A major obstacel to the study of mammalian development, and to the practical application of knowledge gained from it in the clinic during therapeutic in vitro fertilisation and embryo transfer (IVF‐ET), is the propensity of embryos to become retarded or arrested during their culture in vitro. The precise developmental cell cycle in which embryos arrest or delay is characteristic for the species and coincides with the earliest period of embryonic gene expression. Much evidence reviewed here implicates free oxygen radicals (...) (FORs) in the process of arrest. Thus, studies on the development of mouse preimplantation embryos in vitro have shown that (i) FORs are elevated in vitro, but not in vivo, at the time at which embryos become arrested or delayed, (ii) systems for removing reactive oxygen species to limit the formation of hydroxy radicals are present, although they have not yet been assessed quantitatively and may differ qualitatively from those in adult cells, (iii) metabolic and possibly genetic adaptations to oxidative damage are evident, (iv) published procedures for overcoming in vitro arrest are explicable in terms of FOR‐mediated damage or responses and (v) the arrest or delay of most embryos in vitro can be reduced or prevented experimentally by addition of metal chelators to limit hydroxy radical formation and lipid hydroperoxidation. (shrink)

Normal fetal development is dependent upon an intricate exchange between mother and embryo. Several maternal and embryonic elements can influence this intimate interaction, including genetic, environmental or epigenetic factors, and have a significant impact on embryo development. The interaction of the genetic program of both mother and embryo, within the uterine environment, can shape the development of an individual. Accumulating data from animal models indicate that prenatal events may well initiate long‐term changes in the expression of the embryo genetic program, (...) which persist, or may only become apparent, much later in the individual's life. Also, environmental conditions during prenatal development may prompt the adoption of different developmental pathways, leading to alternative life histories. This review focuses on environmental factors, specifically maternally derived molecules, to illustrate how they can influence in utero embryonic development and, by extension, adult life. (shrink)

The mammalian blastocyst exhibits a high capacity for aerobic glycolysis, a metabolic characteristic of tumours. It has been considered that aerobic glycolysis is a means to ensure a high carbon flux to fulfil biosynthetic demands. Here, alternative explanations for this pattern of metabolism are considered. Lactate creates a microenvironment of low pH around the embryo to assist the disaggregation of uterine tissues to facilitate trophoblast invasion. Further it is proposed that lactate acts as a signalling molecule (especially at the (...) reduced oxygen tension present at implantation) to elicit bioactive VEGF recruitment from uterine cells, to promote angiogenesis. Finally it is suggested that the region of high lactate/low pH created by the blastocyst modulates the activity of the local immune response, helping to create immune tolerance. Consequently, the mammalian blastocyst offers a model to study the role of microenvironments, and how metabolites and pH are used in signalling. (shrink)

Animal models play a crucial role in fundamental and medical research. Progress in the fields of drug discovery, regenerative medicine and cancer research among others are heavily dependent on in vivo models to validate in vitro observations, and develop new therapeutic approaches. However, conventional rodent and large animal experiments face ethical, practical and technical issues that limit their usage. The chick embryo represents an accessible and economical in vivo model, which has long been used in developmental biology, gene expression analysis (...) and loss/gain of function experiments. It is also an established model for tissue/cell transplantation, and because of its lack of immune system in early development, the chick embryo is increasingly recognised as a model of choice for mammalian biology with new applications for stem cell and cancer research. Here, we review novel applications of the chick embryo model, and discuss future developments of this in vivo model for biomedical research. (shrink)

Here we review and discuss the link between regeneration capacity and tumor suppression comparing mammals (embryos versus adults) with highly regenerative vertebrates. Similar to mammal embryo morphogenesis, in amphibians (essentially newts and salamanders) the reparative process relies on a precise molecular and cellular machinery capable of sensing abnormal signals and actively reprograming or eliminating them. As the embryo's evil twin, tumor also retains common functional attributes. The immune system plays a pivotal role in maintaining a physiological balance to provide surveillance (...) against tumor initiation or to support its initiation and progression. We speculate that susceptibility to cancer development in adult mammals may be determined by the loss of an advanced regenerative capability during evolution and believe that gaining mechanistic insights into how regenerative capacity linked to tumor suppression is postnatally lost in mammals might illuminate an as yet unrecognized route to cancer treatment. (shrink)

Pregnancy has been traditionally defined as the period during which developing embryos are incubated in the body after egg–sperm union. Despite strong similarities between viviparity in mammals and other vertebrate groups, researchers have historically been reluctant to use the term pregnancy for non‐mammals in recognition of the highly developed form of viviparity in eutherians. Syngnathid fishes (seahorses and pipefishes) have a unique reproductive system, where the male incubates developing embryos in a specialized brooding structure in which they are aerated, osmoregulated, (...) protected and likely provisioned during their development. Recent insights into physiological, morphological and genetic changes associated with syngnathid reproduction provide compelling evidence that male incubation in these species is a highly specialized form of reproduction akin to other forms of viviparity. Here, we review these recent advances, highlighting similarities and differences between seahorse and mammalian pregnancy. Understanding the changes associated with the parallel evolution of male pregnancy in the two major syngnathid lineages will help to identify key innovations that facilitated the development of this unique form of reproduction and, through comparison with other forms of live bearing, may allow the identification of a common set of characteristics shared by all viviparous organisms. BioEssays 29:884–896, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

Thomas Henry Huxley designated three men as the finest intellects of 19th century natural history: his dear friend Charles Darwin; his most worthy opponent Georges Cuvier; and Karl Ernst von Baer, who discovered the mammalian egg cell in 1827 and wrote the founding treatise of modern embryology in 1828. Of these three, posterity has largely forgotten von Baer, who suffered a severe mental breakdown in the 1830's, but then recovered and moved to Russia (not uncommon for a German-speaking Estonian (...) national), where he enjoyed a distinguished second university career, largely in anthropology and lasting well into the 1870's. (shrink)

Many principles of eukaryotic DNA replication and its relationship to transcription have been revealed by studying the replication of animal virus chromosomes. Now microinjection of viral DNA into eggs and embryos is providing clues about regulation of chromosomal replication and transcription during early mammalian development.

