Successful development and birth of mice cultivated in vitro as early as early embryos

The road to gene manipulation in the mouse: Jean Brachet Memorial Lecture of the International Society of Differentiation (delivered June 21, 2023 at Cold Spring Harbor Laboratory)

Genetic manipulation in mammals has progressed rapidly in the past decade with the advent of CRISPR-Cas gene editing tools, promising profound impacts on the understanding of human development, health and disease. However, many years of research in divergent fields of experimental embryology, genetics, reproduction, molecular biology and transgenic technology laid the groundwork and have played critical roles for this progress. This article details various threads of research and the central role of the laboratory mouse that came together in reaching this point, all from the perspective of a scientist whose research was deeply immersed in the field.

Making human eggs in a dish: are we close?

In the space of 50 years, we have seen incredible achievements in human reproductive medicine. With these leaps forward, it is no wonder that there is a major interest in women's reproductive health research, including extension of reproductive lifespan. Substantial effort is currently being made to address this challenge, including from the commercial sector. In vitro gametogenesis (IVG) in mice is a spectacular breakthrough and has the potential to offer hope to women with intractable infertility. However, with such lofty goals, some reflection may be called for: mastering all of the techniques required for complete and safe IVG in women is likely to be extraordinarily difficult.

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.

The history and future of in vitro fertilization in the United States: the complex interrelationships among basic science, human medicine, and politics

Although much of the foundational basic scientific and clinical research was conducted in the United States, the first in vitro fertilization (IVF) birth occurred in the United Kingdom. Why? For centuries, all research surrounding the field of "reproduction" has elicited bipolar passionate responses by the American public, and the issue of "test tube babies" has been no different. The history of conception in the United States is defined by complex interrelationships among scientists, clinicians, and politically charged decisions by various branches of the US government. With a focus on research in the United States, this review summarizes the early scientific and clinical advances important to the development of IVF and then addresses the potential future developments in IVF. We also consider what future advances are possible in the United States given the current regulations, laws, and funding.

Regulation of posterior Hox genes by sex steroids explains vertebral variation in inbred mouse strains

A series of elegant embryo transfer experiments in the 1950s demonstrated that the uterine environment could alter vertebral patterning in inbred mouse strains. In the intervening decades, attention has tended to focus on the technical achievements involved and neglected the underlying biological question: how can genetically homogenous individuals have a heterogenous number of vertebrae? Here I revisit these experiments and, with the benefit of knowledge of the molecular-level processes of vertebral patterning gained over the intervening decades, suggest a novel hypothesis for homeotic transformation of the last lumbar vertebra to the adjacent sacral type through regulation of Hox genes by sex steroids. Hox genes are involved in both axial patterning and development of male and female reproductive systems and have been shown to be sensitive to sex steroids in vitro and in vivo. Regulation of these genes by sex steroids and resulting alterations to vertebral patterning may hint at a deep evolutionary link between the ribless lumbar region of mammals and the switch from egg-laying to embryo implantation. An appreciation of the impact of sex steroids on Hox genes may explain some puzzling aspects of human disease, and highlights the spine as a neglected target for in utero exposure to endocrine disruptors.

From lessons on the long-term effects of the preimplantation environment on later health to a "modified ART-DOHaD" animal model

Background

At its earliest stages, mammalian embryonic development is apparently simple but vulnerable. The environment during the preimplantation period, which only lasts a couple of days, has been implicated in adult health, extending to such early stages the concept of the developmental origin of health and disease (DOHaD).

Methods

In this review, we first provide a brief history of assisted reproductive technology (ART) focusing on in vitro culture and its outcomes during subsequent development mainly in mice and humans. Further, we introduce the "MEM mouse," a novel type 2 diabetes mouse model generated by in vitro culture of preimplantation embryos in alpha minimum essential medium (αMEM).

Main findings

The association between ART and its long-term effects has been carefully examined for its application in human infertility treatment. The "MEM mouse" develops steatohepatitis and kidney disease with diabetes into adulthood.

Conclusion

The close association between the environment of preimplantation and health in postnatal life is being clarified. The approach by which severe mouse phenotypes are successfully induced by manipulating the environment of preimplantation embryos could provide new chronic disease animal models, which we call "modified ART-DOHaD" animal models. This will also offer insights into the mechanisms underlying their long-term effects.

