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Liane Russell primarily worked in the areas of mammalian genetics, mutagenesis and teratogenesis. Two of her major contributions are the '14-day rule' in radiology and the role of the Y chromosome in mammalian sex determination. Earlier in her career, she studied the effect of radiation on mouse embryonic development. This study highlighted the critical period during development that is most susceptible to the teratogenic effects of radiation. Based on this study, she recommended that whenever possible, exposure of women of child-bearing age to radiation should be restricted to the first two weeks after their last menstrual period to avoid potential damage to their embryos, assuming that the women may be in early pregnancies and unaware of it. This is now known as the '14-day rule'. Later on, along with her colleagues, she showed that in mammals, it is the Y chromosome that determines sex during development and that it is quite different from sex determination in fruitflies, which was well-worked out at that time. This pioneering work has been expanded widely by various researchers to gain an in-depth understanding of the mechanism of sex determination in mammals, especially in humans and its connection with male sterility. Along with her husband and colleagues, she developed the 'Mouse House' at Oak Ridge National Laboratory with the capacity to keep a quarter of a million mice at a time. It lasted for almost 60 years, generating a wealth of information in the fields of basic genetics, cytogenetics, chromosomal behaviour, reproductive biology, mutagenesis and teratogenesis in mammalian development. [ABSTRACT FROM AUTHOR]

Retrotransposons are invasive genetic elements that constitute substantial portions of mammalian genomes. They have the potential to influence nearby gene expression through their cis-regulatory sequences, reverse transcription machinery, and the ability to mold higher-order chromatin structures. Due to their multifaceted functions, it is crucial for host fitness to maintain strict regulation of these parasitic sequences to ensure proper growth and development. This review explores how subsets of retrotransposons have undergone evolutionary exaptation to enhance the complexity of mammalian genomes. It also highlights the significance of regulating these elements, drawing on recent studies conducted in human and murine systems. [ABSTRACT FROM AUTHOR]

Retrotransposons are invasive genetic elements that constitute substantial portions of mammalian genomes. They have the potential to influence nearby gene expression through their cis-regulatory sequences, reverse transcription machinery, and the ability to mold higher-order chromatin structures. Due to their multifaceted functions, it is crucial for host fitness to maintain strict regulation of these parasitic sequences to ensure proper growth and development. This review explores how subsets of retrotransposons have undergone evolutionary exaptation to enhance the complexity of mammalian genomes. It also highlights the significance of regulating these elements, drawing on recent studies conducted in human and murine systems.

The first few days of embryonic development in eutherian mammals are dedicated to the specification and elaboration of the extraembryonic tissues. However, where the fetus ends and its adnexa begins is not always as self-evident during the early stages of development, when the definitive body axes are still being laid down, the germ layers being specified and a discrete form or bodyplan is yet to emerge. Function, anatomy, histomorphology and molecular identities have been used through the history of embryology, to make this distinction. In this review, we explore them individually by using specific examples from the early embryo. While highlighting the challenges of drawing discrete boundaries between embryonic and extraembryonic tissues and the limitations of a binary categorization, we discuss how basing such identity on fate is the most universal and conceptually consistent. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'. [ABSTRACT FROM AUTHOR]

Tsukushi (TSK), a leucine-rich peptidoglycan in the extracellular compartment, mediates multiple signaling pathways that are critical for development and metabolism. TSK regulates signaling pathways that eventually control cellular communication, proliferation, and cell fate determination. Research on TSK has become more sophisticated in recent years, illustrating its involvement in the physiology and pathophysiology of neural, genetic, and metabolic diseases. In a recent study, we showed that TSK therapy reversed the pathophysiological abnormalities of the hydrocephalic (a neurological disorder) brain in mice. This review summarizes the roles of TSK in key signaling processes in the mammalian development, disorders, and evaluating its possible therapeutic and diagnostic potential. [ABSTRACT FROM AUTHOR]