Fezf1 and Fezf2 are highly conserved transcription factors that were first identified by their specific expression in the anterior neuroepithelium of Xenopus and zebrafish embryos. These proteins share an N‐terminal domain with homology to the canonical engrailed repressor motif and a C‐terminal DNA binding domain containing six C2H2 zinc‐finger repeats. Over a decade of study indicates that the Fez proteins play critical roles during nervous system development in species as diverse as fruit flies and mice. Herein we discuss recent progress (...) in understanding the functions of Fezf1 and Fezf2 in neurogenesis and cell fate specification during mammalian nervous system development. Going forward we believe that efforts should focus on understanding how expression of these factors is precisely regulated, and on identifying target DNA sequences and interacting partners. Such knowledge may reveal the mechanisms by which Fezf1 and Fezf2 accomplish both independent and redundant functions across diverse tissue and cell types. (shrink)

We review here the data indicating a role for epidermal growth factor receptor (EGF receptor) signalling in early mouse development. Embryonic development of the metazoan embryo generally begins with the formation of a cystic structure and epithelial layers that subsequently form anlagen of the definitive body parts and organs. For the mammalian embryo, this cystic structure is a blastocyst whose wall consists of trophectoderm, the first epithelium to develop during mammalian embryogenesis. The onset of expression and function of (...) EGF receptors is coincident with the onset of trophectoderm development. Modulating EGF receptor expression and function modulates trophectoderm differentiation, leading to the hypothesis that functional EGF receptors participate in the induction of trophectoderm development and perhaps of other embryonic epithelial derivatives such as nervous tissues. (shrink)

Intracellular pH (pHi) regulation is a homeostatic function of all cells. Additionally, the plasma membrane‐based transporters controlling pHi are involved in growth factor activation, cell proliferation and salt transport – all processes active in early embryos. pHi regulation in the early embryos of many species exhibits unique features: in mouse preimplantation embryos, mechanisms for correcting excess acid apparently are inactive, while excess base is removed by the mechanism common in differentiated cells. Additionally, unlike differentiated cells, mouse preimplantation embryos are highly (...) permeable to H+ until the blastocyst stage, where the epithelial cells surrounding the embryo are impermeable. In several non‐mammalian species, of which the best‐studied is sea urchin, cytoplasmic alkalinization at fertilization is necessary for development of the embryo, and elevated pHi must be maintained during early development. Thus, pHi regulatory mechanisms appear to be important for early embryo development in many species. (shrink)

Recent studies of early development in a number of ivertebrate and vertebrate species have suggested that growth factors and their receptors may play important roles in differentiation as well as cell proliferation. In the mouse embryo, the expression of the receptors for insulin and insulin‐like growth factors I and II (IGF‐I and ‐II) are temporally regulated. The ontogeny of receptor and ligand expression within the insulin and IGF gene family suggests that the very earliest stages of mammalian embryogenesis may (...) be subject to regulation by autocrine and paracrine factors from maternal and embryonic sources. (shrink)

The development of the human embryo from the time of fertilization through the eighth post-fertilization week is described for medical policy purposes. During pre-implantation stages, differentiation occurs between precursors of embryonic and extra-embryonic structures. During implantation formation of a fore-hind axis begins within the inner cell mass. By the end of the eighth week, head, face, hands, and feet are suggestive as to species-recognition but not yet definitive. Data from laboratory studies of non-human mammalian embryos elucidate important aspects of (...) human embryonic development. Keywords: embryos, development, differentiation, genome CiteULike Connotea Del.icio.us What's this? (shrink)

This review uses nutritional markers of normal and impaired development to address the question; what makes a viable mammalian preimplantation embryo? Resolution of this question is important to ensure the long‐term safety of embryo‐based biotechnologies in man and domestic animals, the optimisation of embryo production and culture conditions and the development of methods to select viable embryos for replacement. After considering the nutrition of embryos and somatic cells, and the phenomenon of caloric restriction, it is concluded that preimplantation embryo (...) survival is best served by a relatively low level of metabolism; a situation achieved by reducing the concentrations of nutrients in culture media and encouraging the use endogenous resources. BioEssays 24:845–849, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

Certain interventions now permit the derivation of mammalian gametes from stem cells cultivated from either somatic cells or embryos. These gametes can be used in an indefinite cycle of conception in vitro, gamete derivation, conception in vitro, and so on, producing genetic generations that live only in vitro. One commentator has described this prospect for human beings as eugenics, insofar as it would allow for the selection and development of certain traits in human beings. This commentary not only offers (...) this topic for discussion, it also wades into the ethical fray over the practice. Several possible lines of objection can be raised against this practice, but these accounts are by and large insufficient as an ethical analysis of this possible, future way of conceiving human children. (shrink)

The mode and timing of germ-cell specification has been studied in diverse organisms, however, the molecular mechanism regulating germ-cell-fate determination remains to be elucidated. In some model organisms, maternal germ-cell determinants play a key role. In mouse embryos, some germ-line-specific gene products exist as maternal molecules and play critical roles in a pluripotential cell population at preimplantation stages. From those cells, primordial germ cells (PGCs) are specified by extracellular signaling mediated by tissue, as well as cell–cell interaction during gastrulation. Thus, (...) establishment of germ-cell lineage in mammalian embryos appears to be regulated by a multistep process, including formation and maintenance of a pluripotential cell population, as well as specification of PGCs. PGCs can be generated from pluripotential embryonic stem (ES) cells in a simple monolayer culture in which tissue interaction does not occur. This raises the possibility that ES cells, as well as, possibly, pluripotential cells in preimplantation embryos, are more closely related to the PGC precursors than pluripotential cells after implantation. BioEssays 27:136–143, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

Modeling metastasis in animal systems has been an important focus for developing cancer therapeutics. Xenopus laevis is a well‐established model, known for its use in identifying genetic mechanisms underlying diseases and disorders in humans. Prior literature has suggested that the drug, ivermectin, can be used in Xenopus to induce melanocytes to convert into a metastatic melanoma‐like state, and thus could be ideal for testing possible melanoma therapies in vivo. However, there are notable inconsistencies between ivermectin studies in Xenopus and the (...) application of ivermectin in mammalian systems, that are relevant to cancer and melanoma research. In this review, we examine the ivermectin‐induced phenotypes in Xenopus, and we explore the current uses of ivermectin in human research. We conclude that while ivermectin may be a useful drug for many biomedical purposes, it is not ideal to induce a metastatic melanocyte phenotype in Xenopus for testing the effects of potential melanoma therapeutics. (shrink)