Cryopreservation Protocols for Genetically Engineered Mice

Protocols for cryopreservation of mouse embryos and sperm are important for preserving genetically engineered mice (GEMs) used in research to study human development and diseases. Embryo cryopreservation is mainly carried out using either of two protocols: controlled gradual cooling or vitrification. Sperm cryopreservation protocols include two methodologies that are commonly referred to as JAX and CARD. Quality-control measures are necessary to ensure that GEMs are properly cryopreserved so that they can be retrieved for future use. An archiving system is also important in keeping proper records of frozen sperm and embryos. Frozen embryos and sperm are now preferred over live mice for shipping to distant locations. This article describes detailed protocols used in cryopreservation of mouse embryos and sperm, as well as their retrieval to live mice. © 2021 U.S. Government. Sperm cryopreservation Basic Protocol 1: JAX protocol for sperm cryopreservation Support Protocol 1: JAX protocol for making sperm cryopreservation medium Basic Protocol 2: JAX protocol for IVF of mouse sperm Alternate Protocol 1: Modified CARD protocol for sperm cryopreservation Support Protocol 2: CARD protocol for making sperm cryopreservation medium Alternate Protocol 2: CARD protocol for IVF of mouse sperm Embryo cryopreservation Basic Protocol 3: Cryopreserving and thawing 2-cell embryos Alternate Protocol 3: Cryopreserving and thawing 8-cell to morula-stage embryos Surgical transfer of embryos Basic Protocol 4: Infundibulum transfer of 2-cell to morula-stage embryos.

Ex Vivo Culture for Preimplantation Mouse Embryo to Analyze Pluripotency

A couple of days after fertilization of a mouse oocyte by a sperm, two sequential cell differentiation events segregate pluripotent cells that can be identified by the presence of specific markers. Early mammalian embryos are relatively easy to recover as they are not yet implanted in the uterus matrix. Several decades of experimentation have enabled to find appropriate media to culture them, and therefore provide an excellent way to test different experimental setups such as the use of signaling inhibitors. We provide here a commonly used protocol to culture preimplantation embryos as well as a method to detect pluripotent cells in blastocysts.

Truth in science: experimental design and the legacy of John D Biggers, PhD., DSc

The current article presents a brief historical perspective on Professor John D Biggers, PhD, DSc. who died on 7 April, 2018. His interests covered reproductive physiology, embryo culture, cryobiology, sperm preservation, statistics and experimental design, and the history and ethics of human reproductive biology. Emphasis is placed on John Biggers' approach to the development of media for the culture of mammalian preimplantation embryos and to correct several minor misconceptions that have arisen in recent years regarding some of his studies. Much can be learned from his detailed approach to scientific investigation and experimental design. His scientific accomplishments and seminal contributions are important, but the tapestry of his life and legacy continue to be woven through the many students, fellows, and collaborators with whom he worked with over many years. The present article builds on a previous conversation that Michael Summers and Catherine Racowsky had with John Biggers that was published in 2008 [1].

Mouse embryo assay for human in vitro fertilization quality control: a fresh look

The mouse embryo assay (MEA) has been used in the field of human in vitro fertilization (IVF) for multiple purposes such as developing embryo culture media, quality control within the laboratory, and procedural training and proficiency testing for embryology staff. In addition, manufacturing companies use the MEA as a means of quality control for the development of embryo culture media and medical devices and to meet the standards of testing for FDA approval of new products. It has long been considered by embryologists and laboratory scientists whether the MEA is an accurate or sensitive test in the quality assessment of culture media and medical devices or if use of this testing is more an obligation. There is no uniformly accepted gold standard method for IVF lab quality control or FDA approval. This review aims to revisit the role of the use of mouse embryos in the formulation of IVF media for clinical use and the different methods of employing the mouse embryo assay for quality control. In addition, we will review the use of the MEA as an important adjunct in the training for embryology staff and fellows in training in reproductive endocrinology and infertility (REI), as well as alternatives to the use of the MEA for these purposes.

Human in vitro fertilisation and developmental biology: a mutually influential history

This article describes the origins and development of in vitro fertilisation (IVF) and how it was influenced by, and influenced, basic research in developmental biology. It describes the technical and social challenges that confronted the pioneers in this field of study, and the considerable progress that has been made since those early days. It also considers how IVF has contributed, and continues to contribute, to our understanding of early human development.

Modeling Mammalian Commitment to the Neural Lineage Using Embryos and Embryonic Stem Cells

Early mammalian embryogenesis relies on a large range of cellular and molecular mechanisms to guide cell fate. In this highly complex interacting system, molecular circuitry tightly controls emergent properties, including cell differentiation, proliferation, morphology, migration, and communication. These molecular circuits include those responsible for the control of gene and protein expression, as well as metabolism and epigenetics. Due to the complexity of this circuitry and the relative inaccessibility of the mammalian embryo in utero, mammalian neural commitment remains one of the most challenging and poorly understood areas of developmental biology. In order to generate the nervous system, the embryo first produces two pluripotent populations, the inner cell mass and then the primitive ectoderm. The latter is the cellular substrate for gastrulation from which the three multipotent germ layers form. The germ layer definitive ectoderm, in turn, is the substrate for multipotent neurectoderm (neural plate and neural tube) formation, representing the first morphological signs of nervous system development. Subsequent patterning of the neural tube is then responsible for the formation of most of the central and peripheral nervous systems. While a large number of studies have assessed how a competent neurectoderm produces mature neural cells, less is known about the molecular signatures of definitive ectoderm and neurectoderm and the key molecular mechanisms driving their formation. Using pluripotent stem cells as a model, we will discuss the current understanding of how the pluripotent inner cell mass transitions to pluripotent primitive ectoderm and sequentially to the multipotent definitive ectoderm and neurectoderm. We will focus on the integration of cell signaling, gene activation, and epigenetic control that govern these developmental steps, and provide insight into the novel growth factor-like role that specific amino acids, such as L-proline, play in this process.

