may change by 10-30%; embryo/fetal volume increases over a billionfold from conception to birth. Data describing these physiological changes in the human are available from the literature. Human embryo/fetal growth can be well described using the Gompertz equation. By contrast, very little of these same types of data is available for the laboratory animal. In the rodent there is a dearth of information during organogenesis as to embryo weights, and even less organ or tissue weight or volume data during embryonic or fetal periods. Allometric modeling offers a reasonable choice to extrapolate (approximately) from humans to animals; validation, however, is confined to comparisons with limited data during the late embryonic and fetal periods of development (after gestation d 11 in the rat and mouse). Embryonic weight measurements are limited by the small size of the embryo and the current state of technology. However, the application of the laser scanning confocal microscope (LSCM) to optically section intact embryos offers the capability of precise structural measurements and computer-generated three-dimensional reconstruction of early embryos. Application of these PBPK models of pregnancy in laboratory animal models at teratogenically sensitive periods of development provides exposure values at specific target tissues. These exposures provide fundamentally important data to help design and interpret molecular probe investigations into mechanisms of teratogenesis.