Mouse whole embryo culture: Evaluating the requirement for rat serum as culture medium

Shifts in embryonic oxygen levels cue heterochrony in limb initiation

Heterochrony, or the alteration of developmental timing, is an important mechanism of evolutionary change. Avian species display synchronized growth of the forelimbs and hindlimbs, while mammalian species show delayed hindlimb development. We find that mammalian limb heterochrony is evident from the start of limb bud formation, and is associated with heterochronic expression of T-box transcription factors. This heterochronic shift is not due to changes in cis-regulatory sequences controlling T-box gene expression, but unexpectedly, is dependent upon differential oxygen levels to which avian and mammalian embryos are exposed prior to limb initiation, mediated, at least partially, by an NFKB transcription factor, cRel. Together, these results provide mechanistic understanding of an important example of developmental heterochrony and exemplify how the maternal environment regulates timing during embryonic development.

Ex utero embryogenesis of non-human primate embryos and beyond

Understanding cellular and molecular processes underlying the human early post-implantation development represents one of the most fundamental questions in development and stem cell biology. As embryos implant into the uterus a week after fertilization, human development beyond the blastocyst stage is extremely difficult to study due to the inaccessibility of embryos and ethical concerns. The advents in the human embryo in vitro culture system provide an easily accessible, tractable, and perturbable platform to dissect key developmental events of human early embryonic development. However, these studies stopped around gastrulation to technical and ethical limitations, and our understanding of human gastrulation and early organogenesis remains poor. As closely related species to humans, non-human primates (NHPs) are suitable surrogate species to interrogate mechanisms underpinning human embryonic development. Here, we review the most recent advances in embryo in vitro culture systems of NHP and discuss their potential optimization strategies and applications.

Morphological phenotyping after mouse whole embryo culture

Morphological phenotyping of the mouse embryo is described at neurulation stages, primarily as a guide to evaluating the outcome of whole embryo cultures between embryonic days 8.5 and 9.5. During this period, neural tube closure is initiated and progresses to completion in the cranial region. Spinal closure is still underway at the end of the culture period. The focus of this article is particularly on phenotyping that can be performed at the bench, using a stereomicroscope. This involves assessment of embryonic health, through observation and scoring of yolk sac blood circulation, measurement of developmental stage by somite counting, and determination of crown-rump length as a measure of growth. Axial rotation ("turning") can also be assessed using a simple scoring system. Neural tube closure assessment includes: 1) determining whether closure has been initiated at the Closure 1 site; 2) evaluating the complex steps of cranial neurulation including initiation at Closure sites 2 and 3, and completion of closure at the anterior and hindbrain neuropores; 3) assessment of spinal closure by measurement of posterior neuropore length. Interpretation of defects in neural tube closure requires an appreciation of, first, the stages that particular events are expected to be completed and, second, the correspondence between embryonic landmarks, for example, somite position, and the resulting adult axial levels. Detailed embryonic phenotyping, as described in this article, when combined with the versatile method of whole embryo culture, can form the basis for a wide range of experimental studies in early mouse neural development.

Ex utero monkey embryogenesis from blastocyst to early organogenesis

The third and fourth weeks of gestation in primates are marked by several developmental milestones, including gastrulation and the formation of organ primordia. However, our understanding of this period is limited due to restricted access to in vivo embryos. To address this gap, we developed an embedded 3D culture system that allows for the extended ex utero culture of cynomolgus monkey embryos for up to 25 days post-fertilization. Morphological, histological, and single-cell RNA-sequencing analyses demonstrate that ex utero cultured monkey embryos largely recapitulated key events of in vivo development. With this platform, we were able to delineate lineage trajectories and genetic programs involved in neural induction, lateral plate mesoderm differentiation, yolk sac hematopoiesis, primitive gut, and primordial germ-cell-like cell development in monkeys. Our embedded 3D culture system provides a robust and reproducible platform for growing monkey embryos from blastocysts to early organogenesis and studying primate embryogenesis ex utero.

Effect of maternal pregestational diabetes mellitus on congenital heart diseases

BACKGROUND

The increasing population of diabetes mellitus in adolescent girls and women of childbearing age contributes to a large number of pregnancies with maternal pregestational diabetes mellitus. Congenital heart diseases are a common adverse outcome in mothers with pregestational diabetes mellitus. However, there is little systematic information between maternal pregestational diabetes mellitus and congenital heart diseases in the offspring.

DATA SOURCES

Literature selection was performed in PubMed. One hundred and seven papers were cited in our review, including 36 clinical studies, 26 experimental studies, 31 reviews, eight meta-analysis articles, and six of other types.

RESULTS

Maternal pregestational diabetes mellitus poses a high risk of congenital heart diseases in the offspring and causes variety of phenotypes of congenital heart diseases. Factors such as persistent maternal hyperglycemia, oxidative stress, polymorphism of uncoupling protein 2, polymorphism of adiponectin gene, Notch 1 pathway, Nkx2.5 disorders, dysregulation of the hypoxia-inducible factor 1, and viral etiologies are associated with the occurrence of congenital heart diseases in the offspring of mothers with pregestational diabetes mellitus. Treatment options including blood sugar-reducing, anti-oxidative stress drug supplements and exercise can help to prevent maternal pregestational diabetes mellitus from inducing congenital heart diseases.

