Ontogeny of the hypothalamo-pituitary-adrenocortical axis in the chicken embryo: a review

Corticosterone release in very young siblicidal seabird chicks (Rissa tridactyla) is sensitive to environmental variability and responds rapidly and robustly to external challenges

In birds, patterns of development of the adrenocortical response to stressors vary among individuals, types of stressors, and species. Since there are benefits and costs of exposure to elevated glucocorticoids, this variation is presumably a product of selection such that animals modulate glucocorticoid secretion in contexts where doing so increases their fitness. In this study, we evaluated hypothalamo-pituitary-adrenal (HPA) activity in first-hatched free-living seabird nestlings that engage in intense sibling competition and facultative siblicide (black-legged kittiwakes, Rissa tridactyla). We sampled 5 day old chicks (of the ∼45 day nestling period), a critical early age when food availability drives establishment of important parent-offspring and intra-brood dynamics. We experimentally supplemented parents with food ("supplemented") and measured chick baseline corticosterone secretion and capacity to rapidly increase corticosterone in response to an acute challenge (handling and 15 min of restraint in a bag). We also used topical administration of corticosterone to evaluate the ability of chicks to downregulate physiologically relevant corticosterone levels on a short time scale (minutes). We found that 5 day old chicks are not hypo-responsive but release corticosterone in proportion to the magnitude of the challenge, showing differences in baseline between parental feeding treatments (supplemented vs non-supplemented), moderate increases in response to handling, and a larger response to restraint (comparable to adults) that also differed between chicks from supplemented and control nests. Topical application of exogenous corticosterone increased circulating levels nearly to restraint-induced levels and induced downregulation of HPA responsiveness to the acute challenge of handling. Parental supplemental feeding did not affect absorbance/clearance or negative feedback. Thus, while endogenous secretion of corticosterone in young chicks is sensitive to environmental context, other aspects of the HPA function, such as rapid negative feedback and/or the ability to clear acute elevations in corticosterone, are not. We conclude that 5 day old kittiwake chicks are capable of robust adrenocortical responses to novel challenges, and are sensitive to parental food availability, which may be transduced behaviorally, nutritionally, or via maternal effects. Questions remain about the function of such rapid, large acute stress-induced increases in corticosterone in very young chicks.

Pre-natal exposure to glucocorticoids causes changes in developmental circadian clock gene expression and post-natal behaviour in the Japanese quail

The embryonic environment is critical in shaping developmental trajectories and consequently post-natal phenotypes. Exposure to elevated stress hormones during this developmental stage is known to alter a variety of post-natal phenotypic traits, and it has been suggested that pre-natal stress can have long term effects on the circadian rhythm of glucocorticoid hormone production. Despite the importance of the circadian system, the potential impact of developmental glucocorticoid exposure on circadian clock genes, has not yet been fully explored. Here, we showed that pre-natal exposure to corticosterone (CORT, a key glucocorticoid) resulted in a significant upregulation of two key hypothalamic circadian clock genes during the embryonic period in the Japanese quail (Coturnix japonica). Altered expression was still present 10 days into post-natal life for both genes, but then disappeared by post-natal day 28. At post-natal day 28, however, diel rhythms of eating and resting were influenced by exposure to pre-natal CORT. Males exposed to pre-natal CORT featured an earlier acrophase, alongside spending a higher proportion of time feeding. Females exposed to pre-natal CORT featured a less pronounced shift in acrophase and spent less time eating. Both males and females exposed to pre-natal CORT spent less time inactive during the day. Pre-natal CORT males appeared to feature a delay in peak activity levels. Our novel data suggest that these circadian clock genes and aspects of diurnal behaviours are highly susceptible to glucocorticoid disruption during embryonic development, and these effects are persistent across developmental stages, at least into early post-natal life.