We have discovered that a single sperm protein, phospholipase C‐zeta (PLCζ), can stimulate intracellular Ca2+ signalling in the unfertilized oocyte (‘egg’) culminating in the initiation of embryonic development. Upon fertilization by a spermatozoon, the earliest observed signalling event in the dormant egg is a large, transient increase in free Ca2+ concentration. The fertilized egg responds to the intracellular Ca2+ rise by completing meiosis. In mammalian eggs, the Ca2+ signal is delivered as a train of long‐lasting cytoplasmic Ca2+ oscillations that (...) begin soon after gamete fusion and persist beyond the completion of meiosis. Sperm PLCζ effects Ca2+ release from egg intracellular stores by hydrolyzing the membrane lipid PIP2 and consequent stimulation of the inositol 1,4,5‐trisphosphate (InsP3) receptor Ca2+‐signalling pathway, leading to egg activation and early embryogenesis. Recent advances have refined our understanding of how PLCζ induces Ca2+ oscillations in the egg and also suggest its potential dysfunction as a cause of male infertility. (shrink)

Bioethical debates such as those surrounding the manipulation of human embryos are often based on metaphysical assumptions that lack a foundation in the natural sciences. In this paper we support a gradualist position whereby the embryo progressively takes on the form and associated ethical significance of a human being. We support this position by introducing a concept of biological structure or form to show how the gradualist position has its metaphysical foundations in modern biology. The conceptual basis for form and (...) structure are outlined and their compatibility with and basis in current empirical biology is demonstrated by some recent advances in our understanding of the processes of development from single cell to organism. We then briefly explore the ethical significance of accepting a form or structure based conception of biology for the status of the early embryo. (shrink)

The mammalian egg employs a wide spectrum of epigenome modification machinery to reprogram the sperm nucleus shortly after fertilization. This event is required for transcriptional activation of the paternal/zygotic genome and progression through cleavage divisions. Reprogramming of paternal nuclei requires replacement of sperm protamines with canonical and non‐canonical histones, covalent modification of histone tails, and chemical modification of DNA (notably oxidative demethylation of methylated cytosines). In this essay we highlight the role maternal histone variants play during developmental reprogramming following (...) fertilization. We discuss how reduced maternal histone variant incorporation in somatic nuclear transfer experiments may explain the reduced viability of resulting embryos and how knowledge of repressive and activating maternal factors may be used to improve somatic cell reprogramming. (shrink)

Is the mammalian embryo/fetus a part of the organism that gestates it? According to the containment view, the fetus is not a part of, but merely contained within or surrounded by, the gestating organism. According to the parthood view, the fetus is a part of the gestating organism. This paper proceeds in two stages. First, I argue that the containment view is the received view; that it is generally assumed without good reason; and that it needs substantial support if (...) it is to be taken seriously. Second, I argue that the parthood view derives considerable support from a range of biological and physiological considerations. I tentatively conclude in favour of the parthood view, and end by identifying some of the interesting questions it raises. (shrink)

Elselijn Kingma argues that, in cases of mammalian placental pregnancy, the foster (roughly, the post-implantation embryo/foetus) is part of the gravida (the pregnant organism). But she does not consider the possibility of proper overlap. I show that this generates a number of serious problems for her argument and trace the oversight to a quite general issue within the literature on biological individuality. Doing so provides an opportunity to pull apart and clarify the relations between some importantly distinct questions concerning (...) organismality and organismic parthood, and to identify the implications that this has for those who want to draw on this literature when making metaphysical or mereological arguments. (shrink)

Recent molecular studies of mammalian sexual determination have been focused on gene expression in the gonadal ridge at the time of appearance of sexual dimorphism: the critical time defined by the ‘Jost principle’. Three lines of evidence suggest that, instead, sex determination may start shortly after conception: (1) the XY preimplantation embryo usually develops more rapidly than the XX preimplantation embryo (this phenotype has been linked to the Y chromosome and will be termed ‘Growth factor Y’); (2) the gene (...) for testis determination, SRY/Sry, and the closely linked genes ZFY/Zfy and Smcy, are transcribed in the preimplantation embryo; and (3) male and female preimplantation embryos are antigenically distinguishable, indicating sex differences in gene expression. The data to support these assertions are reviewed. Possible relationships of these three phenomena to each other and to sex differentiation are discussed. Similarities in mechanisms of sexual determination between marsupial and eutherian mammals are hypothesized. Problems with interpreting male sexual differentiation as being solely due to testosterone and Müllerian inhibiting substance (MIS) are discussed. (shrink)