Culture of Preimplantation Rabbit Embryos

It is surprising that so little attention is currently given to in vitro culture of preimplantation rabbit embryos, even though the rabbit is the only laboratory animal in which there is very considerable embryo growth before implantation, resulting in a 300-fold increase in protein content of embryonic cells during the preimplantation period and the formation of more than a 100,000 cells in the blastocyst. This growth pattern explains why blastocyst formation in vitro has an absolute requirement for amino acids, and vitamins, particularly inositol, are esssential for blastocyst growth. A semi-defined medium supplemented with 1.5% BSA (variously known as BSM II or modified F10) was developed at Cornell University at the end of the 1960s and allowed the systematic investigation of the requirements for development of 1-cell rabbit embryos to blastocysts. However, the requirements for in vitro blastocyst growth comparable to in vivo growth still remain an unsolved problem. Citrate, often found as a contaminant in serum albumin, may have an essential role in rabbit blastocyst growth, which would fit in with its role in the development of serum-free media for culture of various types of mammalian cells.A comprehensive account of the methodology is given to enable a researcher with experience culturing embryos of a different species to work on the rabbit embryo. This account covers medium preparation, hormonal stimulation of superovulation, natural breeding/artificial insemination, and collection of embryos of different stages from 1-cell to blastocyst either after euthanasia or under anesthesia. Peculiarities of the rabbit embryo such as the presence of the mucoprotein coat and its effects on behavior of cultured and transferred embryos are described. Suggestions are made for future avenues of research.

Development of bovine embryos in vitro in coculture with murine mesenchymal stem cells and embryonic fibroblasts

Despite the progress on development of new culture media, in vitro-produced embryos still display lower quality when compared to the in vivo-produced counterparts. Coculture has been reconsidered as an alternative to improve embryo quality. Mesenchymal stem cells (MSC) and murine embryonic fibroblasts (MEF) have been extensively used as feeder layers due to their capacity to release growth factors. In the present study we investigated the effect of these feeder layers in oocyte maturation and/or embryo development under in vitro conditions. Oocytes were matured in control (CTRL) conditions or in coculture with MSC or MEF. In vitro fertilization and embryo culture until fourth day were performed in CTRL condition for all groups. Embryos from fourth day on were then cultured until the eighth day in CTRL or in coculture system. No significant differences for metaphase II stage and apoptosis in oocytes were found among the groups. There was also no difference among the groups when we evaluated blastocyst formation on the seventh and eighth day, with exception of a higher hatched blastocyst rate in the group maturated and cultivated in CTRL condition when compared to the group matured and cocultured with MSC. Also no difference was observed in the number of cells in the whole embryos, in the inner cell mass, in the trophoblast and at apoptotic stage on the eighth day. We conclude that coculture with MSC or MEF during maturation and/or embryo development do not enhance the in vitro production of bovine embryos.

High Potassium Concentration during Culturing of Early Mammalian Embryos: Normal or Extreme Situation?

Analysis of the element composition of oviduct and uterine fluid in mammals showed high potassium concentrations in the early embryo microenvironment in vivo. The results of early embryogenesis of mammals in vitro in the presence of high potassium concentrations are discussed. The data are summarized in accordance with the conditions of experimentally modeled pre-implantation development. Comparative assessment of the quality of embryo development until the blastocyst stage in vitro proved the embryos more successfully developed at potassium concentrations close to those registered in the oviductal fluid.

Nuclear Localization of Mitochondrial TCA Cycle Enzymes as a Critical Step in Mammalian Zygotic Genome Activation

Transcriptional control requires epigenetic changes directed by mitochondrial tricarboxylic acid (TCA) cycle metabolites. In the mouse embryo, global epigenetic changes occur during zygotic genome activation (ZGA) at the 2-cell stage. Pyruvate is essential for development beyond this stage, which is at odds with the low activity of mitochondria in this period. We now show that a number of enzymatically active mitochondrial enzymes associated with the TCA cycle are essential for epigenetic remodeling and are transiently and partially localized to the nucleus. Pyruvate is essential for this nuclear localization, and a failure of TCA cycle enzymes to enter the nucleus correlates with loss of specific histone modifications and a block in ZGA. At later stages, however, these enzymes are exclusively mitochondrial. In humans, the enzyme pyruvate dehydrogenase is transiently nuclear at the 4/8-cell stage coincident with timing of human embryonic genome activation, suggesting a conserved metabolic control mechanism underlying early pre-implantation development.