Investigating the potential role of the pituitary adenylate cyclase-activating polypeptide (PACAP) in regulating the ubiquitin signaling pathway in poultry

The physiological processes in animal production are regulated through biologically active molecules like peptides, proteins, and hormones identified through the development of the fundamental sciences and their application. One of the main polypeptides that plays an essential role in regulating physiological responses is the pituitary adenylate cyclase-activating polypeptide (PACAP). PACAP belongs to the glucagon/growth hormone-releasing hormone (GHRH)/vasoactive intestinal proteins (VIP) family and regulates feed intake, stress, and immune response in birds. Most of these regulations occur after PACAP stimulates the cAMP signaling pathway, which can regulate the expression of genes like MuRF1, FOXO1, Atrogin 1, and other ligases that are essential members of the ubiquitin system. On the other hand, PACAP stimulates the secretion of CRH in response to stress, activating the ubiquitin signaling pathway that plays a vital role in protein degradation and regulates oxidative stress and immune responses. Many studies conducted on rodents, mammals, and other models confirm the regulatory effects of PACAP, cAMP, and the ubiquitin pathway; however, there are no studies testing whether PACAP-induced cAMP signaling in poultry regulates the ubiquitin pathway. Besides, it would be interesting to investigate if PACAP can regulate ubiquitin signaling during stress response via CRH altered by HPA axis stimulation. Therefore, this review highlights a summary of research studies that indicate the potential interaction of the PACAP and ubiquitin signaling pathways on different molecular and physiological parameters in poultry species through the cAMP and stress signaling pathways.

In ovo injection of testosterone to yolk sac modulates early posthatching development and physiology of male chick in broilers

The aim of this study was to investigate the effects of in ovo testosterone injection into the yolk sac of embryos on physiology and development of broiler chicks during the early posthatching period. A total of 1,010 hatching eggs were obtained from the Ross genotype. Trial design was conducted with a noninjected group (control) and injection groups in which 100 µL sesame oil, or 100 µL sesame oil + 0.50 µmol testosterone were injected into the yolk sac of the embryo on d 6 or d 12 of incubation. Testosterone hormone level was measured in the egg yolk and albumen at onset of incubation, in the yolk sac on d 19 of incubation and in the residual yolk sac at hatching. Weights of chick, yolk sac and organ, morphological traits (body length, lengths of bilateral traits and beak length), asymmetrical development of bilateral morphological traits and body mass index were measured at hatching and on d 7 after hatching. Testosterone, corticosterone and growth hormone levels were determined in blood plasma obtained from male chicks at hatching and on d 7 of chick age. Chick weight was not affected, plasma testosterone level and brain weight decreased, while body mass index, plasma corticosterone and growth hormone levels increased by administering 0.50 µmol testosterone on d 12 of embryonic age. However, plasma testosterone and growth hormone levels did not change, chick weight increased, while plasma corticosterone level and the chick body length decreased by administering 0.50 µmol testosterone on d 6 of embryonic age. A significant interaction between chick age and in ovo testosterone administration resulted in an increase in lung weight of chicks. In conclusion, this study found that in ovo testosterone administered at different embryonic ages due to age-specific effects of testosterone in the yolk sac of embryo modulates development related to physiological parameters of male broiler chicks during early posthatching period.

Extraembryonic metabolism of corticosterone protects against effects of exposure

When females experience stress during reproduction, developing embryos can be exposed to elevated levels of glucocorticoids, which can permanently affect offspring development, physiology, and behavior. However, the embryo can regulate exposure to glucocorticoids. In placental species, the placenta regulates embryonic exposure to maternal steroids via metabolism. In a comparable way, recent evidence has shown the extraembryonic membranes of avian species also regulate embryonic exposure to a number of maternal steroids deposited in the yolk via metabolism early in development. However, despite the known effects of embryonic exposure to glucocorticoids, it is not yet understood how glucocorticoids are metabolized early in development. To address this knowledge gap, we injected corticosterone into freshly laid chicken (Gallus gallus) eggs and identified corticosterone metabolites, located metabolomic enzyme transcript expression, tracked metabolomic enzyme transcript expression during the first six days of development, and determined the effect of corticosterone and metabolites on embryonic survival. We found that yolk corticosterone was metabolized before day four of development into two metabolites: 5β-corticosterone and 20β-corticosterone. The enzymes, AKR1D1 and CBR1 respectively, were expressed in the extraembryonic membranes. Expression was dynamic during early development, peaking on day two of development. Finally, we found that corticosterone exposure is lethal to the embryos, yet exposure to the metabolites is not, suggesting that metabolism protects the embryo. Ultimately, we show that the extraembryonic membranes of avian species actively regulate their endocrine environment very early in development.