Introduction A Brief Conceptual Framework for Biology PART ONE: UNDERSTANDING NATURE 1. The Antecedents of Scientific Thought Animism, Totemism, and Shamanism The Paleolithic View Mesopotamia Egypt 2. Aristotle and the Greek View of Nature The Science of Animal Biology The Parts of Animals The Classification of Animals The Aristotelian System Basic Questions 3. Those Rational Greeks? Theophrastus and the Science of Botany The Roman Pliny Hippocrates, the Father of Medicine Erasistratus Galen of Pergamum The Greek Miracle 4. The Judeo-Christian Worldview (...) The Bishop of Hippo Scholastic Thought Islamic Science Books on Beasts Antecedents of a Revolution 5. The Revival of Science Andreas Vesalius and the Study of Structure William Harvey and the Study of Function Sir Francis Bacon’s Great Instauration Induction, Hypothesis, Deduction The Very Small--Animalcules Robert Hooke and the Discovery of Cells 6. Figur’d Stones and Plastick Virtue Marine Life on Mountain Tops? Figured Stones of Unknown Creatures Baron Cuvier Quarries of the Paris Basin Catastrophism and Uniformitarianism William Smith and the Geological Column Understanding Nature in 1850 PART TWO: THE GROWTH OF EVOLUTIONARY THOUGHT 7. The Paradigm of Evolution First Questions The Paradigm of Natural Theology First Answers 8. Testing Darwins Hypotheses Have Life Forms Changed over Time? Do Species Evolve into Different Species over Time? Has There Been Time Enough for Evolution? Is Natural Selection the Mechanism of Change? The Genetic Basis of Natural Selection Accounting for the Diversity of Life 9. In the Light of Evolution Comparative Anatomy Embryonic Development Classification Microstructure Molecular Processes 10. Life over Time The Origin of Life The Rise of Multicelled Organisms What Is a Phylum? Burgess Shale Metazoans Early Evolution of the Vertebrates The Age of Dinosaurs Birds, Mammals, and Flowering Plants The Rancho La Brea Tar Pits Human Evolution The Role of Extinction in Evolution PART THREE: CLASSICAL GENETICS 11. Pangenesis What Is the Question? Hippocrates and Aristotle The Darwinian Answer Assembling the Data Formulating the Hypothesis by Induction Galton’s Rabbits 12. The Cell Theory The Discovery of Cells: Robert Hooke Schwann and Cells in Animals Gametes as Cells Omnis cellula e cellula? The Technology of Cell Research 13. The Hypothesis of Chromosomal Continuity The Ephemeral Nucleus Schneider, Flemming, and Cell Division The Chromosomes and inheritance Gamete Formation Fertilization 14. Mendel and the Birth of Genetics Model for Monohybrid Crosses Model for Dihybrid Crosses Mendel’s Laws Initial Opposition to Mendelism 15. Genetics + Cytology: 1900-1910 Sutton’s Model The Cytological Basis of Mendel’s Laws Boveri and Abnormal Chromosome Sets Variations in Mendelian Ratios The Discovery of Sex Chromosomes 16. The Genetics of the Fruit Fly Morgan’s First Hypothesis Morgan’s Second Hypothesis The Fly Room Linkage and Crossing-Over The Cytological Proof of Crossing-Over Mapping the Chromosomes The Final Proof The Determinants of Sex The Conceptual Foundations of Classical Genetics 17. The Structure and Function of Genes One Gene, One Enzyme The Substance of Inheritance The Watson-Crick Model of DNA Genes and the Synthesis of Proteins The Genetic Code PART FOUR: THE ENIGMA OF DEVELOPMENT 18. First Principles The Peripatetic Stagirite The Death and Rebirth of Scientific Thought Harvey and Malpighi A Two-Millennial Summing Up Preformation versus Epigenesis 19. The Century of Discovery Von Baer’s Discovery of the Mammalian Ovum Darwin’s Contribution to Embryology Haeckel and Recapitulation 20. Descriptive Embryology Germ Layers The External Development of the Amphibian Embryo The Internal Development of the Amphibian Embryo 21. The Dawn of Analytical Embryology His, Roux, and Mosaic Development Driesch and Regulative Development Novelty in Development Cell Lineage Nucleus or Cytoplasm? Fin de Siècle 22. Interactions during Development Amphibian Organizers Secondary Organizers The Reacting Tissue The Chemical Nature of the Organizer Putting It All Together Conclusion Further Reading References Illustration Credits Index. (shrink)

What is the metaphysical relationship between the fetus/embryo and the pregnant organism? In this paper I apply a substance metaphysics view developed by Barry Smith and Berit Brogaard to argue, on the basis of topological connectedness, that fetuses/embryos are Lady-Parts: part of the maternal organism up until birth. This leaves two options. Either mammalian organisms begin at birth, or we revise our conception of organisms such that mammalian organisms can be part of other mammals. The first option has (...) some advantages: it is numerically neat; aligns with an intuitive picture of organisms as physically distinct individuals; and ties ‘coming into existence’ to a suitably recognisable and important event: birth. But it denies that the fetus survives birth, or that human organisms existed prior to their birth. The second option allows us to recognise that human organisms exist prior to and survive their birth, but at a cost: it leaves the question of when an organism comes into existence unanswered, and demands potentially far-reaching conceptual revision across a range of domains. (shrink)

We report on the deliberations of an interdisciplinary group of experts in science, law, and philosophy who convened to discuss novel ethical and policy challenges in stem cell research. In this report we discuss the ethical and policy implications of safety concerns in the transition from basic laboratory research to clinical applications of cell-based therapies derived from stem cells. Although many features of this transition from lab to clinic are common to other therapies, three aspects of stem cell biology pose (...) unique challenges. First, tension regarding the use of human embryos may complicate the scientific development of safe and effective cell lines. Second, because human stem cells were not developed in the laboratory until 1998, few safety questions relating to human applications have been addressed in animal research. Third, preclinical and clinical testing of biologic agents, particularly those as inherently complex as mammalian cells, present formidable challenges, such as the need to develop suitable standardized assays and the difficulty of selecting appropriate patient populations for early phase trials. We recommend that scientists, policy makers, and the public discuss these issues responsibly, and further, that a national advisory committee to oversee human trials of cell therapies be established. **NB we did not reccommend a NAC, we think it might be appropriate**. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Who or What is the Preembryo?S.J. Richard A. McCormick (bio)IntroductionAlthough widely used by scientists, the term "preembryo" has raised some suspicions. Histopathologist Michael Jarmulowicz (1990), for example, asserts that the term was adopted by the American Fertility Society (AFS) and the Voluntary Licensing Authority (VLA) in Britain "as an exercise of linguistic engineering to make human embryo research more palatable to the general public."I cannot speak for the VLA, (...) but I can clarify that this was not the motive of the AFS. As a member of the AFS ethics committee that discussed and adopted this terminology, I know that our discussion was not tied to embryo research or moral issues. The term "preembryo" was adopted because the earliest stages of mammalian development primarily involve establishment of the nonembryonic trophoblast, rather than the formation of the embryo. As Clifford Grobstein (1988, p. 61), a distinguished basic scientist who also sat on the AFS committee notes:The scientific rationale for the term preembryo... is its greater accuracy in characterizing the initial phase of mammalian and human development.... The status implications of such a change in terminology were not at issue in the discussion.This is not to say that the moral status of the preembryo is not a concern worthy of independent consideration. It is an important question with significant practical implications, especially in the context of in vitro fertilization. At stake are issues such as: (1) what may be done to preembryos before transfer; (2) whether all preembryos must be transferred; and (3) what may be done to preembryos that are not transferred (Ethics Committee of the American Fertility Society (AFS) 1986, p. 29S). Furthermore, emergency treatment of rape cases may also depend on such a determination. [End Page 1]Cryopreservation, which entails freezing unused preembryos for possible later use, is one example of a procedure that raises issues associated with the moral status of the preembryo. Cryopreservation decreases the number of stimulated egg recovery cycles a woman must undergo and increases flexibility in transfer protocols. But thawing of the preembryos is not a completely benign process; significant numbers of the preembryos die in the process and injury cannot be excluded.In addition, down the road a year or so lies preimplantation genetic diagnosis, now being tested in several centers of reproductive research. This would involve the removal of one or more cells from a preembryo developing in vitro. The DNA from these cells would be studied and then only genetically normal preembryos would be transferred. With the growth in knowledge about single gene defects and the rapid advance of amplification technology, which allows gene visualization, it is not difficult to imagine scientists embarking on a variety of micromanipulations of the preembryo for the sake of gaining knowledge.Not all research with gametes involves preembryos, but clearly some experiments and procedures await resolution of the moral status of the preembryo. I contend in this paper that the moral status—and specifically the controversial issue of personhood—is related to attainment of developmental individuality (being the source of one individual). This contrasts with the view that holds that personhood occurs earlier, at the point of genetic uniqueness. I believe that an embryo that has developed to the point where it can be one individual and one individual only, differs in moral status from a preembryo that has not, even if in many cases we may choose to treat them similarly.The Scientific UnderpinningsTwo things must be kept in mind—and in a sense, in tension—in turning to science for help in assessing the status of the preembryo. First, science is not competent to decide the question of personhood or ensoulment. As the Catholic document, Declaration on Procured Abortion, which I will return to later, notes: "It is a philosophical problem" (Congregation for the Doctrine of the Faith (CDF) 1974, p. 10, endnote 19). On the other hand, another Catholic document, Donum vitae(CDF 1987), notes that the conclusions of science provide a "valuable indication" for reason's deliberations about personhood. We might say that if science cannot decide the question of personhood, neither can it be decided without science.For my review of embryological science... (shrink)

