Haeckel's ABC of evolution and development

One of the central, unresolved controversies in biology concerns the distribution of primitive versus advanced characters at different stages of vertebrate development. This controversy has major implications for evolutionary developmental biology and phylogenetics. Ernst Haeckel addressed the issue with his Biogenetic Law, and his embryo drawings functioned as supporting data. We re-examine Haeckel's work and its significance for modern efforts to develop a rigorous comparative framework for developmental studies. Haeckel's comparative embryology was evolutionary but non-quantitative. It was based on developmental sequences, and treated heterochrony as a sequence change. It is not always clear whether he believed in recapitulation of single characters or entire stages. The Biogenetic Law is supported by several recent studies -- if applied to single characters only. Haeckel's important but overlooked alphabetical analogy of evolution and development is an advance on von Baer. Haeckel recognized the evolutionary diversity in early embryonic stages, in line with modern thinking. He did not necessarily advocate the strict form of recapitulation and terminal addition commonly attributed to him. Haeckel's much-criticized embryo drawings are important as phylogenetic hypotheses, teaching aids, and evidence for evolution. While some criticisms of the drawings are legitimate, others are more tendentious. In opposition to Haeckel and his embryo drawings, Wilhelm His made major advances towards developing a quantitative comparative embryology based on morphometrics. Unfortunately His's work in this area is largely forgotten. Despite his obvious flaws, Haeckel can be seen as the father of a sequence-based phylogenetic embryology.     

Gastrulation and the establishment of the three germ layers in the early horse conceptus

Experimental studies and field surveys suggest that embryonic loss during the first 6 weeks of gestation is a common occurrence in the mare. During the first 2 weeks of development, a number of important cell differentiation events must occur to yield a viable embryo proper containing all three major germ layers (ectoderm, mesoderm, and endoderm). Because formation of the mesoderm and primitive streak are critical to the development of the embryo proper, but have not been described extensively in the horse, we examined tissue development and differentiation in early horse conceptuses using a combination of stereomicroscopy, light microscopy, and immunohistochemistry. Ingression of epiblast cells to form the mesoderm was first observed on day 12 after ovulation; by Day 18 the conceptus had completed a series of differentiation events and morphologic changes that yielded an embryo proper with a functional circulation. While mesoderm precursor cells were present from Day 12 after ovulation, vimentin expression was not detectable until Day 14, suggesting that initial differentiation of mesoderm from the epiblast in the horse is independent of this intermediate filament protein, a situation that contrasts with other domestic species. Development of the other major embryonic germ layers was similar to other species. For example, ectodermal cells expressed cytokeratins, and there was a clear demarcation in staining intensity between embryonic ectoderm and trophectoderm. Hypoblast showed clear α1-fetoprotein expression from as early as Day 10 after ovulation, and seemed to be the only source of α1-fetoprotein in the early conceptus.     

被子植物胚胎发育的分子调控

被子植物的胚胎发育受到精确的遗传调控。从双受精开始到种子成熟, 胚胎发育经历了合子激活、细胞分裂与分化、极性建立、模式形成、器官发生和储藏物质累积等重要过程。过去20年来的分子遗传学研究鉴定了很多调控胚胎发生的基因,为了解胚胎形成的分子机理提供了大量信息。本文对这一领域的主要研究进展进行了简要评述, 重点阐述了植物的早期胚胎发生过程, 对尚未解决的科学问题及未来发展方向进行了综合分析。     

Cellular mechanisms of implantation in domestic farm animals

In order for pregnancy to be established, the conceptus of domestic animals must signal its presence upon arrival in the uterus, a process known as maternal recognition of pregnancy. The conceptus derived signal(s) prevent(s) the structural and functional demise of the corpus luteum to ensure the maintenance of a uterine environment that supports implantation and embryonic development. Implantation is a remarkable event that has been described as a biological paradox because an adhesive interaction is formed between two apical surfaces of epithelial cell types that are typically covered by non-adhesive glycoproteins. In domestic animals (such as pigs, horses, sheep, does and cows), the implantation process is not invasive as it is in most other mammalian species. This review describes the interaction between the conceptus (embryo and surrounding membranes) and the uterine epithelial surface in domestic farm animals and ability of the conceptus to control the lifespan of the corpus luteum to maintain pregnancy.     