During the first days of mammalian development, the embryo forms the blastocyst, the structure responsible for implanting the mammalian embryo. Consisting of an epithelium enveloping the pluripotent inner cell mass and a fluid-filled lumen, the blastocyst results from a series of cleavage divisions, morphogenetic movements, and lineage specification. Recent studies have identified the essential role of actomyosin contractility in driving cytokinesis, morphogenesis, and fate specification, leading to the formation of the blastocyst. However, the preimplantation development of contractility mutants has not been characterized. Here, we generated single and double maternal-zygotic mutants of non-muscle myosin II heavy chains (NMHCs) to characterize them with multiscale imaging. We found that Myh9 (NMHC II-A) is the major NMHC during preimplantation development as its maternal-zygotic loss causes failed cytokinesis, increased duration of the cell cycle, weaker embryo compaction, and reduced differentiation, whereas Myh10 (NMHC II-B) maternal-zygotic loss is much less severe. Double maternal-zygotic mutants for Myh9 and Myh10 show a much stronger phenotype, failing most of the attempts of cytokinesis. We found that morphogenesis and fate specification are affected but nevertheless carry on in a timely fashion, regardless of the impact of the mutations on cell number. Strikingly, even when all cell divisions fail, the resulting single-celled embryo can initiate trophectoderm differentiation and lumen formation by accumulating fluid in increasingly large vacuoles. Therefore, contractility mutants reveal that fluid accumulation is a cell-autonomous process and that the preimplantation program carries on independently of successful cell division.

Vertebrate genomes are marked by notably high levels of 5-cytosine DNA methylation (5meC). The clearest function of DNA methylation among members of the subphylum is repression of potentially deleterious transposable elements (TEs). However, enrichment in the bodies of protein coding genes and pericentromeric heterochromatin indicate an important role for 5meC in those genomic compartments as well. Moreover, DNA methylation plays an important role in silencing of germline-specific genes. Impaired function of major components of DNA methylation machinery results in lethality in fish, amphibians and mammals. Despite such apparent importance, mammals exhibit a dramatic loss and regain of DNA methylation in early embryogenesis prior to implantation, and then again in the cells specified for the germline. In this minireview we will highlight recent studies that shine light on two major aspects of embryonic DNA methylation reprogramming: (1) The mechanism of DNA methylation loss after fertilization and (2) the protection of discrete loci from ectopic DNA methylation deposition during reestablishment. Finally, we will conclude with some extrapolations for the evolutionary underpinnings of such extraordinary events that seemingly put the genome under unnecessary risk during a particularly vulnerable window of development.

Mammalian neonates are born at a wide range of maturity levels. Altricial newborns are born with limited sensory agency and require extensive parental care. In contrast, precocial neonates are relatively mature physically and often capable of independent function shortly after birth. In extant mammals, placental newborns vary from altricial to precocial, while marsupials and monotremes are all extremely altricial at birth. Bears (family Ursidae) have one of the lowest neonatal−maternal mass ratios in placental mammals, and are thought to also have the most altricial placental newborns. In particular, giant pandas (Ailuropoda melanoleuca) are thought to be exceptionally altricial at birth, and possibly marsupial‐like. Here we used micro‐computer (micro‐computed) tomography scanning to visualize the skeletal anatomy of ursid neonates and compare their skeletal maturity with the neonates of other caniform outgroups. Specifically, we asked whether ursid neonates have exceptionally altricial skeletons at birth compared with other caniform neonates. We found that most bear neonates are similar to outgroup neonates in levels of skeletal ossification, with little variation in degree of ossification between ursine bears neonates (i.e. bears of the subfamily Ursinae). Perinatal giant pandas, however, have skeletal maturity levels most similar to a 42−45‐day‐old beagle fetus (~70% of total beagle gestation period). No bear exhibits the skeletal heterochronies seen in marsupial development. With regards to skeletal development, ursine bears are not exceptionally altricial relative to other caniform outgroups, but characterized largely by the drastic difference between newborn and adult body sizes. A review on the existing hypotheses for ursids' unique reproductive strategy suggests that the extremely small neonatal−maternal mass ratio of ursids may be related to the recent evolution of large adult body size, while life history characteristics retained an ancestral condition. A relatively short post‐implantation gestation time may be the proximal mechanism behind the giant panda neonates' small size relative to maternal size and altricial skeletal development at birth. [ABSTRACT FROM AUTHOR]

