Yolk sac development, function and role in rodent pregnancy

Early Vascular Developmental Toxicity and Underlying Mechanisms of 1-Bromo-3,6-dichlorocarbazole (1-B-36-CCZ) in Zebrafish Larvae

Polyhalogenated carbazoles (PHCZs) are emerging persistent organic pollutants that have attracted widespread attention due to their environmental occurrence and potential ecological risks. 1-Bromo-3,6-dichlorocarbazole (1-B-36-CCZ), which is a typical homolog of PHCZs produced as a byproduct in the dye industry, has been widely detected in various environmental media. In this study, we employed an integrated approach using an in vivo zebrafish model and network toxicology methods to systematically evaluate the vascular developmental toxicity of 1-B-36-CCZ and elucidate its underlying mechanisms. The experimental results revealed that the 96 h-LC50 of 1-B-36-CCZ in zebrafish larvae was 4.52 mg/L. Sublethal exposures (0.045-45 μg/L) significantly induced an increase in heart rate (p < 0.05) and an enlargement of the pericardial edema area (p < 0.01). Using Tg(flk:eGFP) transgenic zebrafish embryos to assess vascular toxicity at concentrations of 0, 0.045, 0.45, 4.5, and 45 μg/L, we observed that 1-B-36-CCZ exposure significantly reduced the length and anastomosis rate of intersegmental vessels (ISVs) at 30 hpf, and inhibited the development of the common cardinal vein (CCV) at 48 and 72 hpf as well as the subintestinal vessel (SIV) at 72 hpf. Quantitative PCR (qPCR) analysis further revealed that the expression of key angiogenic genes (flk, kdr, and vegfa) was significantly downregulated, thus corroborating the phenotypic observations. Moreover, a "compound-target-pathway" network model predicted that SRC kinase is a key molecular target for 1-B-36-CCZ action. Enrichment analysis of target protein-coding genes and verapamil replication experiments indicated that 1-B-36-CCZ may cause damage to early vascular development in zebrafish larvae by altering intracellular calcium ion content through the activation of the SRC-mediated calcium ion signaling pathway. This study provides new experimental evidence for elucidating the toxic mechanisms of PHCZ-type pollutants and offers a theoretical basis for environmental health risk assessments.

Origin, fate and function of extraembryonic tissues during mammalian development

Extraembryonic tissues have pivotal roles in morphogenesis and patterning of the early mammalian embryo. Developmental programmes mediated through signalling pathways and gene regulatory networks determine the sequence in which fate determination and lineage commitment of extraembryonic tissues take place, and epigenetic processes allow the memory of cell identity and state to be sustained throughout and beyond embryo development, even extending across generations. In this Review, we discuss the molecular and cellular mechanisms necessary for the different extraembryonic tissues to develop and function, from their initial specification up until the end of gastrulation, when the body plan of the embryo and the anatomical organization of its supporting extraembryonic structures are established. We examine the interaction between extraembryonic and embryonic tissues during early patterning and morphogenesis, and outline how epigenetic memory supports extraembryonic tissue development.

Impaired yolk sac NAD metabolism disrupts murine embryogenesis with relevance to human birth defects

Congenital malformations can originate from numerous genetic or non-genetic factors but in most cases the causes are unknown. Genetic disruption of nicotinamide adenine dinucleotide (NAD) de novo synthesis causes multiple malformations, collectively termed Congenital NAD Deficiency Disorder (CNDD), highlighting the necessity of this pathway during embryogenesis. Previous work in mice shows that NAD deficiency perturbs embryonic development specifically when organs are forming. While the pathway is predominantly active in the liver postnatally, the site of activity prior to and during organogenesis is unknown. Here, we used a mouse model of human CNDD and assessed pathway functionality in embryonic livers and extraembryonic tissues via gene expression, enzyme activity and metabolic analyses. We found that the extra-embryonic visceral yolk sac endoderm exclusively synthesises NAD de novo during early organogenesis before the embryonic liver takes over this function. Under CNDD-inducing conditions, visceral yolk sacs had reduced NAD levels and altered NAD-related metabolic profiles, affecting embryo metabolism. Expression of requisite pathway genes is conserved in the equivalent yolk sac cell type in humans. Our findings show that visceral yolk sac-mediated NAD de novo synthesis activity is essential for mouse embryogenesis and its perturbation causes CNDD. As mouse and human yolk sacs are functionally homologous, our data improve the understanding of human congenital malformation causation.