Artificial cloning of domestic animals

Domestic animals can be cloned using techniques such as embryo splitting and nuclear transfer to produce genetically identical individuals. Although embryo splitting is limited to the production of only a few identical individuals, nuclear transfer of donor nuclei into recipient oocytes, whose own nuclear DNA has been removed, can result in large numbers of identical individuals. Moreover, clones can be produced using donor cells from sterile animals, such as steers and geldings, and, unlike their genetic source, these clones are fertile. In reality, due to low efficiencies and the high costs of cloning domestic species, only a limited number of identical individuals are generally produced, and these clones are primarily used as breed stock. In addition to providing a means of rescuing and propagating valuable genetics, somatic cell nuclear transfer (SCNT) research has contributed knowledge that has led to the direct reprogramming of cells (e.g., to induce pluripotent stem cells) and a better understanding of epigenetic regulation during embryonic development. In this review, I provide a broad overview of the historical development of cloning in domestic animals, of its application to the propagation of livestock and transgenic animal production, and of its scientific promise for advancing basic research.

Single cells get together: High-resolution approaches to study the dynamics of early mouse development

Embryonic development is a complex and highly dynamic process during which individual cells interact with one another, adopt different identities and organize themselves in three-dimensional space to generate an entire organism. Recent technical developments in genomics and high-resolution quantitative imaging are making it possible to study cellular populations at single-cell resolution and begin to integrate different inputs, for example genetic, physical and chemical factors, that affect cell differentiation over spatial and temporal scales. The preimplantation mouse embryo allows the analysis of cell fate decisions in vivo with high spatiotemporal resolution. In this review we highlight how the application of live imaging and single-cell resolution analysis pipelines is providing an unprecedented level of insight on the processes that shape the earliest stages of mammalian development.

In vitro culture and in vitro fertilization techniques for prairie voles (Microtus ochrogaster)

Prairie vole (Microtus ochrogaster) is a highly social animal and is a commonly used animal model for neuropsychopharmacological and psychiatric studies. To date, only a few reports on the development of transgenic prairie voles which was primarily due to the suboptimal development of assisted reproductive technology (ART) in prairie voles. Limitations in ART further hinder the development of genetically modified prairie voles such as the application of conventional gene targeting technologies using embryonic stem (ES) or induced pluripotent stem (iPS) cells to generate chimeric prairie voles. Moreover, recent advancement in genome-editing tools such as transcription activator-like effector nuclease (TALEN) and clustered regulatory interspaced short palindromic repeat (CRISPR)/Cas technology provide an unprecedented opportunity to create gene targeting animal model and the development of ART in prairie voles is necessary for future development of novel transgenic prairie vole model. We have established efficient method for in vitro embryo culture and sperm cryopreservation with high fertilization rate. In G-1 PLUS and G-2 PLUS sequential culture condition, 81.0% (# of Blastocysts/total n) of one-cell embryos developed to blastocysts. In contrary, no embryos were developed to blastocyst stage in KSOM medium (0/total # of embryos in culture). In vitro fertilization rate using fresh and frozen-thawed sperm was 32.6% and 29.3%, respectively. This is the first report of IVF using cryopreserved prairie vole sperm. We employed mouse IVF methods in prairie voles and optimize culture conditions using human G-1/G-2 PLUS sequential culture method that resulted in high embryonic development rate. The development in vole reproductive technology will facilitate the generation of transgenic voles in the future.

ACE consensus meeting report: culture systems

The UK Association of Clinical Embryologists held a workshop on Culture Systems for assisted conception in Sheffield on 22 May 2013. The meeting was organised in the light of the availability of numerous commercial products for the culture of human preimplantation embryos in vitro and the absence of data comparing the performance of these products. Expert opinions were presented, along with survey data provided by participating IVF Centres. The workshop highlighted the lack of a sound evidence base to support the selection of any one commercial product over another, and raised concerns over the lack of information defining precisely the composition of media, and the potential for adverse long-term effects of such products following their use in assisted conception.