Proper patterns of genome-wide DNA methylation, mediated by DNA methyltransferases DNMT1, -3A and -3B, are essential for embryonic development and genomic stability in mammalian cells. The de novo DNA methyltransferase DNMT3B is of particular interest because it is frequently overexpressed in tumor cells and is mutated in immunodeficiency, centromere instability and facial anomalies (ICF) syndrome. In order to gain a better understanding of DNMT3B, in terms of the targeting of its methylation activity and its role in genome stability, we biochemically purified endogenous DNMT3B from HeLa cells. DNMT3B co-purifies and interacts, both in vivo and in vitro, with several components of the condensin complex (hCAP-C, hCAP-E and hCAP-G) and KIF4A. Condensin mediates genome-wide chromosome condensation at the onset of mitosis and is critical for proper segregation of sister chromatids. KIF4A is proposed to be a motor protein carrying DNA as cargo. DNMT3B also interacts with histone deacetylase 1 (HDAC1), the co-repressor SIN3A and the ATP-dependent chromatin remodeling enzyme hSNF2H. Further more, DNMT3B co-localizes with condensin and KIF4A on condensed chromosomes throughout mitosis. These studies therefore reveal the first direct link between the machineries regulating DNA methylation and mitotic chromosome condensation in mammalian cells.

The caudal lateral epiblast of mammalian embryos harbours bipotent progenitors that contribute to the spinal cord and the paraxial mesoderm in concert with the body axis elongation. These progenitors, called neural mesodermal progenitors (NMPs), are identified as cells that co-express Sox2 and T/brachyury, a criterion used to derive NMP-like cells from embryonic stem cells in vitro. However, unlike embryonic NMPs, these progenitors do not selfrenew. Here, we find that the protocols that yield NMP-like cells in vitro initially produce a multipotent population that, in addition to NMPs, generates progenitors for the lateral plate and intermediate mesoderm. We show that epiblast stem cells (EpiSCs) are an effective source of these multipotent progenitors, which are further differentiated by a balance between BMP and Nodal signalling. Importantly, we show that NMP-like cells derived from EpiSCs exhibit limited self-renewal in vitro and a gene expression signature like their embryonic counterparts. [ABSTRACT FROM AUTHOR]

Early mammalian development is crucially dependent on the establishment of oxidative energy metabolism within the trophectoderm (TE) lineage. Unlike the inner cell mass, TE cells enhance ATP production via mitochondrial oxidative phosphorylation (OXPHOS) and this metabolic preference is essential for blastocyst maturation. However, molecular mechanisms that regulate establishment of oxidative energy metabolism in TE cells are incompletely understood. Here, we show that conserved transcription factor TEAD4, which is essential for pre-implantation mammalian development, regulates this process by promoting mitochondrial transcription. In developing mouse TE and TE-derived trophoblast stem cells (TSCs), TEAD4 localizes to mitochondria, binds to mitochondrial DNA (mtDNA) and facilitates its transcription by recruiting mitochondrial RNA polymerase (POLRMT). Loss of TEAD4 impairs recruitment of POLRMT, resulting in reduced expression of mtDNA-encoded electron transport chain components, thereby inhibiting oxidative energy metabolism. Our studies identify a novel TEAD4-dependent molecular mechanism that regulates energy metabolism in the TE lineage to ensure mammalian development. [ABSTRACT FROM AUTHOR]

GATA transcription factors are implicated in establishing cell fate during mammalian development. In early mammalian embryos, GATA3 is selectively expressed in the extraembryonic trophoblast lineage and regulates gene expression to promote trophoblast fate. However, trophoblast-specific GATA3 function is dispensable for early mammalian development. Here, using dual conditional knockout mice, we show that genetic redundancy of Gata3 with paralog Gata2 in trophoblast progenitors ensures the successful progression of both pre- and postimplantation mammalian development. Stage-specific gene deletion in trophoblasts reveals that loss of both GATA genes, but not either alone, leads to embryonic lethality prior to the onset of their expression within the embryo proper. Using ChIP-seq and RNA-seq analyses, we define the global targets of GATA2/GATA3 and show that they directly regulate a large number of common genes to orchestrate stem versus differentiated trophoblast fate. In trophoblast progenitors, GATA factors directly regulate BMP4, Nodal and Wnt signaling components that promote embryonic-extraembryonic signaling crosstalk, which is essential for the development of the embryo proper. Our study provides genetic evidence that impairment of trophoblast-specific GATA2/GATA3 function could lead to early pregnancy failure. [ABSTRACT FROM AUTHOR]