Reflections on the golden anniversary of the first embryo transfer to produce a calf

In the belief that the vitality of a discipline benefits from a knowledge of its roots, events surrounding the first embryo transfer to produce a live calf (by Willett and his colleagues in Wisconsin fifty years ago) have been reviewed. That first success is discussed in the context of those times (including a brief review of the first international conference on embryo transfer, held in Texas in 1949) and in relation to subsequent events that led to the founding of the International Embryo Transfer Society in 1974.     

Developmental changes in endogenous retinoids during pregnancy and embryogenesis in the mouse.

Vitamin A and its analogs (retinoids) have acquired particular significance in embryonic development since the discovery that retinoic acid (RA) possesses properties of an endogenous morphogen and that embryonic tissues contain specific nuclear receptors for RA. Since the mammalian embryo does not synthesize RA de novo but rather must acquire it directly or in a precursor form from the maternal circulation, we sought to establish the relationship between levels of RA, retinol, and retinyl esters in the maternal system and their acquisition by the embryo, particularly during organogenesis in the mouse. Results indicate profound changes in maternal vitamin A levels during pregnancy in the mouse. These changes were characterized by a large, transient decrease in plasma retinol levels coincident with the period of organogenesis (e.g. gestational Days 9-14), and an apparent increase in mobilization from hepatic stores to the conceptus. During organogenesis, the embryo exhibited a steady increase in retinol levels with little increase in retinyl esters and virtually no change in RA. Analysis of retinoid accumulation patterns in the embryonic liver indicate that functional onset of vitamin A storage occurs by mid-organogenesis. In contrast, placental levels of these retinoids remained unchanged throughout organogenesis. Analysis of the conceptus as a developmental unit revealed that during early organogenesis the majority of retinoids are contained in the placenta (8-fold more than in the embryo). However, by mid-organogenesis the retinoid content of the embryo exceeds that of the placenta. Together, these results provide evidence that pregnancy in the mouse is accompanied by pronounced alterations in maternal retinoid homeostasis that occur coincident with the period of high embryonic sensitivity to exogenous retinoids.     

Hypoblast controls mesoderm generation and axial patterning in the gastrulating rabbit embryo

Gastrulation in higher vertebrate species classically commences with the generation of mesoderm cells in the primitive streak by epithelio-mesenchymal transformation of epiblast cells. However, the primitive streak also marks, with its longitudinal orientation in the posterior part of the conceptus, the anterior-posterior (or head-tail) axis of the embryo. Results obtained in chick and mouse suggest that signals secreted by the hypoblast (or visceral endoderm), the extraembryonic tissue covering the epiblast ventrally, antagonise the mesoderm induction cascade in the anterior part of the epiblast and thereby restrict streak development to the posterior pole (and possibly initiate head development anteriorly). In this paper we took advantage of the disc-shape morphology of the rabbit gastrula for defining the expression compartments of the signalling molecules Cerberus and Dickkopf at pre-gastrulation and early gastrulation stages in a mammal other than the mouse. The two molecules are expressed in novel expression compartments in a complementary fashion both in the hypoblast and in the emerging primitive streak. In loss-of-function experiments, carried out in a New-type culturing system, hypoblast was removed prior to culture at defined stages before and at the beginning of gastrulation. The epiblast shows a stage-dependent and topographically restricted susceptibility to express Brachyury, a T-box gene pivotal for mesoderm formation, and to transform into (histologically proven) mesoderm. These results confirm for the mammalian embryo that the anterior-posterior axis of the conceptus is formed first as a molecular prepattern in the hypoblast and then irrevocably fixed, under the control of signals secreted from the hypoblast, by epithelio-mesenchymal transformation (primitive streak formation) in the epiblast.Edited by D. Tautz     

Trophoblast biology: Forum introduction

In mammals, a carefully orchestrated dialogue between the mother and conceptus (embryo/fetus and associate extraembryonic membranes) is initiated during the peri-implantation period of pregnancy as the trophoblast develops, functions to signal pregnancy recognition, and initiates implantation. The purpose of this Forum is to highlight comparative aspects of trophoblast morphogenesis and function in mammals.     