The human gestational sac as a choriovitelline placenta during early pregnancy; the secondary yolk sac and organoid models

The yolk sac is phylogenetically the oldest of the extra-embryonic membranes and plays important roles in nutrient transfer during early pregnancy in many species. In the human this function is considered largely vestigial, in part because the secondary yolk sac never makes contact with the inner surface of the chorionic sac. Instead, it is separated from the chorion by the fluid-filled extra-embryonic coelom and attached to the developing embryo by a relatively long vitelline duct. The coelomic fluid is, however, rich in nutrients and key co-factors, including folic acid and anti-oxidants, derived from maternal plasma and the endometrial glands. Bulk sequencing has recently revealed the presence of transcripts encoding numerous transporter proteins for these ligands. Mounting evidence suggests the human secondary yolk sac plays a pivotal role in the transfer of histotrophic nutrition during the critical phase of organogenesis but also of chemicals such as medical drugs and cotinine. We therefore propose that the early placental villi, coelomic cavity and yolk sac combine to function physiologically as a choriovitelline placenta during the first weeks of pregnancy. We have derived organoids from the mouse yolk sac as proof-of-principle of a model system that could be used to answer many questions concerning the functional capacity of the human yolk sac as a maternal-fetal exchange interface during the first trimester of pregnancy.

Physiological accumulation of lipid droplets in the newborn liver during breastfeeding is driven by TLR4 ligands

The liver plays a central role in fat storage, but little is known about physiological fat accumulation during early development. Here we investigated a transient surge in hepatic lipid droplets observed in newborn mice immediately after birth. We developed a novel model to quantify liver fat content without tissue processing. Using high-resolution microscopy assessed the spatial distribution of lipid droplets within hepatocytes. Lugol's iodine staining determined the timing weaning period, and milk deprivation experiments investigated the relationship between milk intake and fat accumulation. Lipidomic analysis revealed changes in the metabolic profile of the developing liver. Finally, we investigated the role of Toll-like receptor 4 (TLR4) signaling in fat storage using knockout mice and cell-specific deletion strategies. Newborn mice displayed a dramatic accumulation of hepatic lipid droplets within the first 12 h after birth, persisting for the initial two weeks of life. This pattern coincided with exclusive milk feeding and completely abated by the third week, aligning with weaning. Importantly, the observed fat accumulation shared characteristics with established models of pathological steatosis, suggesting potential biological relevance. Lipid droplets were primarily localized within the cytoplasm of hepatocytes. Milk deprivation experiments demonstrated that milk intake is the primary driver of this transient fat accumulation. Lipidomic analysis revealed significant changes in the metabolic profile of newborn livers compared to adults. Interestingly, several highly abundant lipids in newborns were identified as putative ligands for TLR4. Subsequent studies using TLR4-deficient mice and cell-specific deletion revealed that TLR4 signaling, particularly within hepatocytes, plays a critical role in driving fat storage within the newborn liver. Additionally, a potential collaboration between metabolic and immune systems was suggested by the observed effects of myeloid cell-specific TLR4 ablation. This study demonstrates a unique phenomenon of transient hepatic fat accumulation in newborn mice driven by milk intake and potentially regulated by TLR4 signaling, particularly within hepatocytes.

B Cell Responses to the Placenta and Fetus

Pregnancy has fascinated immunologists ever since Peter Medawar's observation that reproduction runs contrary to the founding tenets of immunology. During healthy pregnancy, maternal B cells interact with antigens of the foreign conceptus (placenta and fetus) yet do not elicit rejection. Instead, robust and redundant fetomaternal tolerance pathways generally prevent maternal B cells and antibodies from harming the placenta and fetus. Fetomaternal tolerance is not absolute, and unfortunately there exist several pregnancy complications that arise from breaks therein. Here, important historic and recent developments in the field of fetomaternal tolerance pertaining to maternal B cells and antibodies are reviewed. General rules from which to conceptualize humoral tolerance to the placenta and fetus are proposed. Significant but underexplored ideas are highlighted and topics for future research are suggested, findings from which are predicted to provide insight into the fundamental nature of tolerance and bolster efforts to combat immune-mediated pregnancy complications.

