Mouse Whole Embryo Culture

A 3D "sandwich" co-culture system with vascular niche supports mouse embryo development from E3.5 to E7.5 in vitro

BACKGROUND

Various methods for ex utero culture systems have been explored. However, limitations remain regarding the in vitro culture platforms used before implanting mouse embryos and the normal development of mouse blastocysts in vitro. Furthermore, vascular niche support during mouse embryo development from embryonic day (E) 3.5 to E7.5 is unknown in vitro.

METHODS

This study established a three-dimensional (3D) "sandwich" vascular niche culture system with in vitro culture medium (IVCM) using human placenta perivascular stem cells (hPPSCs) and human umbilical vein endothelial cells (hUVECs) as supportive cells (which were seeded into the bottom layer of Matrigel) to test mouse embryos from E3.5 to E7.5 in vitro. The development rates and greatest diameters of mouse embryos from E3.5 to E7.5 were quantitatively determined using SPSS software statistics. Pluripotent markers and embryo transplantation were used to monitor mouse embryo quality and function in vivo.

RESULTS

Embryos in the IVCM + Cells (hPPSCs + hUVECs) group showed higher development rates and greater diameters at each stage than those in the IVCM group. Embryos in the IVCM + Cells group cultured to E5.5 morphologically resembled natural egg cylinders and expressed specific embryonic cell markers, including Oct4 and Nanog. These features were similar to those of embryos developed in vivo. After transplantation, the embryos were re-implanted in the internal uterus and continued to develop to a particular stage.

CONCLUSIONS

The 3D in vitro culture system enabled embryo development from E3.5 to E7.5, and the vascularization microenvironment constructed by Matrigel, hPPSCs, and hUVECs significantly promoted the development of implanted embryos. This system allowed us to further study the physical and molecular mechanisms of embryo implantation in vitro.

Establishment of an in vitro method of rabbit embryo toxicity with toxicokinetics study

This report introduces a novel method, rabbit whole embryo culture (WEC) combined with toxicokinetics (TK), for toxicity testing. Rodent WEC has been extensively used for in vitro screening of developmental toxicity. To improve the reliability of in vitro data, it is important to consider TK and species specificity. To test the utility and effectiveness of this method, we investigated the toxic effect of thalidomide on rabbit embryos and its behavior in test systems both in vitro and in vivo under the same experimental condition. The data showed that thalidomide induced embryo malformations such as embryonic brain hypoplasia, short limb buds, and declined embryonic growth both in vitro and in vivo. The toxic effect increased with the increasing exposure of the embryo to thalidomide. In addition, we observed similar toxic effects and exposure-effect relationships in vivo and in vitro. Therefore, we preliminarily conclude that this new method can effectively predict and explain thalidomide toxicity. Furthermore, we investigated the behavior of test compounds in the WEC system for the first time, and this method is expected to be an important technique for in vitro toxicity study after extensive verification.

Refinement of inducible gene deletion in embryos of pregnant mice

CreERT2‐mediated gene recombination is widely applied in developmental biology research. Activation of CreERT2 is typically achieved by injection of tamoxifen in an oily vehicle into the peritoneal cavity of mid‐gestation pregnant mice. This can be technically challenging and adversely impacts welfare. Here we characterize three refinements to this technique: Pipette feeding (not gavage) of tamoxifen, ex vivo CreERT2 activation in whole embryo culture and injection of cell‐permeable TAT‐Cre into Cre‐negative cultured embryos. We demonstrate that pipette feeding of tamoxifen solution to the mother on various days of gestation reliably activates embryonic CreERT2, illustrated here using β‐Actin^CreERT2, Sox2^CreERT2, T^CreERT2, and Nkx1.2^CreERT2. Pipette feeding of tamoxifen induces dose‐dependent recombination of Rosa26^mTmG reporters when administered at E8.5. Activation of two neuromesodermal progenitor‐targeting Cre drivers, T^CreERT2, and Nkx1.2^CreERT2, produces comparable neuroepithelial lineage tracing. Dose‐dependent CreERT2 activation can also be achieved by brief exposure to 4OH‐tamoxifen in whole embryo culture, allowing temporal control of gene deletion and eliminating the need to treat pregnant mice. Rosa26^mTmG reporter recombination can also be achieved regionally by injecting TAT‐Cre into embryonic tissues at the start of culture. This allows greater spatial control over Cre activation than can typically be achieved with endogenous CreERT2, for example by injecting TAT‐Cre on one side of the midline. We hope that our description and application of these techniques will stimulate refinement of experimental methods involving CreERT2 activation for gene deletion and lineage tracing studies. Improved temporal (ex vivo treatment) and spatial (TAT‐Cre injection) control of recombination will also allow previously intractable questions to be addressed.