From the Farm to the Lab: How Chicken Embryos Contribute to the Field of Teratology

A scientific case for revisiting the embryonic chicken model in biomedical research

The availability of fertilised chicken eggs and the accessibility and rapid development of the avian embryo, have been utilised in biomedical scientific research to make fundamental discoveries including of developmental processes that are common to all vertebrates, advances in teratology, the understanding of tumour growth and metastasis, angiogenesis, cancer drug assessment and vaccine development as well as advances in understanding avian specific biology. However, recent innovations in chicken transgenesis, genome engineering and surrogate host technology in chickens have only been utilised in a few of these fields of research, specifically some areas of developmental biology, avian sex determination and immunology. To understand why other biomedical fields have not adopted modern chicken transgenic tools, we investigated the non-technical summaries of projects granted in the UK under the Animals (Scientific Procedures) Act 1986 between 2017 and 2023 to assess when and how chicken embryos are used in research, and if they were considered as a Replacement model for other species.

Cereblon E3 ligase complex genes are expressed in tissues sensitive to thalidomide in chicken and zebrafish embryos but are unchanged following thalidomide exposure

Thalidomide is an infamous drug used initially as a sedative until it was tragically discovered it has highly teratogenic properties. Despite this it is now being used to successfully treat a range of clinical conditions including erythema nodosum leprosum (ENL) and multiple myeloma (MM). Cereblon (CRBN), a ubiquitin ligase, is a binding target of thalidomide for both its therapeutic and teratogenic activities and forms part of an CRL4-E3 ubiquitin ligase complex with the proteins Damaged DNA Binding protein 1 (DDB1) and Cullin-4A (CUL4A). This complex mediates degradation of the zinc-finger transcription factors Ikaros (IKZF1) and Aiolos (IKZF3), to mediate thalidomide's anti-myeloma response. To better understand the importance of CRBN and its binding partners for thalidomide teratogenesis here we analysed the expression patterns of CRBN and some of its known E3 complex binding partners in wildtype and thalidomide-treated chicken and zebrafish embryos. CRBN and DDB1 are expressed in many tissues throughout development including those that are thalidomide-sensitive while CUL4A and targets of the CRL4-CRBN E3 Ligase Complex IKZF1 and IKZF3 are expressed at different timepoints and in fewer tissues in the body than CRBN. Furthermore, IKZF3 is expressed in tissues of the eye that CRBN is not. However, although we observed rapid changes to the chicken yolk-sac membrane vasculature following thalidomide exposure, we did not detect CRL4-CRBN E3 Ligase Complex expression in the yolk-sac membrane vessels. Furthermore, we did not detect any changes in CRBN, DDB1, CUL4, IKZF1 and IKZF3 expression following thalidomide exposure in chicken and zebrafish embryos. These findings demonstrate that the anti-angiogenic activities of thalidomide may occur independent of CRBN and that thalidomide does not regulate CRL4-CRBN E3 Ligase Complex gene expression at the mRNA level.

Protective effects of CoQ10 and L-carnitine against antidepressant-induced mitochondrial dysfunction and teratogenicity in chicken embryos

Fluoxetine (FXT) and alprazolam (APZ), widely used for mental disorders, have poorly studied adverse effects on mitochondrial function, including oxidative phosphorylation, electron transport, and membrane permeability. This study represents the first investigation using a chick embryo model (HH-stage 10, day 1.5) to analyze the teratogenic effects of FXT and APZ and explore the protective potential of coenzyme Q10 (CoQ10) and L-carnitine (CNT). Administration of FXT (10 μM) and APZ (1 μM) resulted in high teratogenic rates of 53 % and 80 %, respectively, predominantly manifesting as lipid myopathy in hatching muscles, characterized by lipid accumulation, myofibril disruption, inflammation, and edema. Gene expression analysis revealed upregulation of acetyl-CoA carboxylase (ACC) and downregulation of carnitine palmitoyltransferase 1 (CPT1), leading to impaired lipid peroxidation and excessive reactive oxygen species (ROS) production. Markers of oxidative stress, including superoxide dismutase (SOD), hydrogen peroxide (H2O2), and nitric oxide (NO), were significantly elevated, correlating with glutathione (GSH) depletion and mitochondrial ultrastructural damage, resulting in reduced ATP production. Notably, co-administration of CoQ10 and CNT with FXT or APZ significantly improved teratogenic and mortality rates and reduced oxidative stress levels. Specifically, CoQ10 (2 μM) in the FXT group significantly reduced SOD, H2O2, and NO levels, while co-treatment with CNT and CoQ10 (2 μM) in the APZ group significantly alleviated NO levels. This pioneering study highlights the novel and crucial potential of CoQ10 and CNT as nutritional supplements to mitigate mitochondrial damage and antioxidant system imbalance caused by FXT and APZ, providing an innovative strategy for clinical application.