Homeobox genes encode a group of DNA binding regulatory proteins whose key function occurs in the spatial-temporal organization of genome during embryonic development and differentiation. The role of these Hox genes during ontogenesis makes it an important model for research. HoxA5 is a member of Hox gene family playing a central role during axial body patterning and morphogenesis. DNA modification studies have shown that the function of Hox genes is partly governed by the methylation-mediated gene expression regulation. Therefore the study aimed to investigate the role of epigenetic events in regulation of tissue-specific expression pattern of HoxA5 gene during mammalian development. The methodology adopted were sodium bisulfite genomic DNA sequencing, quantitative real-time PCR and chromatin-immunoprecipitation (ChIP). Methylation profiling of HoxA5 gene promoter shows higher methylation in adult as compared to fetus in various somatic tissues of mouse being highest in adult spleen. However q-PCR results show higher expression during fetal stages being highest in fetal intestine followed by brain, liver and spleen. These results clearly indicate a strict correlation between DNA methylation and tissue-specific gene expression. The findings of chromatin-immunoprecipitation (ChIP) have also reinforced that epigenetic event like DNA methylation plays important role in the regulation of tissue specific expression of HoxA5. [ABSTRACT FROM AUTHOR]

Vertebrate Hox genes encode transcription factors operating during the development of multiple organs and structures. However, the evolutionary mechanism underlying this remarkable pleiotropy remains to be fully understood. Here, we show that Hoxd8 and Hoxd9, two genes of the HoxD complex, are transcribed during mammary bud (MB) development. However, unlike in other developmental contexts, their coexpression does not rely on the same regulatory mechanism. Hoxd8 is regulated by the combined activity of closely located sequences and the most distant telomeric gene desert. On the other hand, Hoxd9 is controlled by an enhancer-rich region that is also located within the telomeric gene desert but has no impact on Hoxd8 transcription, thus constituting an exception to the global regulatory logic systematically observed at this locus. The latter DNA region is also involved in Hoxd gene regulation in other contexts and strongly interacts with Hoxd9 in all tissues analyzed thus far, indicating that its regulatory activity was already operational before the appearance of mammary glands. Within this DNA region and neighboring a strong limb enhancer, we identified a short sequence conserved in therian mammals and capable of enhancer activity in the MBs. We propose that Hoxd gene regulation in embryonic MBs evolved by hijacking a preexisting regulatory landscape that was already at work before the emergence of mammals in structures such as the limbs or the intestinal tract. [ABSTRACT FROM AUTHOR]

Embryonic stem cells (ESC) are both a potential source of cells for tissue replacement therapies and an accessible tool to model early embryonic development. Chemical factors such as soluble growth factors and insoluble components of the extracellular matrix are known to affect the differentiation of murine ESCs. However, there is also evidence to suggest that undifferentiated cells can both sense the mechanical properties of their environment and differentiate accordingly. By growing ESCs on flexible polydimethylsiloxane substrates with varying stiffness, we tested the hypothesis that substrate stiffness can influence ESC differentiation. While cell attachment was unaffected by the stiffness of the growth substrate, cell spreading and cell growth were all increased as a function of substrate stiffness. Similarly, several genes expressed in the primitive streak during gastrulation and implicated in early mesendoderm differentiation, such as Brachyury, Mixl1 and Eomes, were upregulated in cell cultures on stiffer compared to softer substrates. Finally, we demonstrated that osteogenic differentiation of ESCs was enhanced on stiff substrates compared to soft substrates, illustrating that the mechanical environment can play a role in both early and terminal ESC differentiation. Our results suggest a fundamental role for mechanosensing in mammalian development and illustrate that the mechanical environment should be taken into consideration when engineering implantable scaffolds or when producing therapeutically relevant cell populations in vitro.