Off to a good start: The importance of the placental exchange surface - Lessons from the mouse

The role of the chorio-allantoic placenta as the critical nutrient- and oxygen-supplying organ to nourish the demands of the fetus has been well recognized. This function relies on the successful establishment of the placental feto-maternal exchange unit, or interhaemal barrier, across which all nutrients as well as waste products must pass to cross from the maternal to the fetal blood circulation, or vice versa, respectively. As a consequence, defects in the establishment of this elaborate interface lead to fetal growth retardation or even embryonic lethality, depending on the severity of the defect. Beyond this essential role, however, it has also emerged that the functionality of the feto-maternal interface dictates the proper development of specific embryonic organs, with tightest links observed to the formation of the heart. In this article, we build on the foundational strength of the mouse as experimental model in which the placental causality of embryonic defects can be genetically proven. We discuss in detail the formation of the interhaemal barrier that makes up the labyrinth layer of the murine placenta, including insights into drivers of its formation and the interdependence of the cell types that make up this essential interface, from in vivo and in vitro data using mouse trophoblast stem cells. We highlight mouse genetic tools that enable the elucidation of cause-effect relationships between defects driven by either the trophoblast cells of the placenta or by embryonic cell types. We specifically emphasize gene knockouts for which a placental causality of embryonic heart defects has been demonstrated. This in-depth perspective provides much-needed insights while highlighting remaining gaps in knowledge that are essential for gaining a better understanding of the multi-facetted roles of the placenta in setting us up for a healthy start in life well beyond nutritional support alone.

Preimplantation trophectoderm: A 'quick-fix' protector for embryo survival?

The trophectoderm (TE) epithelium forms the outer layer of the mammalian blastocyst and generates the blastocoel through vectorial transport. Its differentiation during cleavage, studied mainly in mouse, is integrated with blastocyst morphogenesis with key roles for cell polarisation, asymmetric cell divisions, cell signalling, regulatory transcription factors and cellular inheritance. The TE provides a physical and cellular protection to the emerging lineages of the embryo essential for the integrity of blastocyst development. Here, two examples of TE differentiation are considered in some detail where this immediate protective function for embryo survival is assessed: (i) cellular processes from TE at the polar-mural junctional zone in the early blastocyst that later form filopodia traversing the blastocoel, and (ii) the endocytic system which matures and polarises during differentiation. Understanding the broad role for TE in regulating early morphogenesis and environmental protection of the embryo, including these two examples, have clinical as well as biological relevance.

Amniotic membrane, a novel bioscaffold in cardiac diseases: from mechanism to applications

Cardiovascular diseases represent one of the leading causes of death worldwide. Despite significant advances in the diagnosis and treatment of these diseases, numerous challenges remain in managing them. One of these challenges is the need for replacements for damaged cardiac tissues that can restore the normal function of the heart. Amniotic membrane, as a biological scaffold with unique properties, has attracted the attention of many researchers in recent years. This membrane, extracted from the human placenta, contains growth factors, cytokines, and other biomolecules that play a crucial role in tissue repair. Its anti-inflammatory, antibacterial, and wound-healing properties have made amniotic membrane a promising option for the treatment of heart diseases. This review article examines the applications of amniotic membrane in cardiovascular diseases. By focusing on the mechanisms of action of this biological scaffold and the results of clinical studies, an attempt will be made to evaluate the potential of using amniotic membrane in the treatment of heart diseases. Additionally, the existing challenges and future prospects in this field will be discussed.