The avian embryo as a time-honoured animal model in developmental, biomedical and agricultural research

Avian embryos have been at the core of embryological, morphological, physiological and biochemical/molecular research, especially involving research in three primary areas: developmental, biomedical and agricultural research. As developmental models, the avian embryo-especially that of the chicken-has been the single most used embryo model, perhaps in part from the combination of large size, ease of access and prior knowledge base. Developmental research with avian embryos has included organ system studies of the heart, vasculature, lungs, kidneys, nervous system, etc., as well as integrated physiological processes including gas-exchange, acid-base and ion/water regulation. In terms of translational research, avian embryos have modelled vascular development, based on the easily accessible chorioallantoic membrane under the eggshell. This same respiratory organ has enabled toxicological studies of how pollutants affect vertebrate development. Investigation of the transition to pulmonary breathing and the associated emergence of respiratory control has also relied heavily upon the avian embryo. In addition to developmental and biomedical investigations, the avian embryo has been studied intensively due to the huge importance of domesticated birds as a food source. Consequently, the effects of environment (including temperature, humidity, noise levels and photoperiod) during incubation on subsequent post-hatch phenotype are being actively investigated.This article is part of the theme issue 'The biology of the avian respiratory system'.

Analysis of bioactive compounds of hen egg components at the first half of incubation

A comparative analysis of mass-volume characteristics of structural and morphological components of hen eggs before incubation and on the fifth (HH25-HH27) and 10th (HH36) days of incubation was carried out. During incubation, egg weight decreased by 9.25% (10 days), mainly due to a decrease in albumen weight (35.8%). The ratio of lipid-soluble fraction (LSF) and water-soluble fraction (WSF) in the mixed components and yolk did not change significantly. The total amount of solids in the mixed substances practically does not change during incubation. Antiradical activity of substances increased significantly by more than three times on the fifth day and additionally by 38.6% at the 10th day in relation to the fifth day. Total antioxidant activity increased by 18.9% on the fifth day and by 24.3% on the 10th day, compared to eggs before incubation. Transformation of the main components of WSF and LSF of albumin, yolk, and chicken embryo (CE) was studied using high-performance liquid chromatography and gas chromatography with mass spectrometry. On the 10th day, an increase in the number of high-molecular proteins is recorded, which indicates the activation of enzymatic processes of transformation of the main albumen proteins into proteins of organs and tissues of CE. This may cause an increase in the biological activity of substances. It was found that in conditions of in vitro digestion, antiradical activity increases by two times, anti-inflammatory activity increases by 2.4 times, and an angiotensin-converting enzyme inhibitory effect occurs in the mixed components of a 10-day incubation egg.

Protective Role of Vitamin B6 Against Teratogenic Effects Induced by Lead in Chick Embryo

BACKGROUND

Heavy metals like lead (Pb) have been used by humans for a very long time, but throughout the industrial revolution, their use expanded, increasing exposure to the metal. Lead, however, has no biological purpose in the human body and is hazardous when it gets into soft tissues and organs. Lead is still used in a variety of industries, including battery manufacturing and car maintenance, despite efforts to limit its usage.

OBJECTIVE

This study investigates the teratogenic and morphometric effects of lead on chick embryos and the potential ameliorative effects of vitamin B6.

METHODS

Two hundred fertilized eggs from the golden black chicken were divided into four groups: control, lead acetate, vitamin B6, and lead + vitamin B6.