In birds and frogs, species pairs retain the capacity to produce viable hybrids for tens of millions of years, an order of magnitude longer than mammals. What accounts for these differences in relative rates of pre- and postzygotic isolation? We propose that reproductive mode is a critically important but previously overlooked factor in the speciation process. Viviparity creates a post-fertilization arena for genomic conflicts absent in egg-laying species. With viviparity, conflict can arise between: mothers and embryos; sibling embryos in the (...) womb, and maternal and paternal genomes within individual embryos. Such intra- and intergenomic conflicts result in perpetual antagonistic coevolution, thereby accelerating interpopulation postzygotic isolation. In addition, by generating intrapopulation genetic incompatibility, viviparity-driven conflict favors polyandry and limits the potential for precopulatory divergence. Mammalian diversification is characterized by rapid evolution of incompatible feto-maternal interactions, asymmetrical postzygotic isolation, disproportionate effects of genomically-imprinted genes, and “F2 hybrid enhancement.” The viviparity-driven conflict hypothesis provides a parsimonious explanation for these patterns in mammalian evolution. BioEssays 22:938–946, 2000. © 2000 John Wiley & Sons, Inc. (shrink)

The germ track is the cellular path by which genes are transmitted to future generations whereas somatic cells die with their body and do not leave direct descendants. Transposable elements (TEs) evolve to be silent in somatic cells but active in the germ track. Thus, the performance of most bodily functions by a sequestered soma reduces organismal costs of TEs. Flexible forms of gene regulation are permissible in the soma because of the self‐imposed silence of TEs, but strict licensing of (...) transcription and translation is maintained in the germ track to control proliferation of TEs. Delayed zygotic genome activation (ZGA) and maternally inherited germ granules are adaptations that enhance germ‐track security. Mammalian embryos exhibit very early ZGA associated with extensive mobilization of retroelements. This window of vulnerability to retrotransposition in early embryos is an indirect consequence of evolutionary conflicts within the mammalian genome over postzygotic maternal provisioning. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Who or What is the Preembryo?S.J. Richard A. McCormick (bio)IntroductionAlthough widely used by scientists, the term "preembryo" has raised some suspicions. Histopathologist Michael Jarmulowicz (1990), for example, asserts that the term was adopted by the American Fertility Society (AFS) and the Voluntary Licensing Authority (VLA) in Britain "as an exercise of linguistic engineering to make human embryo research more palatable to the general public."I cannot speak for the VLA, (...) but I can clarify that this was not the motive of the AFS. As a member of the AFS ethics committee that discussed and adopted this terminology, I know that our discussion was not tied to embryo research or moral issues. The term "preembryo" was adopted because the earliest stages of mammalian development primarily involve establishment of the nonembryonic trophoblast, rather than the formation of the embryo. As Clifford Grobstein (1988, p. 61), a distinguished basic scientist who also sat on the AFS committee notes:The scientific rationale for the term preembryo... is its greater accuracy in characterizing the initial phase of mammalian and human development.... The status implications of such a change in terminology were not at issue in the discussion.This is not to say that the moral status of the preembryo is not a concern worthy of independent consideration. It is an important question with significant practical implications, especially in the context of in vitro fertilization. At stake are issues such as: (1) what may be done to preembryos before transfer; (2) whether all preembryos must be transferred; and (3) what may be done to preembryos that are not transferred (Ethics Committee of the American Fertility Society (AFS) 1986, p. 29S). Furthermore, emergency treatment of rape cases may also depend on such a determination. [End Page 1]Cryopreservation, which entails freezing unused preembryos for possible later use, is one example of a procedure that raises issues associated with the moral status of the preembryo. Cryopreservation decreases the number of stimulated egg recovery cycles a woman must undergo and increases flexibility in transfer protocols. But thawing of the preembryos is not a completely benign process; significant numbers of the preembryos die in the process and injury cannot be excluded.In addition, down the road a year or so lies preimplantation genetic diagnosis, now being tested in several centers of reproductive research. This would involve the removal of one or more cells from a preembryo developing in vitro. The DNA from these cells would be studied and then only genetically normal preembryos would be transferred. With the growth in knowledge about single gene defects and the rapid advance of amplification technology, which allows gene visualization, it is not difficult to imagine scientists embarking on a variety of micromanipulations of the preembryo for the sake of gaining knowledge.Not all research with gametes involves preembryos, but clearly some experiments and procedures await resolution of the moral status of the preembryo. I contend in this paper that the moral status—and specifically the controversial issue of personhood—is related to attainment of developmental individuality (being the source of one individual). This contrasts with the view that holds that personhood occurs earlier, at the point of genetic uniqueness. I believe that an embryo that has developed to the point where it can be one individual and one individual only, differs in moral status from a preembryo that has not, even if in many cases we may choose to treat them similarly.The Scientific UnderpinningsTwo things must be kept in mind—and in a sense, in tension—in turning to science for help in assessing the status of the preembryo. First, science is not competent to decide the question of personhood or ensoulment. As the Catholic document, Declaration on Procured Abortion, which I will return to later, notes: "It is a philosophical problem" (Congregation for the Doctrine of the Faith (CDF) 1974, p. 10, endnote 19). On the other hand, another Catholic document, Donum vitae(CDF 1987), notes that the conclusions of science provide a "valuable indication" for reason's deliberations about personhood. We might say that if science cannot decide the question of personhood, neither can it be decided without science.For my review of embryological science... (shrink)