CONCLUSIONS

Our review contributes to a better understanding of the association between maternal pregestational diabetes mellitus and congenital heart diseases in the offspring and to a profound thought of the mechanism, preventive and therapeutic measurements of congenital heart diseases caused by maternal pregestational diabetes mellitus.

Cannabidiol impairs neural tube closure in mouse whole embryo culture

BACKGROUND

Cannabidiol (CBD) is a nonpsychoactive constituent of cannabis widely available as a dietary supplement. Previous reports that it impairs the retinoid, sonic hedgehog, and folate metabolism pathways raise concern that it may impair closure of the embryonic neural tube (NT), producing NT defects including spina bifida and exencephaly.

METHODS

We undertook teratogenicity testing of CBD in mouse whole embryo culture.

RESULTS

At concentrations that do not diminish embryo viability, growth, or axial rotation, CBD dose-dependently impairs cranial NT closure, increasing the proportion of embryos that develop exencephaly. It concomitantly diminishes closure of the spinal NT, the posterior neuropore (PNP), producing longer neuropores at the end of culture which is a hallmark of spina bifida risk. Exposure to CBD does not disrupt the formation of long F-actin cables in surface ectoderm cells flanking the PNP or folding of the neuroepithelium at predictable hinge points. At the cellular level, CBD exposure does not alter proliferation or apoptosis of the spinal neuroepithelium.

DISCUSSION

Thus, CBD acts selectively as a neuroteratogen predisposing to spina bifida and exencephaly in mouse whole embryo culture at exposure levels not associated with overt toxicity. Large-scale testing of CBD's effects on NT closure, particularly in at-risk groups, is warranted to inform its marketing to women of childbearing age.

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.

Say NO to ROS: Their Roles in Embryonic Heart Development and Pathogenesis of Congenital Heart Defects in Maternal Diabetes

Congenital heart defects (CHDs) are the most prevalent and serious birth defect, occurring in 1% of all live births. Pregestational maternal diabetes is a known risk factor for the development of CHDs, elevating the risk in the child by more than four-fold. As the prevalence of diabetes rapidly rises among women of childbearing age, there is a need to investigate the mechanisms and potential preventative strategies for these defects. In experimental animal models of pregestational diabetes induced-CHDs, upwards of 50% of offspring display congenital malformations of the heart, including septal, valvular, and outflow tract defects. Specifically, the imbalance of nitric oxide (NO) and reactive oxygen species (ROS) signaling is a major driver of the development of CHDs in offspring of mice with pregestational diabetes. NO from endothelial nitric oxide synthase (eNOS) is crucial to cardiogenesis, regulating various cellular and molecular processes. In fact, deficiency in eNOS results in CHDs and coronary artery malformation. Embryonic hearts from diabetic dams exhibit eNOS uncoupling and oxidative stress. Maternal treatment with sapropterin, a cofactor of eNOS, and antioxidants such as N-acetylcysteine, vitamin E, and glutathione as well as maternal exercise have been shown to improve eNOS function, reduce oxidative stress, and lower the incidence CHDs in the offspring of mice with pregestational diabetes. This review summarizes recent data on pregestational diabetes-induced CHDs, and offers insights into the important roles of NO and ROS in embryonic heart development and pathogenesis of CHDs in maternal diabetes.

Mouse whole embryo culture: Evaluating the requirement for rat serum as culture medium

Background

Whole embryo culture is a valuable research method in mammalian developmental biology and birth defects research, enabling longitudinal studies of explanted organogenesis‐stage rodent embryos. Rat serum is the primary culture medium, and can sustain growth and development over limited periods as in utero. However, the cost, labor, and time to produce culture serum are factors limiting the uptake of the methodology. The goal of replacing or at least reducing rat serum usage in culture would be in accordance with the principles of “replacement, reduction, and refinement” of animals in research (the 3Rs).

Methods

We performed cultures of mouse embryos for 24 hr from embryonic day 8.5 in serum‐free media or in rat serum diluted with defined media, compared with 100% rat serum. Developmental parameters scored after culture included yolk sac circulation, dorsal axial length, somite number, protein content, and completion of cranial neural tube closure.

Results

A literature review revealed use of both serum‐free and diluted rat serum‐based media in whole embryo culture studies, but with almost no formal comparisons of culture success against 100% rat serum. Two serum‐free media were tested, but neither could sustain development as in 100% rat serum. Dilution of rat serum 1:1 with Glasgow Minimum Essential Medium plus defined supplements supported growth and development as well as whole rat serum, whereas other diluent media yielded substandard outcomes.

Conclusion

Rat serum usage cannot be avoided, to achieve high quality mouse embryo cultures, but rat usage can be reduced using medium containing diluted serum.