RESULTS

On the 14th day, embryos were analyzed. Significant reductions in body weight and size were observed in the lead-exposed group (33.93 ± 1.27 g) compared to the control (41.12 ± 0.97 g). Pronounced deformities included rudimentary beaks, protruding eyes, tridactyl limbs, hydrocephaly, and neck deformities. Appendicular deformities like phocomelia, amelia, and abnormal phalanges growth were also noted. Vitamin B6 demonstrated therapeutic benefits, significantly improving mean embryo weight in the Lead + Vitamin B6 group (42.37 ± 0.99 g). The lead-exposed group showed a reduction in maxilla length (3.61 ± 1.30 mm) compared to the Lead + Vitamin B6 group (7.57 ± 0.79 mm). This group also showed reduced severity of muscular dystrophy and bone thinning, with signs of recovery in beak and bone sizes.

CONCLUSIONS

The study highlights vitamin B6's beneficial impact in mitigating lead's toxic effects on chick embryonic development.

Expanding understanding of chick embryo's nervous system development at HH22-HH41 embryonic stages using X-ray microcomputed tomography

Assessing the embryotoxicity and teratogenicity of various substances and processes is crucial due to their complexity and resource intensity. The chicken embryo (CE) serves an ideal model for simulating the first months of mammalian embryonic development. This makes the CE a reliable model for testing teratogenic effects, particularly in relation to the nervous system (NS), which experiences developmental abnormalities second in frequency only to cardiovascular teratogenic disorders. Microcomputed tomography (μCT) is a promising method for studying these processes. The advantages of μCT include relatively high research speed, diagnostic accuracy, high resolution and the ability to visualize the entire internal 3D structure of an object while preserving for other types of research. At the same time, there are practically no available databases of normative μCT data, both qualitative and quantitative, which would act as a starting point for screening detection of abnormalities in the development of the NS. In this study, we present a simple method for obtaining very detailed quantitative sets of 2D and 3D μCT data of NS structures of the CE (Gallus Gallus domesticus) at HH22-HH41 embryonic stages with contrasting by 1% phosphotungstic acid. The results of μCT demonstrate the exact boundaries, high general and differentiated contrast of the main and specific structures of the NS of CE, which are quantitatively and qualitatively similar to results of histological analysis. Calculations of the X-ray density and volume of the main structures of the NS at constant exponential growth are presented. In addition to the increase in linear dimensions, significant changes in the structures of various parts of the brain were identified and visualized during the CE development at HH22 to HH41 embryonic stages. The data presented establish the first methodology for obtaining normative data, including subtle localized differences in the NS in CE embryogenesis. The data obtained open up new opportunities for modern embryology, teratology, pharmacology and toxicology.

A new standardization for the use of chicken embryo: selection of target from the phage display library and infection

The filamentous bacteriophage M13KO7 (M13) is the most used in phage display (PD) technology and, like other phages, has been applied in several areas of medicine, agriculture, and in the food industry. One of the advantages is that they can modulate the immune response in the presence of pathogenic microorganisms, such as bacteria and viruses. This study evaluated the use of phage M13 in the chicken embryos model. We inoculated 13-day-old chicken embryos with Salmonella Pullorum (SP) and then evaluated survival for the presence of phage M13 or E. coli ER2738 (ECR) infected with M13. We found that the ECR bacterium inhibits SP multiplication in 0.32 (M13-infected ECR) or 0.44 log UFC/mL (M13-uninfected ECR) and that the ECR-free phage M13 from the PD library can be used in chicken embryo models. This work provides the use of the chicken embryo as a model to study systemic infection and can be employed as an analysis tool for various peptides that M13 can express from PD selection. KEY POINTS: • SP-infected chicken embryo can be a helpful model of systemic infection for different tests. • Phage M13 does not lead to embryonic mortality or cause serious injury to embryos. • Phage M13 from the PD library can be used in chicken embryo model tests.