The zona pellucida is an extracellular coat that surrounds mammalian eggs and early embryos. This insoluble matrix separates germ from somatic cells during folliculogenesis and plays critical roles during fertilization and early development. The mouse and human zona pellucida contain three glycoproteins (ZP1 or ZPB, ZP2, ZP3), the primary structures of which have been deduced by molecular cloning. Targeted mutagenesis of endogenous mouse genes and transgenesis with human homologues provide models to investigate the roles of individual zona components. Collectively, (...) the genetic data indicate that no single mouse zona pellucida protein is obligatory for taxon‐specific sperm binding and that two human proteins are not sufficient to support human sperm binding. An observed post‐fertilization persistence of mouse sperm binding to “humanized” zona pellucida correlates with uncleaved ZP2. These observations are consistent with a model for sperm binding in which the supramolecular structure of the zona pellucida necessary for sperm binding is modulated by the cleavage status of ZP2. BioEssays 26:29–38, 2004. Published 2003 Wiley Periodicals, Inc. (shrink)

X chromosome inactivation is the mammalian answer to the dilemma of dosage compensation between males and females. The study of this fascinating form of chromosome-wide gene regulation has yielded surprising insights into early development and cellular memory. In the past few months, three papers1-3 reported unexpected findings about the paternal X chromosome (Xp). All three studies agree that the Xp is imprinted to become inactive earlier than ever suspected during embryonic development. Although apparently incomplete, this early form of inactivation (...) insures dosage compensation throughout development. Silencing of the Xp persists in cells of extraembryonic tissues, but it is erased and followed by random X inactivation in cells of the embryo proper. These findings challenge several aspects of the current view of X inactivation during early development and may have profound impact on studies of pluripotency and epigenetics. BioEssays 26:821–824, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Kennedy Institute of Ethics Journal 12.3 (2002) 305-323 [Access article in PDF] Scope Note 41 Bioethics and Cloning, Part I Susan Cartier Poland and Laura Jane Bishop This is Part One of a two part Scope Note on Bioethics and Cloning. Part Two will be published in the December 2002 issue of the Kennedy Institute of Ethics Journal and as a separate reprint. Contents For Parts 1 And 2 (...) Part 1Part 2I.History of the ScienceI.Ethical PerspectivesII.Historical CommentaryII.Religious PerspectivesIII.Animal WelfareIII.Fiction and FilmIV.Laws and Legal Literature"Clone" is a collective noun (I, Stewart 1997, p. 771). A collective noun names a group and is composed of individual members, such as "herd" or "flock." Derived from the Greek kl¯on for twig or slip, "clone" was first used in biology to describe the aggregate of asexually produced progeny of an individual, either naturally occurring or man-made; the definition later expanded to mean a group of genetically identical members who originate from a single ancestor. Thus, a clone is a group that results from the process of asexual reproduction.In nature, cloning occurs either by parthenogenesis (reproduction without fertilization) or by fission (embryo splitting). In a pioneering experiment in 1952, Robert Briggs and Thomas J. King (I, 1952), cancer researchers at the Institute for Cancer Research (today part of the Fox Chase Cancer Center) in Philadelphia, demonstrated on frogs another method of cloning by transplanting nuclei into enucleated cells, a technique called "nuclear transplantation," which John B. Gurdon and colleagues (I, 1958, 1975) used to create clones of frogs.Then, in the late 1970s and early 1980s, two back-to-back frauds in cloning occurred. First, the cloning of a human described by American science journalist David Rorvik in the 1978 book In His Image was called a hoax by a court in [End Page 305] 1982. Second, the cloning of three mice, announced by German wunderkind Karl Illmensee to scientific circles in 1979 (and to the public in 1981), could not be proven to be true, according to convincing evidence by McGrath and Solter (I) published in 1984.With their pronouncement that mammalian cloning was impossible, McGrath and Solter essentially stopped cloning research and funding for such biological research. So by the mid-1980s, instead of further investigating cloning, developmental biologists studied embryology related to in vitro fertilization, while molecular biologists studied recombinant DNA under the newly relaxed NIH guidelines. These two strands of scientific investigation came together in the work of Ian Wilmut, who created in vitro embryos using nuclear transplantation to reproduce genetically engineered cattle. Wilmut was seeking a way to increase the successful birth rate when, in the 1990s, he produced identical twin sheep, Megan and Morag, cloned from an embryonic cell, followed by another sheep, Dolly, cloned from an adult cell. The birth of a mammal from an embryo whose genetic material came from an adult cell firmly re-established the possibility of cloning from adults. Prior to Dolly's birth, scientists believed that adult cells lost the ability to dedifferentiate—i.e., to become unspecialized—and so could not direct the development of an embryo through to birth.The 1997 announcement of Dolly's birth initiated the present widespread discussion of cloning. Because of its potentially vast applications across human health, medicine, agriculture, and business/economics, cloning raises a host of issues including those concerning: genetics and genetic engineering, patenting, animal rights and/or animal welfare (especially for transgenic, research, or cloned animals), organ transplantation, reproduction, eugenics, pharmacology (drug production using transgenic animals), research practices, informed consent, stem cell research, women's rights, children's rights, parental rights and responsibilities, and theology and philosophy. It fosters questions about the meaning and purpose of life, the limits of human powers to intervene in research and therapy, and what it means to be human or animal. In addition, environmentalists and organic farmers raise concerns about the effect of novel animals and plants on the environment, while nutritionists and the lay public wonder... (shrink)