The effects of pre-hatch elevated corticosterone and post-hatch restrictive food availability on the HPA axis development of mallard ducks (Anas platyrhynchos)

Environmental changes can be stressors (altered habitat and food supply, climate change, etc.) to wild animals. Stressors trigger the hypothalamic pituitary adrenal (HPA) axis to release corticosterone (CORT) which modifies energy homeostasis. During nesting, stressed females can deposit increased concentrations of CORT into eggs, altering egg viability and offspring characteristics, constituting a significant mechanism regulating population productivity in subsequent generations. In this study, increased maternal disposition of CORT was mimicked through a 15 ng/g in ovo injection of CORT into mallard duck eggs. Growth and HPA axis function were measured during post-hatch development. For growth, changes in mass were compared at hatch, 7 weeks and 11 weeks. The HPA axis was assessed at seven weeks by measuring CORT at baseline, followed by restraint stress, dexamethasone (negative feedback) and ACTH (maximal adrenal capacity) challenges. At eleven weeks of age, ducks were subjected to a 6-day 25% feed reduction to simulate a poor quality environment to evaluate response to a chronic stressor by comparing CORT at baseline and after restraint stress. Growth and CORT concentration did not differ between treatments at seven weeks or after feed restriction (11 weeks). The CORT dosage administered did not appear to affect HPA axis development in ducklings. Mallards are a highly adaptable species and may have overcome any early alterations to their phenotype. Further research is needed to determine the effects of increased maternal CORT on growth and the development of the HPA axis in ducks. SUMMARY STATEMENT: This study examines how maternal stress (simulated through elevated corticosterone in ovo) and post-hatch chronic stressors (food restriction) affect the development of the HPA axis in a precocial bird.

Mapping of corticotropin-releasing factor, receptors, and binding protein mRNA in the chicken telencephalon throughout development

Understanding the neural mechanisms that regulate the stress response is critical to know how animals adapt to a changing world and is one of the key factors to be considered for improving animal welfare. Corticotropin-releasing factor (CRF) is crucial for regulating physiological and endocrine responses, triggering the activation of the sympathetic nervous system and the hypothalamo-pituitary-adrenal axis (HPA) during stress. In mammals, several telencephalic areas, such as the amygdala and the hippocampus, regulate the autonomic system and the HPA responses. These centers include subpopulations of CRF containing neurons that, by way of CRF receptors, play modulatory roles in the emotional and cognitive aspects of stress. CRF binding protein also plays a role, buffering extracellular CRF and regulating its availability. CRF role in activation of the HPA is evolutionary conserved in vertebrates, highlighting the relevance of this system to help animals cope with adversity. However, knowledge on CRF systems in the avian telencephalon is very limited, and no information exists on detailed expression of CRF receptors and binding protein. Knowing that the stress response changes with age, with important variations during the first week posthatching, the aim of this study was to analyze mRNA expression of CRF, CRF receptors 1 and 2, and CRF binding protein in chicken telencephalon throughout embryonic and early posthatching development, using in situ hybridization. Our results demonstrate an early expression of CRF and its receptors in pallial areas regulating sensory processing, sensorimotor integration and cognition, and a late expression in subpallial areas regulating the stress response. However, CRF buffering system develops earlier in the subpallium than in the pallium. These results help to understand the mechanisms underlying the negative effects of noise and light during prehatching stages in chicken, and suggest that stress regulation becomes more sophisticated with age.

In ovo corticosterone exposure does not influence yolk steroid hormone relative abundance or skeletal muscle development in the embryonic chicken

In ovo corticosterone (CORT) exposure reportedly reduces growth and alters body composition traits in meat-type chickens. However, the mechanisms governing alterations in growth and body composition remain unclear but could involve myogenic stem cell commitment, and/or the presence of yolk steroid hormones. This study investigated whether in ovo CORT exposure influenced yolk steroid hormone content, as well as embryonic myogenic development in meat-type chickens. Fertile eggs were randomly divided at embryonic day (ED) 11 and administered either a control (CON; 100 µL of 10 mM PBS) or CORT solution (100 µL of 10 mM PBS containing 1 µg CORT) into the chorioallantoic membrane. Yolk samples were collected at ED 0 and ED 5. At ED 15 and hatch, embryos were humanely killed, and yolk and breast muscle (BM) samples were collected. The relative abundance of 15 steroid hormones, along with total lipid content was measured in yolk samples collected at ED 0, ED 5, ED 15, and ED 21. Muscle fiber number, cross-sectional area, and fascicle area occupied by muscle fibers were measured in BM samples collected at hatch. Relative expression of MyoD, MyoG, Pax7, PPARγ, and CEBP/β, and the sex steroid receptors were measured in BM samples collected at hatch. The administration of CORT had a limited effect on yolk steroid hormones. In ovo CORT significantly reduced fascicle area occupied by muscle fibers and CEBP/β expression was increased in CORT exposed birds at hatch. In addition, the quantity of yolk lipid was significantly reduced in CORT-treated birds. In conclusion, in ovo exposure to CORT does not appear to influence early muscle development through yolk steroid hormones in embryonic meat-type chickens however, the results provide a comprehensive analysis of the composition of yolk steroid hormones in ovo at different developmental time points. The findings may suggest increased mesenchymal stem cell commitment to the adipogenic lineage during differentiation and requires further investigation.