Perspectives on chick embryo models in developmental and reproductive toxicity screening

Teratology, the study of congenital anomalies and their causative factors intersects with developmental and reproductive toxicology, employing innovative methodologies. Evaluating the potential impacts of teratogens on fetal development and assessing human risk is an essential prerequisite in preclinical research. The chicken embryo model has emerged as a powerful tool for understanding human embryonic development due to its remarkable resemblance to humans. This model offers a unique platform for investigating the effects of substances on developing embryos, employing techniques such as ex ovo and in ovo assays, chorioallantoic membrane assays, and embryonic culture techniques. The advantages of chicken embryonic models include their accessibility, cost-effectiveness, and biological relevance to vertebrate development, enabling efficient screening of developmental toxicity. However, these models have limitations, such as the absence of a placenta and maternal metabolism, impacting the study of nutrient exchange and hormone regulation. Despite these limitations, understanding and mitigating the challenges posed by the absence of a placenta and maternal metabolism are critical for maximizing the utility of the chick embryo model in developmental toxicity testing. Indeed, the insights gained from utilizing these assays and their constraints can significantly contribute to our understanding of the developmental impacts of various agents. This review underscores the utilization of chicken embryonic models in developmental toxicity testing, highlighting their advantages and disadvantages by addressing the challenges posed by their physiological differences from mammalian systems.

A Comparative Analysis of the Cytotoxic and Vascular Activity Effects of Western Diamondback Rattlesnake (Crotalus atrox) and Eastern Diamondback Rattlesnake (Crotalus adamanteus) Venoms Using a Chick Embryo Model

Crotalus snakebites induce various toxicological effects, encompassing neurological, myotoxic, and cytotoxic symptoms, with potentially fatal outcomes. Investigating venom toxicity is essential for public health, and developing new tools allows for these effects to be studied more comprehensively. The research goals include the elucidation of the physiological consequences of venom exposure and the assessment of toxicity using animal models. Chicken embryos serve as valuable models for assessing venom toxicity through the chick embryotoxicity screening test (CHEST) and the chick chorioallantoic membrane (CAM) assay, particularly useful for evaluating vascular impacts. C. adamanteus venom application resulted in higher embryotoxicity and morphological abnormalities, such as Siamese twins. The CAM assay demonstrated the hemorrhagic effects of venom, varying with venom type and concentration. The irritant potential of both venom types was classified as slight or moderate depending on their concentration. Additionally, acetylcholinesterase (AChE) activity was performed to receive information about organ toxicity. The results show that both venoms induced changes in the whole embryo, heart, and liver weights, but the C. adamanteus venom was identified as more toxic. Specific venom concentrations affected AChE activity in embryonic tissues. These findings underscore the embryotoxic and vasoactive properties of Crotalus venoms, providing valuable insights into their mechanisms of toxicity and potential applications in biomedicine.

Valproic Acid in Pregnancy Revisited: Neurobehavioral, Biochemical and Molecular Changes Affecting the Embryo and Fetus in Humans and in Animals: A Narrative Review

Valproic acid (VPA) is a very effective anticonvulsant and mood stabilizer with relatively few side effects. Being an epigenetic modulator, it undergoes clinical trials for the treatment of advanced prostatic and breast cancer. However, in pregnancy, it seems to be the most teratogenic antiepileptic drug. Among the proven effects are congenital malformations in about 10%. The more common congenital malformations are neural tube defects, cardiac anomalies, urogenital malformations including hypospadias, skeletal malformations and orofacial clefts. These effects are dose related; daily doses below 600 mg have a limited teratogenic potential. VPA, when added to other anti-seizure medications, increases the malformations rate. It induces malformations even when taken for indications other than epilepsy, adding to the data that epilepsy is not responsible for the teratogenic effects. VPA increases the rate of neurodevelopmental problems causing reduced cognitive abilities and language impairment. It also increases the prevalence of specific neurodevelopmental syndromes like autism (ASD) and Attention Deficit Hyperactivity Disorder (ADHD). High doses of folic acid administered prior to and during pregnancy might alleviate some of the teratogenic effect of VPA and other AEDs. Several teratogenic mechanisms are proposed for VPA, but the most important mechanisms seem to be its effects on the metabolism of folate, SAMe and histones, thus affecting DNA methylation. VPA crosses the human placenta and was found at higher concentrations in fetal blood. Its concentrations in milk are low, therefore nursing is permitted. Animal studies generally recapitulate human data.