H1 linker histones are key chromatin architectural proteins facilitating the formation of higher order chromatin structures. The H1 family constitutes the most heterogeneous group of histone proteins, with eleven non-allelic H1 variants in mammals. H1 variants differ in their biochemical properties and exhibit significant sequence divergence from one another, yet most of them are highly conserved during evolution from mouse to human. H1 variants are differentially regulated during development and their cellular compositions undergo dramatic changes in embryogenesis, gametogenesis, tissue maturation and cellular differentiation. As a group, H1 histones are essential for mouse development and proper stem cell differentiation. Here we summarize our current knowledge on the expression and functions of H1 variants in mammalian development and stem cell differentiation. Their diversity, sequence conservation, complex expression and distinct functions suggest that H1s mediate chromatin reprogramming and contribute to the large variations and complexity of chromatin structure and gene expression in the mammalian genome. This article is part of a Special Issue entitled: Histone H1, edited by Dr. Albert Jordan. [ABSTRACT FROM AUTHOR]

In mammals, one of the two X chromosomes of female cells is inactivated for dosage compensation between the sexes. X chromosome inactivation is initiated in early embryos by the noncoding Xist RNA. Subsequent chromatin modifications on the inactive X chromosome (Xi) lead to a remarkable stability of gene repression in somatic cell lineages. In mice, reactivation of genes on the Xi accompanies the establishment of pluripotent cells of the female blastocyst and the development of primordial germ cells. Xi reactivation also occurs when pluripotency is established during the reprogramming of somatic cells to induced pluripotent stem cells. The mechanism of Xi reactivation has attracted increasing interest for studying changes in epigenetic patterns and for improving methods of cell reprogramming. Here, we review recent advances in the understanding of Xi reactivation during development and reprogramming and illustrate potential clinical applications. [ABSTRACT FROM AUTHOR]

Studies of replication timing provide a handle into previously impenetrable higher-order levels of chromosome organization and their plasticity during development. Although mechanisms regulating replication timing are not clear, novel genome-wide studies provide a thorough survey of the extent to which replication timing is regulated during most of the early cell fate transitions in mammals, revealing coordinated changes of a defined set of 400–800 kb chromosomal segments that involve at least half the genome. Furthermore, changes in replication time are linked to changes in sub-nuclear organization and domain-wide transcriptional potential, and tissue-specific replication timing profiles are conserved from mouse to human, suggesting that the program has developmental significance. Hence, these studies have provided a solid foundation for linking megabase level chromosome structure to function, and suggest a central role for replication in domain-level genome organization. [ABSTRACT FROM AUTHOR]

Abstract: The four members of the albumin gene family encode the serum transport proteins albumin, α-fetoprotein, α-albumin, and vitamin D-binding protein. These genes are transcribed primarily in the liver with each having a different pattern of developmental expression. The tight linkage of these genes, particularly that of albumin, α-fetoprotein and α-albumin, and their liver-specific expression, has led to the suggestion that these genes share common regulatory elements. To directly examine whether the α-fetoprotein enhancer region could regulate the albumin gene family, expression of these genes was monitored in mice in which this region was deleted by homologous recombination. Our data indicate that this enhancer region is required for α-fetoprotein and albumin activation early in liver development and α-fetoprotein reactivation during liver regeneration, but that albumin, α-albumin, and vitamin D-binding protein expression later in hepatic development is not affected by the absence of these enhancers. We also demonstrate that RNA polymerase II loading on the α-fetoprotein and albumin promoters is reduced in the absence of this enhancer region, indicating a direct role for these enhancers in the assembly of the RNA Polymerase II complex during liver development. [Copyright &y& Elsevier]