Progesterone is critical for the development of mouse embryos

Infertility affects approximately 10-15 % of reproductive-aged couples, and embryo loss due to preimplantation death is common to many mammals. Previous studies showed that a complex series of interactive molecular events are associated with this process, especially hormones (progesterone and estrogens) and growth factors, and are important for the cleavage and differentiation of the blastocysts. Yet, the mechanism of preimplantation embryo development is unclear. Using conditional knockout mice (CKO), we showed the development of blastocyst is tightly controlled by the level of progesterone (P4); furthermore, we found that the time when P4 should increase is also crucial for the formation of blastocysts. In CKO mice whose Lrh1 (liver receptor homolog 1) is deleted under the expression of Cre recombinase driven by progesterone receptor promoter, which reduced P4 synthesis, few of their embryos can reach blastocyst stage. When these CKO mice were supplied with P4 in the afternoon of dpc 1 (day post copulation), most of the embryos can form blastocysts; when CKO mice were supplied with P4 from the morning of dpc1, one-third of the embryos can reach blastocyst stage; however, the supplement of P4 in the morning of dpc 2 made very few of the embryos become blastocysts. We conclude that early exposure to P4 is essential for timely progression of early embryogenesis in the mouse.

IVF culture media: past, present and future

BACKGROUND

The advances in the world of IVF during the last decades have been rapid and impressive and culture media play a major role in this success. Until the 1980s fertility centers made their media in house. Nowadays, there are numerous commercially available culture media that contain various components including nutrients, vitamins and growth factors. This review goes through the past, present and future of IVF culture media and explores their composition and quality assessment.

METHODS

A computerized search was performed in PubMed regarding IVF culture media including results from 1929 until March 2014. Information was gathered from the websites of companies who market culture media, advertising material, instructions for use and certificates of analysis. The regulation regarding IVF media mainly in the European Union (EU) but also in non-European countries was explored.

RESULTS

The keyword 'IVF culture media' gave 923 results in PubMed and 'embryo culture media' 12 068 results dating from 1912 until March 2014, depicting the increased scientific activity in this field. The commercialization of IVF culture media has increased the standards bringing a great variety of options into clinical practice. However, it has led to reduced transparency and comparisons of brand names that do not facilitate the scientific dialogue. Furthermore, there is some evidence suggesting that suboptimal culture conditions could cause long-term reprogramming in the embryo as the periconception period is particularly susceptible to epigenetic alterations. IVF media are now classified as class III medical devices and only CE (Conformité Européene)-marked media should be used in the EU.

CONCLUSION

The CE marking of IVF culture media is a significant development in the field. However, the quality and efficiency of culture media should be monitored closely. Well-designed randomized controlled trials, large epidemiological studies and full transparency should be the next steps. Reliable, standardized models assessing multiple end-points and post-implantation development should replace the mouse embryo assay. Structured long-term follow-up of children conceived by assisted reproduction technologies and traceability are of paramount importance.

Connections between preimplantation embryo physiology and culture

PURPOSE

To review the history of experimental embryo culture and how culture media that permitted complete preimplantation development in vitro were first discovered, and the physiological insights gained.

METHODS

This article reviews the history of in vitro mammalian embryo culture, in particular the efforts that led to the current generation of successful culture media and how these reflect embryo physiology, highlighting the contributions of Dr. John D. Biggers and his colleagues and students.

RESULTS

The culture of mammalian embryos began about a century ago. However, defined media without biological fluids were only developed in the late 1950s, and the first live young born from cultured embryos, using these media, were produced by McLaren and Biggers in 1958. It wasn’t until the late 1980s, however, that preimplantation mammalian embryos could generally be cultured in vitro from fertilized eggs to blastocysts. These new media led to insights into embryo physiology, including the importance of cell volume homeostasis to early embryo viability.

CONCLUSIONS

The development of successful preimplantation embryo culture media has had a profound effect on assisted reproduction technologies and on research into early embryo physiology.

Utero-tubal embryo transfer and vasectomy in the mouse model

The transfer of preimplantation embryos to a surrogate female is a required step for the production of genetically modified mice or to study the effects of epigenetic alterations originated during preimplantation development on subsequent fetal development and adult health. The use of an effective and consistent embryo transfer technique is crucial to enhance the generation of genetically modified animals and to determine the effect of different treatments on implantation rates and survival to term. Embryos at the blastocyst stage are usually transferred by uterine transfer, performing a puncture in the uterine wall to introduce the embryo manipulation pipette. The orifice performed in the uterus does not close after the pipette has been withdrawn, and the embryos can outflow to the abdominal cavity due to the positive pressure of the uterus. The puncture can also produce a hemorrhage that impairs implantation, blocks the transfer pipette and may affect embryo development, especially when embryos without zona are transferred. Consequently, this technique often results in very variable and overall low embryo survival rates. Avoiding these negative effects, utero-tubal embryo transfer take advantage of the utero-tubal junction as a natural barrier that impedes embryo outflow and avoid the puncture of the uterine wall. Vasectomized males are required for obtaining pseudopregnant recipients. A technique to perform vasectomy is described as a complement to the utero-tubal embryo transfer.