Exploring the Expression and Function of cTyro3, a Candidate Zika Virus Receptor, in the Embryonic Chicken Brain and Inner Ear

The transmembrane protein Axl was proposed as an entry receptor for Zika virus (ZIKV) infection in vitro, but conflicting results from in vivo studies have made it difficult to establish Axl as a physiologically relevant ZIKV receptor. Both the functional redundancy of receptors and the experimental model used can lead to variable results. Therefore, it can be informative to explore alternative animal models to analyze ZIKV receptor candidates as an aid in discovering antivirals. This study used chicken embryos to examine the role of chicken Tyro3 (cTyro3), the equivalent of human Axl. Results show that endogenous cTyro3 mRNA expression overlaps with previously described hot spots of ZIKV infectivity in the brain and inner ear. We asked if ectopic expression or knockdown of cTyro3 influenced ZIKV infection in embryos. Tol2 vectors or replication-competent avian retroviruses were used in ovo to introduce full-length or truncated (presumed dominant-negative) cTyro3, respectively, into the neural tube on embryonic day two (E2). ZIKV was delivered to the brain 24 h later. cTyro3 manipulations did not alter ZIKV infection or cell death in the E5/E6 brain. Moreover, delivery of truncated cTyro3 variants to the E3 otocyst had no effect on inner ear formation on E6 or E10.

Bridging the In Vitro to In Vivo gap: Using the Chick Embryo Model to Accelerate Nanoparticle Validation and Qualification for In Vivo studies

We postulate that nanoparticles (NPs) for use in therapeutic applications have largely not realized their clinical potential due to an overall inability to use in vitro results to predict NP performance in vivo. The avian embryo and associated chorioallantoic membrane (CAM) has emerged as an in vivo preclinical model that bridges the gap between in vitro and in vivo, enabling rapid screening of NP behavior under physiologically relevant conditions and providing a rapid, accessible, economical, and more ethical means of qualifying nanoparticles for in vivo use. The CAM is highly vascularized and mimics the diverging/converging vasculature of the liver, spleen, and lungs that serve as nanoparticle traps. Intravital imaging of fluorescently labeled NPs injected into the CAM vasculature enables immediate assessment and quantification of nano-bio interactions at the individual NP scale in any tissue of interest that is perfused with a microvasculature. In this review, we highlight how utilization of the avian embryo and its CAM as a preclinical model can be used to understand NP stability in blood and tissues, extravasation, biocompatibility, and NP distribution over time, thereby serving to identify a subset of NPs with the requisite stability and performance to introduce into rodent models and enabling the development of structure-property relationships and NP optimization without the sacrifice of large populations of mice or other rodents. We then review how the chicken embryo and CAM model systems have been used to accelerate the development of NP delivery and imaging agents by allowing direct visualization of targeted (active) and nontargeted (passive) NP binding, internalization, and cargo delivery to individual cells (of relevance for the treatment of leukemia and metastatic cancer) and cellular ensembles (e.g., cancer xenografts of interest for treatment or imaging of cancer tumors). We conclude by showcasing emerging techniques for the utilization of the CAM in future nano-bio studies.

Towards Tabula Gallus

The Tabula Gallus is a proposed project that aims to create a map of every cell type in the chicken body and chick embryos. Chickens (Gallus gallus) are one of the most recognized model animals that recapitulate the development and physiology of mammals. The Tabula Gallus will generate a compendium of single-cell transcriptome data from Gallus gallus, characterize each cell type, and provide tools for the study of the biology of this species, similar to other ongoing cell atlas projects (Tabula Muris and Tabula Sapiens/Human Cell Atlas for mice and humans, respectively). The Tabula Gallus will potentially become an international collaboration between many researchers. This project will be useful for the basic scientific study of Gallus gallus and other birds (e.g., cell biology, molecular biology, developmental biology, neuroscience, physiology, oncology, virology, behavior, ecology, and evolution). It will eventually be beneficial for a better understanding of human health and diseases.