Stage-specific formation of the equine blastocyst capsule is instrumental to hatching and to embryonic survival in vivo

Early embryonic development in the horse is characterized by the formation of an unusual acellular glycoprotein "capsule" between the trophectoderm and the overlying zona pellucida. This structure is first detected between days 6 and 7 after ovulation and completely envelops the spherical conceptus until as late as day 23 of gestation. In the present study, a micromanipulator was used to remove the capsule from 15 embryos on day 6-7 after ovulation. None of these denuded embryos developed into ultrasonographically detectable pregnancies after surgical transfer into recipient mares whereas four of six control embryos handled and transferred similarly but without capsule removal developed normally, thereby demonstrating clearly a role for the capsule in embryonic survival. In addition, observation of the embryonic investments after embryo collection and during micromanipulation led to the hypothesis that hatching of the horse embryo from its zona pellucida is assisted by the force of the expanding capsule, which causes the attenuating zona to literally burst open.     

Prostaglandin‐endoperoxide synthase 2 is not required for preimplantation ovine conceptus development in sheep

Conceptus development and elongation is required for successful pregnancy establishment in ruminants and is coincident with the production of interferon τ (IFNT) and prostaglandins (PGs). In both the conceptus trophectoderm and endometrium, PGs are primarily synthesized through a prostaglandin‐endoperoxide synthase 2 (PTGS2) pathway and modify endometrial gene expression and thus histotroph composition in the uterine lumen to promote conceptus growth and survival. Chemical inhibition of PG production by both the endometrium and the conceptus prevented elongation in sheep. However, the contributions of conceptus‐derived PGs to preimplantation conceptus development remain unclear. In this study, CRISPR‐Cas9 genome editing was used to inactivate PTGS2 in ovine embryos to determine the role of PTGS2‐derived PGs in conceptus development and elongation. PTGS2 edited conceptuses produced fewer PGs, but secreted similar amounts of IFNT to their Cas9 control counterparts and elongated normally. Expression of PTGS1 was lower in PTGS2 edited conceptuses, but PPARG expression and IFNT secretion were unaffected. Content of PGs in the uterine lumen was similar as was gene expression in the endometrium of ewes who received either Cas9 control or PTGS2 edited conceptuses. These results support the idea that intrinsic PTGS2‐derived PGs are not required for preimplantation embryo or conceptus survival and development in sheep.     

Selected diseases and conditions associated with bovine conceptus loss in the first trimester

The outcomes of insults to the bovine conceptus depend on the predilection of the insulting agent for the gravid reproductive tract, the virulence of the insult, and the developmental maturity/immune competence of the conceptus at the time of the insult. Agents that are lethal at one time during gestation may be harmless at another, or may have completely different effects (some not so harmless) at different gestational ages. This review discusses some of the known physical-mechanical, physiological, and infectious causes of first trimester bovine conceptus losses, including three infectious agents that have been the subject of recent studies for their potential to transmit disease via embryo transfer.     

Embryological origins of the human individual

The human fertilized egg is not an embryo, but, more accurately, a conceptus, which contains all the information required to establish both the embryo and extraembryonic supporting tissues. The fertilized eggs of placental mammals, including humans, are entirely dependent upon the female's uterine environment for development to birth. At least half, possibly more, fertilized eggs, or potential lives, do not survive to birth, the greatest loss thought to occur during preimplantation and implantation. Which conceptuses will be lost and which will progress to birth cannot be predicted. The. preimplantation conceptus exhibits extreme developmental lability. Importantly, twinning can occur throughout the preimplantation and implantation phases, and thus, a single human individual has not emerged from the conceptus during this time period. Once the primitive streak is complete during early postimplantation development, identical twinning no longer occurs and the individual emerges. Thus, emergence of the human individual is a process. No single event thus far known is more important than any other prior to formation of the primitive streak. Formation of the streak is a defining moment in the origin of the individual. All further organization of the fetus occurs around this midline. Thus, by 14 days in the human, the body plan for life is established.     