Effect of photoperiod during incubation on embryonic temperature, hatch traits and performance of two commercial broiler strains

Provision of light during incubation has shown the potential to enhance hatching traits and affect posthatch productivity, physiology, and behavior. In this study, 2 repeated trials were conducted to investigate the effect of photoperiod and strain on the embryo temperature, hatching traits and posthatch growth performance of 2 commercial strains of broilers (Ross 308 and Cobb 500). In each trial, hatching eggs were randomly distributed into 6 incubators with 3 photoperiod treatments: blue LED light for 12 h d^-1 (12L:12D) or 18 h d^-1 (18L:6D) during entire incubation were compared with no illumination condition (DARK). Data were analyzed as a 3 × 2 factorial arrangement with the trial as the blocking factor. Embryos incubated under 12L:12D and 18L:6D had lower air cell temperature (P < 0.05) than the DARK embryos from d 13 of incubation onward except on the day of candling. The response of air cell temperature to periodic illumination differed between 2 strains. Cobb embryos had lower air cell temperature in 12L:12D than those incubated with 18L:6D from d 16 of incubation onward, whereas lower air cell temperature was found in Ross embryos when illuminated with 18L:6D photoperiod compared to those under 12L:12D. The 12L:12D treatment was associated with improved (P < 0.05) navel closure condition of hatchlings. There were no differences in hatchability, embryo mortality, body weight, or length at hatch among photoperiod groups or its combination with strain. No differences in production parameters were found between DARK and illuminated groups. However, 12L:12D had heavier (P < 0.05) body weight on d 14 of age and higher (P < 0.05) body weight gain than 18L:6D from d 7 to 14 of age. The results of this study indicate that providing blue LED light up to 18 h d^-1 has no detrimental effect on production of broilers, however, 12L:12D light regime improved chick quality at hatch compared to DARK and resulted in heavier birds by d 14 compared to 18L:6D.

Embryonic modulation through thermal manipulation and in ovo feeding to develop heat tolerance in chickens

Healthy chickens are necessary to meet the ever-increasing demand for poultry meat. Birds are subjected to numerous stressful conditions under commercial rearing systems, including variations in the environmental temperature. However, it is difficult to counter the effects of global warming on the livestock industry. High environmental temperature is a stressful condition that has detrimental effects on growth and production performance, resulting in decreased feed intake, retarded growth, compromised gut health, enhanced oxidative stress, and altered immune responses. Traditional approaches include nutritional modification and housing management to mitigate the harmful effects of hot environments. Currently, broiler chickens are more susceptible to heat stress (HS) than layer chickens because of their high muscle mass and metabolic rate. In this review, we explored the possibility of in ovo manipulation to combat HS in broiler chickens. Given their short lifespan from hatching to market age, embryonic life is thought to be one of the critical periods for achieving these objectives. Chicken embryos can be modulated through either temperature treatment or nourishment to improve thermal tolerance during the rearing phase. We first provided a brief overview of the harmful effects of HS on poultry. An in-depth evaluation was then presented for in ovo feeding and thermal manipulation as emerging strategies to combat the negative effects of HS. Finally, we evaluated a combination of the two methods using the available data. Taken together, these investigations suggest that embryonic manipulation has the potential to confer heat resistance in chickens.