Abstract: Nedd4 (neural precursor cell expressed developmentally down-regulated gene 4) is an E3 ubiquitin ligase highly conserved from yeast to humans. The expression of Nedd4 is developmentally down-regulated in the mammalian nervous system, but the role of Nedd4 in mammalian neural development remains poorly understood. Here we show that a null mutation of Nedd4 in mice leads to perinatal lethality: mutant mice were stillborn and many of them died in utero before birth (between E15.5–E18.5). In Nedd4 mutant embryos, skeletal muscle fiber sizes and motoneuron numbers are significantly reduced. Surviving motoneurons project axons to their target muscles on schedule, but motor nerves defasciculate upon reaching the muscle surface, suggesting that Nedd4 plays a critical role in fine-tuning the interaction between the nerve and the muscle. Electrophysiological analyses of the neuromuscular junction (NMJ) demonstrate an increased spontaneous miniature endplate potential (mEPP) frequency in Nedd4 mutants. However, the mutant neuromuscular synapses are less responsive to membrane depolarization, compared to the wildtypes. Ultrastructural analyses further reveal that the pre-synaptic nerve terminal branches at the NMJs of Nedd4 mutants are increased in number, but decreased in diameter compared to the wildtypes. These ultrastructural changes are consistent with functional alternation of the NMJs in Nedd4 mutants. Unexpectedly, Nedd4 is not expressed in motoneurons, but is highly expressed in skeletal muscles and Schwann cells. Together, these results demonstrate that Nedd4 is involved in regulating the formation and function of the NMJs through non-cell autonomous mechanisms. [Copyright &y& Elsevier]

Abstract: MicroRNAs are small non-coding RNAs that regulate protein expression by binding 3′UTRs of target mRNAs, thereby inhibiting translation. Similar to siRNAs, miRNAs are cleaved by Dicer. Mouse and ES cell Dicer mutants demonstrate that microRNAs are necessary for embryonic development and cellular differentiation. However, technical obstacles and the relative infancy of this field have resulted in few data on the functional significance of individual microRNAs. We present evidence that miR-17 family members, miR-17-5p, miR-20a, miR-93, and miR-106a, are differentially expressed in developing mouse embryos and function to control differentiation of stem cells. Specifically, miR-93 localizes to differentiating primitive endoderm and trophectoderm of the blastocyst. We also observe high miR-93 and miR-17-5p expression within the mesoderm of gastrulating embryos. Using an ES cell model system, we demonstrate that modulation of these miRNAs delays or enhances differentiation into the germ layers. Additionally, we demonstrate that these miRNAs regulate STAT3 mRNA in vitro. We suggest that STAT3, a known ES cell regulator, is one target mRNA responsible for the effects of these miRNAs on cellular differentiation. [Copyright &y& Elsevier]

During mammalian development, it is essential that the proper epigenetic state is established across the entire genome in each differentiated cell. To date, little is known about the mechanism for establishing epigenetic modifications of individual genes during the course of cellular differentiation. Genome-wide DNA methylation analysis of embryonic stem cells by restriction landmark genomic scanning provides information about cell type- and tissue-specific DNA methylation profiles at tissue-specific methylated regions associated with developmental processes. It also sheds light on DNA methylation alterations following fetal exposure to chemical agents. In addition, analysis of embryonic stem cells deficient in epigenetic regulators will contribute to revealing the mechanism for establishing DNA methylation profiles and the interplay between DNA methylation and other epigenetic modifications. [ABSTRACT FROM AUTHOR]

Abstract: The field of endoplasmic reticulum (ER) stress in mammalian cells has expanded rapidly during the past decade, contributing to understanding of the molecular pathways that allow cells to adapt to perturbations in ER homeostasis. One major mechanism is mediated by molecular ER chaperones which are critical not only for quality control of proteins processed in the ER, but also for regulation of ER signaling in response to ER stress. Here, we summarized the properties and functions of GRP78/BiP, GRP94/gp96, GRP170/ORP150, GRP58/ERp57, PDI, ERp72, calnexin, calreticulin, EDEM, Herp and co-chaperones SIL1 and P58IPK and their role in development and diseases. Many of the new insights are derived from recently constructed mouse models where the genes encoding the chaperones are genetically altered, providing invaluable tools for examining the physiological involvement of the ER chaperones in vivo. [Copyright &y& Elsevier]