A proteomic atlas of ligand–receptor interactions at the ovine maternal–fetal interface reveals the role of histone lactylation in uterine remodeling

Well-orchestrated maternal–fetal cross talk occurs via secreted ligands, interacting receptors, and coupled intracellular pathways between the conceptus and endometrium and is essential for successful embryo implantation. However, previous studies mostly focus on either the conceptus or the endometrium in isolation. The lack of integrated analysis impedes our understanding of early maternal–fetal cross talk. Herein, focusing on ligand–receptor complexes and coupled pathways at the maternal–fetal interface in sheep, we provide the first comprehensive proteomic map of ligand–receptor pathway cascades essential for embryo implantation. We demonstrate that these cascades are associated with cell adhesion and invasion, redox homeostasis, and the immune response. Candidate interactions and their physiological roles were further validated by functional experiments. We reveal the physical interaction of albumin and claudin 4 and their roles in facilitating embryo attachment to endometrium. We also demonstrate a novel function of enhanced conceptus glycolysis in remodeling uterine receptivity by inducing endometrial histone lactylation, a newly identified histone modification. Results from in vitro and in vivo models supported the essential role of lactate in inducing endometrial H3K18 lactylation and in regulating redox homeostasis and apoptotic balance to ensure successful implantation. By reconstructing a map of potential ligand–receptor pathway cascades at the maternal–fetal interface, our study presents new concepts for understanding molecular and cellular mechanisms that fine-tune conceptus–endometrium cross talk during implantation. This provides more direct and accurate insights for developing potential clinical intervention strategies to improve pregnancy outcomes following both natural and assisted conception.     

Polarity of the mouse embryo is anticipated before implantation

In most species, the polarity of an embryo underlies the future body plan and is determined from that of the zygote. However, mammals are thought to be an exception to this; in the mouse, polarity is generally thought to develop significantly later, only after implantation. It has not been possible, however, to relate the polarity of the preimplantation mouse embryo to that of the later conceptus due to the lack of markers that endure long enough to follow lineages through implantation. To test whether early developmental events could provide cues that predict the axes of the postimplantation embryo, we have used the strategy of injecting mRNA encoding an enduring marker to trace the progeny of inner cell mass cells into the postimplantation visceral endoderm. This tissue, although it has an extraembryonic fate, plays a role in axis determination in adjacent embryonic tissue. We found that visceral endoderm cells that originated near the polar body (a marker of the blastocyst axis of symmetry) generally became distal as the egg cylinder formed, while those that originated opposite the polar body tended to become proximal. It follows that, in normal development, bilateral symmetry of the mouse blastocyst anticipates the polarity of the later conceptus. Moreover, our results show that transformation of the blastocyst axis of symmetry into the axes of the postimplantation conceptus involves asymmetric visceral endoderm cell movement. Therefore, even if the definitive axes of the mouse embryo become irreversibly established only after implantation, this polarity can be traced back to events before implantation.     

Conditional Aurora A deficiency differentially affects early mouse embryo patterning

Aurora A is a mitotic kinase essential for cell proliferation. In mice, ablation of Aurora A results in mitotic arrest and pre-implantation lethality, preventing studies at later stages of development. Here we report the effects of Aurora A ablation on embryo patterning at early post-implantation stages. Inactivation of Aurora A in the epiblast or visceral endoderm layers of the conceptus leads to apoptosis and inhibition of embryo growth, causing lethality and resorption at approximately E9.5. The effects on embryo patterning, however, depend on the tissue affected by the mutation. Embryos with an epiblast ablation of Aurora A properly establish the anteroposterior axis but fail to progress through gastrulation. In contrast, mutation of Aurora A in the visceral endoderm, leads to posteriorization of the conceptus or failure to elongate the anteroposterior axis. Injection of ES cells into Aurora A epiblast knockout blastocysts reconstitutes embryonic development to E9.5, indicating that the extra-embryonic tissues in these mutant embryos can sustain development to organogenesis stages. Our results reveal new ways to induce apoptosis and to ablate cells in a tissue-specific manner in vivo. Moreover, they show that epiblast-ablated embryos can be used to test the potency of stem cells.