Historical background of gamete and embryo culture

The first tissue culture media were developed nearly 150 years ago by Ludwig and Ringer. These were simple salt solutions, which were initially based on the chemical properties of blood serum. The second generation of culture media was developed more than a century later, in the 1970s, aiming to mimic the reproductive environment. In the 1990s, simplex optimization was used to design the third group of media, to some extent ignoring existing formulations and principles. Simultaneous with the development of culture media, it became evident that it was necessary to carefully control the culture conditions, including temperature, pH, osmolarity, and air quality. Equally important was the development of instruments and tools specifically designed for cell tissue culture such as the inverted microscope, the incubator, the Petri dish, sterile plasticware, the laminar flow cabinet, and air filtration equipment.

Media composition: salts and osmolality

The main components of embryo culture media are salts, which dissociate into their component inorganic ions in aqueous solution. All embryo culture media contain the same six inorganic ions: Na(+), K(+), Cl(-), Ca(2+), Mg(2+), and SO(4)(2-), while most also contain PO(4)(2-). The salts that are used to formulate embryo culture media can be traced back to classic saline solutions, particularly Krebs-Ringer Bicarbonate (KRB), that were developed for somatic cells in the first half of the twentieth century. The salt and inorganic ion concentrations in the first successful defined mouse embryo culture medium, Whittens medium, were identical to those in KRB. These remained largely unchanged in embryo culture media for decades, with similar levels found in the standard mouse embryo culture medium, M16, formulated in the 1970s. Human embryos were initially cultured in undefined somatic cell media such as Earles and Hams F-10 with serum added. This changed in the mid-1980s, however, with the development of Quinns HTF, a defined medium specifically formulated for human embryo culture, in which the inorganic ion concentrations are similar to those in M16 and Whittens. While these media were useful both for experimental work and clinically, embryos suffered developmental blocks in all of them, with mouse embryos blocking at the 2-cell stage and human embryos at the 4- to 8-cell stage. Starting in the late 1980s, however, mouse embryo culture media were first developed that alleviated these developmental blocks. These media, CZB and KSOM, had much lower osmolalities than previous media, mainly due to lower inorganic ion concentrations. Indeed, lowering total inorganic ion concentration and osmolality proved key to understanding how media that supported complete preimplantation development in vitro can be formulated. A subsequent improvement was the addition of amino acids to culture media for both mouse and human embryos. At least in part, their beneficial effect during the cleavage stages of development is due to the presence in early preimplantation embryos of mechanisms for cell volume regulation that depend on the accumulation of amino acids as organic osmolytes to provide intracellular osmotic support. These amino acids, principally glycine, replace a portion of the intracellular inorganic ions that would otherwise be needed to maintain cell size, preventing the intracellular ionic strength from rising to deleterious levels and blocking development. Thus, the optimum salts levels, osmolality, and amino acid contents of culture media are not independent, but interact strongly because of their roles in cell volume regulation. In the absence of compounds that preimplantation embryos can use as organic osmolytes, embryos will develop only at lower osmolalities and salt concentrations in the medium. However, when organic osmolytes such as some amino acids are present, embryos will develop in culture at higher osmolarities that are similar to those they experience in tubal fluid in vivo.

IVF and embryo transfer: historical origin and development

IVF and embryo transfer for the treatment of human infertility has now resulted in the birth of over 4 million babies. The technique did not arise as a quantum event but was built on the efforts of many earlier workers in the fields of reproductive endocrinology and development. One should remember the famous saying of Isaac Newton: 'If I have seen further than most, it is because I have stood on the shoulder's of giants'. Ethical and moral issues have always arisen when investigators study early mammalian development, particularly human development. This paper documents these earlier studies and also draws attention to the ethical and moral arguments that inevitably arose.

Development and current applications of assisted fertilization

Since the very early establishment of in vitro insemination, it became clear that one of the limiting steps is the achievement of fertilization. Among the different assisted fertilization methods, intracytoplasmic sperm injection emerged as the ultimate technique to allow fertilization with ejaculated, epididymal, and testicular spermatozoa. This work describes the early steps that brought forth the development of intracytoplasmic sperm injection and its role in assisted reproductive techniques. The current methods to select the preferential male gamete will be elucidated and the concerns related to the offspring of severe male factor couples will be discussed.

Culture systems: single step

Culture media to support development of zygotes to the blastocyst stage is based either on a single medium or sequential (two-step) media. Single medium culture either with or without day 3 renewal is associated with simplified laboratory protocols and lower costs compared with sequential medium. There are currently insufficient clinical data to conclude that one system, either single or sequential, is superior to the other with regard to clinical performance. This chapter summarizes the rationale for use of a single medium, along with supporting animal and human data for culturing embryos in a single medium, renewed or not renewed.