The retrotransposon Long Interspersed Element 1 (LINE‐1 or L1) has played a major role in shaping the sequence composition of the mammalian genome. In our recent publication, “Heritable L1 retrotransposition in the mouse primordial germline and early embryo,” we systematically assessed the rate and developmental timing of de novo, heritable endogenous L1 insertions in mice. Such heritable retrotransposition events allow L1 to exert an ongoing influence upon genome evolution. Here, we place our findings in the context of earlier studies, (...) and highlight how our results corroborate, and depart from, previous research based on human patient samples and transgenic mouse models harboring engineered L1 reporter genes. In parallel, we outline outstanding questions regarding the stage‐specificity, regulation, and functional impact of embryonic and germline L1 retrotransposition, and propose avenues for future research in this field. -/- . (shrink)

Not all vertebrates share the familiar system of XX:XY sex determination seen in mammals. In the chicken and other birds, sex is determined by a ZZ:ZW sex chromosome system. Gonadal development in the chicken has provided insights into the molecular genetics of vertebrate sex determination and how it has evolved. Such comparative studies show that vertebrate sex‐determining pathways comprise both conserved and divergent elements. The chicken embryo resembles lower vertebrates in that estrogens play a central role in gonadal sex differentiation. (...) However, several genes shown to be critical for mammalian sex determination are also expressed in the chicken, but their expression patterns differ, indicating functional plasticity. While the genetic trigger for sex determination in birds remains unknown, some promising candidate genes have recently emerged. The Z‐linked gene, DMRT1, supports the Z‐dosage model of avian sex determination. Two novel W‐linked genes, ASW and FET1, represent candidate female determinants. BioEssays 26:120–132, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

Since the decade of the 1970s, and particularly since the first successful test-tube baby in 1978, the development and use of assisted reproductive technologies have grown exponentially. Would-be parents—including those in so-called traditional male-female marriages, unmarried adults, postmenopausal women, and same-sex partnerships—who just over 20 years ago had no recourse for their fertility issues can now pursue their desires to have children with at least a partial, if not, total, genetic and/or biological relationship. Ovulation-stimulating medications, artificial insemination using the sperm (...) of a husband or unrelated donor, in vitro fertilization with embryo transfer, intracytoplasmic sperm injection, and gamete and zygote intrafallopian transfers are but a few of a host of treatment options ranging in complexity, invasiveness, and expense. And on the horizon are genetic techniques such as cloning—which was once considered “pure” science fiction but in 1997 became what some call an inevitability, with the development of mammalian cloning in the form of the now-famous Dolly the sheep. (shrink)

Cells fine‐tune their DNA repair, selecting some regions of the genome in preference to others. In the paradigm case, excision of UV‐induced pyrimidine dimers in mammalian cells, repair is concentrated in transcribed genes, especially in the transcribed strand. This is due both to chromatin structure being looser in transcribing domains, allowing more rapid repair, and to repair enzymes being coupled to RNA polymerases stalled at damage sites; possibly other factors are also involved. Some repair‐defective diseases may involve repair‐transcription coupling: (...) three candidate genes have been suggested.However, preferential excision of pyrimidine dimers is not uniformly linked to transcription. In mammals it varies with species, and with cell differentiation. In Drosophila embryo cells it is absent, and in yeast, the determining factor is nucleosome stability rather than transcription.Repair of other damage departs further from the paradigm, even in some UV‐mimetic lesions. No selectivity is known for repair of the very frequent minor forms of base damage. And the most interesting consequence of selective repair, selective mutagenesis, normally occurs for UV‐induced, but not for spontaneous mutations. The temptation to extrapolate from mammalian UV repair should be resisted. (shrink)

Variation or rearrangement of regulatory genes is responsible for cellular malignant change. These types of chromosomal variations also produce heterochrony or paedomorphic evolution at the organismal level. Analogously, neoplasia represents a cellular macroevolutionary event, and a tumour can be said to be an evolved population of cells. To understand this cellular evolution to malignancy, it may be necessary to go beyond a clonal selection (adaptationist) explanation of neoplastic alteration. In the pericellular environment natural selection consists of the organizational restraints of (...) surrounding cells as well as the host's immunological surveillance and non-specific monocyte-macrophage systems. Indirect evidence suggests that success for the neoplasm depends not upon clonal selection, but solely upon a genetic methodology—the function of which is to elude selection.The author has coined the term cellular heterochrony to illustrate analogic similarities in the molecular modes of speciation between anaplastic cancer cells and the heterochronic evolution of organisms. By reverting to a juvenile (embryonic) repertoire of cellular behaviour a tumour secures its own tenure or niche by usurping the host's armamentarium of selection forces, employing many of the same or similar methods by which implanting and invading tissues of the mammalian embryo forestall maternal detection and rejection. A number of ways by which the tumour blocks, subverts or evades selection are discussed. (shrink)

Drosophila responds to a septic injury by the rapid synthesis of antimicrobial peptides. These molecules are predominantly produced by the fat body, a functional equivalent of mammalian liver, and are secreted into the hemolymph where their concentrations can reach up to 100 μM. Six distinct antibacterial peptides (plus isoforms) and one antifungal peptide have been characterized in Drosophila and their genes cloned. The induction of the gene encoding the antifungal peptide relies on the spätzle/Toll/cactus gene cassette, which is involved (...) in the control of dorsoventral patterning in the embryo, and shows interesting structural and functional similarities with cytokine‐induced activation of NF‐ϰB in mammalian cells. An additional pathway, dependent on the as yet unidentified imd (for immune‐deficiency) gene, is required for the full induction of the antibacterial peptide genes. Mutants deficient for the Toll and imd pathways exhibit a severely reduced survival to fungal and bacterial infections, respectively. Recent data on the molecular mechanisms underlying recognition of non‐self are also discussed in this review. (shrink)