Known vertebrate GATA proteins contain two zinc fingers and are required in development, whereas invertebrates express a class of essential proteins containing one GATA‐type zinc finger. We isolated the gene encoding TRPS1, a vertebrate protein with a single GATA‐type zinc finger. TRPS1 is highly conserved between Xenopus and mammals, and the human gene is implicated in dominantly inherited tricho‐rhino‐phalangeal (TRP) syndromes. TRPS1 is a nuclear protein that binds GATA sequences but fails to transactivate a GATA‐dependent reporter. Instead, TRPS1 potently and specifically represses transcriptional activation mediated by other GATA factors. Repression does not occur from competition for DNA binding and depends on a C‐terminal region related to repressive domains found in Ikaros proteins. During mouse development, TRPS1 expression is prominent in sites showing pathology in TRP syndromes, which are thought to result from TRPS1 haploinsufficiency. We show instead that truncating mutations identified in patients encode dominant inhibitors of wild‐type TRPS1 function, suggesting an alternative mechanism for the disease. TRPS1 is the first example of a GATA protein with intrinsic transcriptional repression activity and possibly a negative regulator of GATA‐dependent processes in vertebrate development. [ABSTRACT FROM AUTHOR]

Most vertebrate organs begin their initial formation by a common, developmentally conserved pattern of inductive tissue interactions between two tissues. The developing tooth germ is a prototype for such inductive tissue interactions and provides a powerful experimental system for elucidation of the genetic pathways involved in organogenesis. Members of the Msx homeobox gene family are expressed at sites of epithelial-mesenchymal interaction during embryogenesis, including the tooth. The important role that Msx genes play in tooth development is exemplified by mice lacking Msx gene function. Msx1 - deficient mice exhibit an arrest in tooth development at the bud stage, while Msx2-deficient mice exhibit late defects in tooth development. The co-expression of Msx, Bmp, Lef1, and Activin βA genes and the coincidence of tooth phenotypes in the various knockout mice suggest that these genes reside within a common genetic pathway. Results summarized here indicate that Msx1 is required for the transmission of Bmp4 expression from dental epithelium to mesenchyme and also for Lef1 expression. In addition, we consider the role of other signaling molecules in the epithelial-mesenchymal interactions leading to tooth formation, the role that transcription factors such as Msx play in the propagation of inductive signals, and the role of extracellular matrix. Last, as a unifying mechanism to explain the disparate tooth phenotypes in Msx1- and Msx2-deficient mice, we propose that later steps in tooth morphogenesis molecularly resemble those in early tooth development. [ABSTRACT FROM AUTHOR]

Post-implantation embryo development commences with a bilaminar disc in most mammals, including humans. Whereas access to early human embryos is limited and subject to greater ethical scrutiny, studies on non-primate embryos developing as bilaminar discs offer exceptional opportunities for advances in gastrulation, the germline, and the basis for evolutionary divergence applicable to human development. Here, we discuss the advantages of investigations in the pig embryo as an exemplar of development of a bilaminar disc embryo with relevance to early human development. Besides, the pig has the potential for the creation of humanized organs for xenotransplantation. Precise genetic engineering approaches, imaging, and single-cell analysis are cost effective and efficient, enabling research into some outstanding questions on human development and for developing authentic models of early human development with stem cells., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