Utility of animal models for human embryo culture development: rodents

Advancements in clinical human embryo culture over the last 30-40 years have been supported by research conducted with embryos from rodent and domestic species. The mouse has been the primary rodent species that has contributed to improved embryo culture outcomes. Numerous parameters applied in the beginning of experiments, during progress of experiments, and as end-point measures provide varying degrees of rigor and interpretive strength and/or complexity. A nonexhaustive discussion of these parameters is presented with important emphasis on experimental design to obtain the greatest power of intraexperimental interpretation of inferior, equivalent, or improved culture conditions in the mouse model. Additionally, data are presented demonstrating the inherent flaw of overinterpretation of interexperimental outcome comparisons and caution of expectations of data translation from the mouse to the human embryo culture scenario. Finally, a materials, methods, and notes discussion enumerates important steps in use of mouse embryos as a bioassay tool, independent of whether they are being used in an experiment focused on quality control or improving culture conditions.

Robert Edwards: the path to IVF

The early influences on Robert Edwards’ approach to the scientific research that led to human IVF are described. His interest as a graduate student in the genetics of early mammalian development stimulated him later to investigate whether the origins of human genetic diseases such as Down, Klinefelter and Turner syndromes might be explained by events during egg maturation. This clinical problem provided the most powerful stimulus to achieve both oocyte maturation and fertilization in vitro in humans. Indeed,preimplantation genetic diagnosis was his main goal until he met Patrick Steptoe in 1968. A re-evaluation of his meeting with Steptoe suggests that initially Steptoe’s laparoscopic skill was of interest for its potential to solve the sperm capacitation problem. Steptoe’simpact on Edwards was twofold. First, Steptoe’s long-held interest in infertility raised this application of IVF higher in Edwards’priorities. Second, Steptoe offered a long-term partnership, in which oocyte collection without in-vitro maturation was a possibility.The professional criticism generated by their work together encouraged Edwards to pursue a deliberate programme of public education about the issues raised and to challenge and develop professional bioethical thought and discourse about reproduction.

The demoiselle of X-inactivation: 50 years old and as trendy and mesmerising as ever

In humans, sexual dimorphism is associated with the presence of two X chromosomes in the female, whereas males possess only one X and a small and largely degenerate Y chromosome. How do men cope with having only a single X chromosome given that virtually all other chromosomal monosomies are lethal? Ironically, or even typically many might say, women and more generally female mammals contribute most to the job by shutting down one of their two X chromosomes at random. This phenomenon, called X-inactivation, was originally described some 50 years ago by Mary Lyon and has captivated an increasing number of scientists ever since. The fascination arose in part from the realisation that the inactive X corresponded to a dense heterochromatin mass called the “Barr body” whose number varied with the number of Xs within the nucleus and from the many intellectual questions that this raised: How does the cell count the X chromosomes in the nucleus and inactivate all Xs except one? What kind of molecular mechanisms are able to trigger such a profound, chromosome-wide metamorphosis? When is X-inactivation initiated? How is it transmitted to daughter cells and how is it reset during gametogenesis? This review retraces some of the crucial findings, which have led to our current understanding of a biological process that was initially considered as an exception completely distinct from conventional regulatory systems but is now viewed as a paradigm “par excellence” for epigenetic regulation.

A potential use of embryonic stem cell medium for the in vitro culture of preimplantation embryos

PURPOSE

To assess the impact of embryonic stem cell culture medium (ESCM) on the pre- and post-implantation development of the mouse embryo, as a mammalian model, in comparison with the conventional culture medium, a potassium simplex optimized medium (KSOM).

METHODS

Development in ESCM versus KSOM was compared in terms of embryo morphology, cleavage, cavitation, hatching, cell number, expression of TE and ICM transcription factors (Cdx2 and Oct4, respectively), implantation, and development in utero.

RESULTS

An enriched medium like ESCM can be beneficial for in vitro embryo development when cultured from the 8-cell stage, as evidenced by promotion of blastocyst development with respect to cavity expansion, hatching, and cell division. Such benefits were not observed when embryos were cultured from the 2-cell stage.

CONCLUSIONS

ESCM may augment in vitro embryo development from the 8-cell stage. Using different culture media at different stages may be beneficial to achieve more effective human in vitro fertilization.

The use of transgenic mouse models in the study of male infertility

Over the past few decades with the rapid advances in embryo and embryonic stem cell manipulation techniques, transgenic mouse models have emerged as a powerful tool for the study of gene function and complex diseases including male infertility. In this review we give a brief history of the development of tools for the production of transgenic mouse models. This spans the advances from early pronuclear injection to the use of targeted embryonic stem cells to produce gene targeted, conditional, and inducible knockout mouse models. Lastly we provide a few examples to illustrate the utility of mouse models in the study of male infertility.

Survival in Vitro of sheep eggs

Transfers were made to recipient ewes of sheep eggs stored in serum at 37° C. for up to 5 hr. (fresh eggs), in serum at room temperature for 24 to 48 hr. (stored eggs), or in Whitten's modified Krebs-Ringer-bicarbonate medium for 24 to 48 hr. (cultured eggs). The success of transfer was judged from the presence of embryos at autopsy 25–30 days after transfer.