Transcription and replication of genes in mammalian cells always requires a promoter or replication origin, respectively, but the ability of enhancers to stimulate these regulatory elements and the interactions that mediate this stimulation are developmentally acquired. The primary function of enhancers is to prevent repression, which appears to result from particular components of chromatin structure. Factors responsible for this repression are present in the maternal nucleus of oocytes and its descendant, the maternal pronucleus of mouse 1‐cell embryos and in (...) mouse 2‐cell embryos, but are absent in the paternal pronucleus. Thus, enhancers are not needed to achieve efficient transcription and replication in paternal pronuclei. However, enhancers, even in the presence of their specific activation protein, are inactive prior to formation of a 2‐cell embryo, suggesting that a coactivator essential for enhancer function is not available until zygotic gene expression begins. Furthermore, enhancer stimulation of transcription appears to be mediated through a promoter transcription factor, but this interaction can change as cells undergo differentiation, switching from a TATA‐box independent to a TATA‐box dependent mode. (shrink)

Polo‐like kinase 1 (PLK1) is a serine/threonine kinase that plays multiple and essential roles during the cell division cycle. Its inhibition in cultured cells leads to severe mitotic aberrancies and cell death. Whereas previous reports suggested that Plk1 depletion in mice leads to a non‐mitotic arrest in early embryos, we show here that the bi‐allelic Plk1 depletion in mice certainly results in embryonic lethality due to extensive mitotic aberrations at the morula stage, including multi‐ and mono‐polar spindles, impaired chromosome segregation (...) and cytokinesis failure. In addition, the conditional depletion of Plk1 during mid‐gestation leads also to severe mitotic aberrancies. Our data also confirms that Plk1 is completely dispensable for mitotic entry in vivo. On the other hand, Plk1 haploinsufficient mice are viable, and Plk1‐heterozygous fibroblasts do not harbor any cell cycle alterations. Plk1 is overexpressed in many human tumors, suggesting a therapeutic benefit of inhibiting Plk1, and specific small‐molecule inhibitors for this kinase are now being evaluated in clinical trials. Therefore, the different Plk1 mouse models here presented are a valuable tool to reexamine the relevance of the mitotic kinase Plk1 during mammalian development and animal physiology. (shrink)

The zona pellucida is an extracellular coat that surrounds all mammalian eggs. It is a porous matrix of interconnected filaments that are assembled from glycoproteins synthesized and secreted by growing oocytes. The zona pellucida is responsible both for species‐specific binding of sperm to unfertilized eggs and inducing bound sperm to undergo the acrosome reaction. The latter enables sperm to penetrate the extracellular coat and fertilize the egg. The zona pellucida also aids in prevention of polyspermy following fertilization and in (...) protection of preimplantation embryos. In mice, several of these important functions can now be ascribed to specific zona pellucida glycoproteins that have been purified and characterized. Furthermore, the enzyme responsible for hatching of embryos from the zona pellucida, just prior to implantation, has been identified and characterized. (shrink)

The recent desperation to clone human embryos may be seriously undermining accepted ethical principles of medical research, with potentially profound wider consequencesIn her editorial in the February 2005 issue of this journal, Nikola Biller-Andorno questioned whether the effort and resources that have been invested in debates about cloning at the United Nations might have been somewhat disproportionate, if a binding universal agreement on reproductive cloning cannot be reached.1 Although most of the overt disagreement has centred around “therapeutic” cloning, rather than (...) the potential use of nuclear transfer for reproduction, it is none the less clear that the delay and ultimate failure to date in achieving consensus on the former has also increased the likelihood of the latter becoming a foreseeable reality in the absence of a legally binding global convention. Whilst the much heralded promise of therapies has now been severely undermined by scandals of fraud,2 the available evidence from various primate studies3–5 and the history of similar work with other mammalian species6,7 have provided little reason to doubt that human reproductive cloning might be possible in principle . I completely agree with Dr Biller-Andorno’s appeal that we need to “foster a genuine, worldwide discourse on bioethical issues” and not let our debate get completely derailed by vested interests, whether politically or economically motivated. If we don’t, we can probably expect dire consequences for the future of biomedical research and its impact on society at large.Prior to the United Nations’ discussions regarding a ban on human cloning in October 2004, the Human Fertilisation and Embryology Authority in Great Britain announced that they had granted their first licence to clone human embryos by nuclear transfer,8 though no other applications had apparently been made following the legalisation …. (shrink)

How different neural crest derivatives differentiate in distinct embryonic locations in the vertebrate embryo is an intriguing issue. Many attempts have been made to understand the underlying mechanism of specific pathway choices made by migrating neural crest cells. In this speculative review we suggest a new mechanism for the regulation of neural crest cell migration patterns in avian and mammalian embryos, based on recent progress in understanding the expression and activity of receptor tyrosine kinases during embryogenesis. Distinct subpopulations of (...) crest‐derived cells express specific receptor tyrosine kinases while residing in a migration staging area. We postulate that the differential expression of receptor tyrosine kinases by specific subpopulations of neural crest cells allows them to respond to localized growth factor ligand activity in the embryo. Thus, the migration pathway taken by neural crest subpopulations is determined by their receptor tyrosine kinase response to the differential localization of their cognate ligand. (shrink)

Members of the transforming growth factor beta (TGFβ) family are pleiotropic cytokines with key roles in tissue morphogenesis and growth. TGFβ1, TGFβ2 and TGFβ3 are abundant in mammalian reproductive tissues, where development and cyclic remodelling continue in post‐natal and adult life. Potential roles for TGFβ have been identified in gonad and secondary sex organ development, spermatogenesis and ovarian function, immunoregulation of pregnancy, embryo implantation and placental development. However, better tools must now be employed to map more precisely essential functions (...) and the regulatory networks governing their activity. Gene ablation and transgenic models are expected to provide novel insights into distinct physiological activities for each TGFβ isoform in normal reproductive function and reproductive pathologies. It is also necessary to consider the mechanisms controlling TGFβ activation from latent precursor forms, and receptor and binding protein expression. Smad intracellular signalling circuitry and modulation by environmental stimuli through cross‐talk with other signal transduction pathways will further constrain TGFβ action. This review examines existing evidence for TGFβ1, TGFβ2 and TGFβ3 regulation of male and female reproductive biology, and highlights prospects for future research. BioEssays 24:904–914, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

The levels of different classes of mitochondrially encoded transcripts are developmentally regulated in sea urchin embryos, as a result of selection between mutually exclusive synthetic pathways. I propose a simple model to explain these observations, based on a dual role for mitochondrial ribosomes and translation factors in RNA synthesis as well as in translation. This effect may be exerted either at the transcriptional or post‐transcriptional level (or both), and is potentially generalizable to mammalian mtDNA and to other systems.