During the first days of mammalian development, the embryo forms the blastocyst, the structure responsible for implanting the mammalian embryo. Consisting of an epithelium enveloping the pluripotent inner cell mass and a fluid-filled lumen, the blastocyst results from a series of cleavage divisions, morphogenetic movements, and lineage specification. Recent studies have identified the essential role of actomyosin contractility in driving cytokinesis, morphogenesis, and fate specification, leading to the formation of the blastocyst. However, the preimplantation development of contractility mutants has not been characterized. Here, we generated single and double maternal-zygotic mutants of non-muscle myosin II heavy chains (NMHCs) to characterize them with multiscale imaging. We found that Myh9 (NMHC II-A) is the major NMHC during preimplantation development as its maternal-zygotic loss causes failed cytokinesis, increased duration of the cell cycle, weaker embryo compaction, and reduced differentiation, whereas Myh10 (NMHC II-B) maternal-zygotic loss is much less severe. Double maternal-zygotic mutants for Myh9 and Myh10 show a much stronger phenotype, failing most of the attempts of cytokinesis. We found that morphogenesis and fate specification are affected but nevertheless carry on in a timely fashion, regardless of the impact of the mutations on cell number. Strikingly, even when all cell divisions fail, the resulting single-celled embryo can initiate trophectoderm differentiation and lumen formation by accumulating fluid in increasingly large vacuoles. Therefore, contractility mutants reveal that fluid accumulation is a cell-autonomous process and that the preimplantation program carries on independently of successful cell division., Competing Interests: MS is employed by Carl Zeiss SAS via a public PhD programme Conventions Industrielles de Formation par la Recherche (CIFRE) co-funded by the Association Nationale de la Recherche et de la Technologie (ANRT). AT, ÖÖ, Ld, OP, DP, JM No competing interests declared, (© 2021, Schliffka et al.)

Vertebrate genomes are marked by notably high levels of 5-cytosine DNA methylation (5meC). The clearest function of DNA methylation among members of the subphylum is repression of potentially deleterious transposable elements (TEs). However, enrichment in the bodies of protein coding genes and pericentromeric heterochromatin indicate an important role for 5meC in those genomic compartments as well. Moreover, DNA methylation plays an important role in silencing of germline-specific genes. Impaired function of major components of DNA methylation machinery results in lethality in fish, amphibians and mammals. Despite such apparent importance, mammals exhibit a dramatic loss and regain of DNA methylation in early embryogenesis prior to implantation, and then again in the cells specified for the germline. In this minireview we will highlight recent studies that shine light on two major aspects of embryonic DNA methylation reprogramming: (1) The mechanism of DNA methylation loss after fertilization and (2) the protection of discrete loci from ectopic DNA methylation deposition during reestablishment. Finally, we will conclude with some extrapolations for the evolutionary underpinnings of such extraordinary events that seemingly put the genome under unnecessary risk during a particularly vulnerable window of development., Competing Interests: The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Greenberg.)

Upon fertilization, terminally differentiated gametes are transformed to a totipotent zygote, which gives rise to an embryo. How parental epigenetic memories are inherited and reprogrammed to accommodate parental-to-zygotic transition remains a fundamental question in developmental biology, epigenetics, and stem cell biology. With the rapid advancement of ultra-sensitive or single-cell epigenome analysis methods, unusual principles of epigenetic reprogramming begin to be unveiled. Emerging data reveal that in many species, the parental epigenome undergoes dramatic reprogramming followed by subsequent re-establishment of the embryo epigenome, leading to epigenetic "rebooting." Here, we discuss recent progress in understanding epigenetic reprogramming and their functions during mammalian early development. We also highlight the conserved and species-specific principles underlying diverse regulation of the epigenome in early embryos during evolution., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Organismal development is defined by progressive transformations that ultimately give rise to distinct tissues and organs. Thus, temporal shifts in ontogeny often reflect key phenotypic differences in phylogeny. Classical theory predicts that interspecific morphological divergence originates towards the end of embryonic or fetal life stages, i.e. the early conservation model. By contrast, the hourglass model predicts interspecific variation early and late in prenatal ontogeny, though with a phylogenetically similar mid-developmental period. This phylotypic period, however, remains challenging to define within large clades such as mammals. Thus, molecular and morphological tests on a mammalian hourglass have not been entirely congruent. Here, we report an hourglass-like pattern for mammalian developmental evolution. By comparing published data on the timing of 74 homologous characters across 51 placental species, we demonstrated that variation in the timing of development decreased late in embryogenesis--when organ formation is highly active. Evolutionary rates of characters related to this timeframe were lowest, coinciding with a phylotypic period that persisted well beyond the pharyngula 'stage'. The trajectory culminated with elevated variation in a handful of fetal and perinatal characters, yielding an irregular hourglass pattern. Our study invites further quantification of ontogeny across diverse amniotes and thus challenges current ideas on the universality of developmental patterns.