Fifteen of 42 fresh eggs, 11 of 23 stored eggs and 3 of 15 cultured eggs were found to have developed after transfer.

The findings are discussed.

Embryo Culture Media, Culture Techniques and Embryo Selection: A Tribute to Wesley Kingston Whitten

This review article gives a brief history of the classical experiments that led to the development of the embryo culture medium and in vitro embryo culture. It proposes that, in view of the outstanding and significant pioneering contributions of Wesley Kingston Whitten to the development of embryo culture medium, he be considered the “Father of Embryo Culture Medium”. Furthermore, it describes the nutritional requirements of early embryos and how these requirements with specific references to carbohydrates, amino acids, phosphates, growth factors, etc, have been utilized to formulate increasingly more complex embryo culture media. This has led to the development of progressively more efficacious embryo culture media including the formulation of completely defined and synthetic protein-free embryo culture medium. The review also describes physical factors, growth factors, insemination methods for the fertilization of oocytes and culture methods affecting embryo growth, development, metabolism, oxygen embryotoxicity and survival. In procedural terms, the review also summarizes the evolution of embryo culture techniques from tube culture to, microdrop culture under oil to co-culture to ultra microdrop culture techniques. It includes techniques of in vitro maturation and for the selection of potentially viable embryos of various developmental stages.

From mouse egg to mouse embryo: polarities, axes, and tissues

This review describes the three classical models (mosaic, positional, and polarization) proposed to explain blastocyst formation and summarizes the evidence concerning them. It concludes that the polarization model incorporates elements of the other two models and best explains most known information. I discuss key requirements of a molecular basis for the generation and stabilization of polarity and identify ezrin/E-cadherin, PAR proteins, and Cdx2 as plausible key molecular players. I also discuss the idea of a network process operating to build cell allocations progressively into committed differences. Finally, this review critically considers the possibility of developmental information being encoded within the oocyte and zygote. No final decision can be reached on a mechanism of action underlying any encoded information, but a cell interaction process model is preferred over one that relies solely on differential inheritance.

Embryo culture: can we perform better than nature?

Culture of preimplantation-stage embryos has always been a key element of laboratory embryology and has contributed substantially to the success of many assisted reproduction procedures. During the past decade, its importance has increased as extended in-vitro embryo culture and single blastocyst transfer have become indispensable parts of the approach to decreasing the chance of multiple pregnancy while preserving the overall efficiency of the treatment. However, in spite of the scientific and commercial challenge stimulating research worldwide to optimize embryo culture conditions, a consensus is missing even in the basic principles, including composition and exchange of media, the required physical and biological environment and even the temperature of incubation. This review attempts to summarize the controversies, demonstrate the fragility of some widely accepted dogmas and generate an open-minded debate towards rapid and efficient optimization. New approaches expanding the traditional frames of mammalian embryo culture are also discussed. Although some researchers suppose that the efficiency of the presently applied in-vitro culture systems have already approached the biological limits, authors are confident that substantial improvement may be achieved that may expand considerably the possibilities of future assisted reproduction in humans.

Cell volume regulation in oocytes and early embryos: connecting physiology to successful culture media

BACKGROUND

Preimplantation embryos are particularly susceptible to in vitro developmental blocks. These could be alleviated by lowering culture medium osmolarity. Because mammalian cells regulate their volumes by adjusting intracellular osmotic pressure, cell volume regulation could be critical to early embryos.

METHODS

We reviewed the literature on cell volume regulation in preimplantation embryos and the effects of increased osmolarity on embryo development, focusing also on the relation with improvements in embryo culture media.

RESULTS

Embryos failed to develop from fertilized oocytes when osmolarity is increased. This could be alleviated by decreasing osmolarity or including certain compounds such as certain amino acids. Early preimplantation mouse embryos require intracellular accumulation of glycine to provide osmotic support and thus control cell volume. The glycine-specific transporter, GLYT1, mediates osmoregulated glycine accumulation in mouse embryos and likely in human embryos. GLYT1 is activated during meiotic maturation starting at ovulation. Prior to this, oocyte size is not independently controlled but instead is determined by strong adhesion between the oocyte plasma membrane and the inner surface of the zona pellucida.

CONCLUSIONS

Early preimplantation embryos are particularly sensitive to increased osmolarity, and require the importation of glycine to regulate their cell volumes using a mechanism unique to early embryos. Cell volume regulation first appears when ovulation is triggered, oocyte zona pellucida adhesion is released, and glycine transport is activated. The requirement for supporting these physiological functions in oocytes and embryos should be taken into account when developing and improving systems for in vitro oocyte maturation and embryo culture.