The developmental pattern related to fatty acid uptake and oxidation in the yolk sac membrane and jejunum during embryogenesis in Muscovy duck

This study aimed to investigate the developmental change of body growth and gene expression related to fatty acid uptake and oxidation in the yolk sac membrane (YSM) and jejunum during embryogenesis in Muscovy ducks. The weights of embryos and yolk sac (YS) (5 embryos per replicate, n = 6) were recorded on embryonic days (E)16, E19, E22, E25, E28, E31, and the day of hatch (DOH). The fat and fatty acid contents in YSM, jejunal histology, and gene expression related to fatty acid metabolism in YSM and jejunum were determined in each sampling time. Among the nonlinear models, the maximum growth is estimated at 2.83 (E22.5), 2.67 (E22.1), and 2.60 (E21.3) g/d using logistic, Gompertz, and Von Bertalanffy models, respectively. The weight of YS, and ether extract-free YS as well as the amounts of fat and fatty acids in YS decreased (P < 0.05) linearly, whereas the villus height, crypt depth, villus height/crypt depth, and musculature thickness in jejunum increased (P < 0.05) linearly during embryogenesis. The mRNA expression of CD36, SLC27A4, and FABP1 related to fatty acid uptake as well as the mRNA and protein expressions of PPARα and CPT1 related to fatty acid oxidation increased in a quadratic manner (P < 0.05) in both YS and jejunum, and the maximum values were achieved during E25 to E28. In conclusion, the maximum growth rate of Muscovy duck embryos was estimated at 2.60 to 2.83 g/d on E21.3 to E23.5, while the accumulations of lipid and fatty acid in YS were decreased in association with the increased absorptive area of morphological structures in jejunum. The gene and protein expression involved in fatty acid metabolism displayed a similar enhancement pattern between YSM and jejunum during E25 to E28, suggesting that fatty acid utilization could be strengthened to meet the energy demand for embryonic development.

The Effects of Different Doses of Canthaxanthin in the Diet of Laying Hens on Egg Quality, Physical Characteristics, Metabolic Mechanism, and Offspring Health

Currently, there is a dearth of in-depth analysis and research on the impact of canthaxanthin on the production performance, egg quality, physical characteristics, and offspring health of laying hens. Furthermore, the metabolic mechanism of cantharidin in the body remains unclear. Therefore, to solve the above issues in detail, our study was conducted with a control group (C group), a low-dose canthaxanthin group (L group), and a high-dose canthaxanthin group (H group), each fed for a period of 40 days. Production performance was monitored during the experiment, in which L and H groups showed a significant increase in ADFI. Eggs were collected for quality analysis, revealing no significant differences in qualities except for yolk color (YC). The YC of the C group almost did not change, ranging from 6.08 to 6.20; however, the trend in YC change in other groups showed an initial intense increase, followed by a decrease, and eventually reached dynamic equilibrium. By detecting the content of canthaxanthin in the yolk, the YC change trend was found to be correlated with canthaxanthin levels in the yolk. The content of unsaturated fatty acid increased slightly in L and H groups. Following the incubation period, the physical characteristics and blood biochemical indices of chicks were evaluated. It was observed that the shank color of chicks in the L and H groups was significantly higher than that in the C group at birth. However, by the 35th day, there were no significant differences in shank color among the three groups. Further investigation into the metabolic mechanism involving canthaxanthin revealed that the substance underwent incomplete metabolism upon entering the body, resulting in its accumulation as well as metabolic by-product accumulation in the yolk. In summary, this study highlighted the importance of understanding canthaxanthin's role in production performance, egg quality, and offspring health, providing valuable insights for breeders to optimize feeding strategies.

Role of microglia in brain development after viral infection

Microglia are immune cells in the brain that originate from the yolk sac and enter the developing brain before birth. They play critical roles in brain development by supporting neural precursor proliferation, synaptic pruning, and circuit formation. However, microglia are also vulnerable to environmental factors, such as infection and stress that may alter their phenotype and function. Viral infection activates microglia to produce inflammatory cytokines and anti-viral responses that protect the brain from damage. However, excessive or prolonged microglial activation impairs brain development and leads to long-term consequences such as autism spectrum disorder and schizophrenia spectrum disorder. Moreover, certain viruses may attack microglia and deploy them as "Trojan horses" to infiltrate the brain. In this brief review, we describe the function of microglia during brain development and examine their roles after infection through microglia-neural crosstalk. We also identify limitations for current studies and highlight future investigated questions.