Long-Lasting Effects of Incubation Temperature During Fetal Development on Subcutaneous Adipose Tissue of Broilers

Increasing evidence indicates that fetal programming may cause permanent effects on offspring adipose tissue and body composition. Previous study showed reduction in newly-hatched broiler chick adiposity by manipulating incubation temperature during fetal development. The present study examined whether incubation temperature during fetal development has long-term effects on post-hatching fat deposition in broilers. Broiler breeder eggs (Cobb-500^®) were incubated under constant low (36°C, LT), control (37.5°C, CT) or high (39°C, HT) temperature from day 13 onward, giving to eggshell temperature of 37.3 ± 0.08°C, 37.8 ± 0.2°C, and 38.8 ± 0.3°C, respectively. Male chicks were reared under recommended temperatures until 42 days old. LT 21 days old broilers exhibited higher blood cholesterol than CT broilers, and higher triglycerids, VLDL, and LDL, and lower HDL than CT and HT broilers. LT broilers presented higher liver cholesterol than CT broilers and lower ether extract percentage than CT broilers. Adipocyte count was lower in the abdomen than in the thigh. Until day 21 of age, feed intake was higher in LT than in HT broilers. At day 42 of age, blood cholesterol and LDL were higher in HT broilers than in CT and LT broilers. Liver cholesterol was higher in LT than in HT broilers. LT treatment reduced neck and increased thigh adipocyte size compared to CT treatment, while the HT treatment reduced abdomen and neck adipocyte size compared to other two treatments and in the thigh compared to LT treatment. In CT broilers, thigh adipocytes were smaller than abdomen and neck adipocytes. HT treatment increased adipocyte number per area in the neck compared to LT and CT treatment, and LT and HT treatments reduced adipocyte count in the thigh compared to CT treatment. CT broilers presented higher adipocyte count in the thigh than the abdomen and neck, while HT broilers presented higher adipocyte count in the neck than the abdomen and thigh. Cell proliferation was lower in the abdomen than in the thigh. The results show incubation temperature manipulation during fetal development has long-term and distinct effects on regional adiposity, and can be used to modulate broiler fat deposition.

Perspective: Chicken Models for Studying the Ontogenetic Origin of Neuropsychiatric Disorders

Nutrients and xenobiotics cross the blood–placenta barrier, potentially depositing in the fetal brain. The prenatal exposure affects the neuroendocrine and microbial development. The mechanism underlying maternal risk factors reprograming the microbiota–gut–brain axis with long-term effects on psychosocial behaviors in offspring is not clear. In humans, it is not possible to assess the nutrient or xenobiotic deposition in the fetal brain and gastrointestinal system for ethical reasons. Moreover, the maternal–fetal microbe transfer during gestation, natural labor, and breast-feeding constitutes the initial gut microbiome in the progeny, which is inevitable in the most widely utilized rodent models. The social predisposition in precocial birds, including chickens, provides the possibility to test behavioral responses shortly after being hatched. Hence, chickens are advantageous in investigating the ontogenetic origin of behaviors. Chicken embryos are suitable for deposition assessment and mechanistic study due to the accessibility, self-contained development, uniform genetic background, robust microbiota, and easy in vivo experimental manipulation compared to humans and rodents. Therefore, chicken embryos can be used as an alternative to the rodent models in assessing the fetal exposure effect on neurogenesis and investigating the mechanism underlying the ontogenetic origin of neuropsychiatric disorders.

Remodeling of Hepatocyte Mitochondrial Metabolism and De Novo Lipogenesis During the Embryonic-to-Neonatal Transition in Chickens

Embryonic-to-neonatal development in chicken is characterized by high rates of lipid oxidation in the late-term embryonic liver and high rates of de novo lipogenesis in the neonatal liver. This rapid remodeling of hepatic mitochondrial and cytoplasmic networks occurs without symptoms of hepatocellular stress. Our objective was to characterize the metabolic phenotype of the embryonic and neonatal liver and explore whether these metabolic signatures are preserved in primary cultured hepatocytes. Plasma and liver metabolites were profiled using mass spectrometry based metabolomics on embryonic day 18 (ed18) and neonatal day 3 (nd3). Hepatocytes from ed18 and nd3 were isolated and cultured, and treated with insulin, glucagon, growth hormone and corticosterone to define hormonal responsiveness and determine their impacts on mitochondrial metabolism and lipogenesis. Metabolic profiling illustrated the clear transition from the embryonic liver relying on lipid oxidation to the neonatal liver upregulating de novo lipogenesis. This metabolic phenotype was conserved in the isolated hepatocytes from the embryos and the neonates. Cultured hepatocytes from the neonatal liver also maintained a robust response to insulin and glucagon, as evidenced by their contradictory effects on lipid oxidation and lipogenesis. In summary, primary hepatocytes from the embryonic and neonatal chicken could be a valuable tool to investigate mechanisms regulating hepatic mitochondrial metabolism and de novo lipogenesis.

Effect of Stressors on the mRNA Expressions of Neurosecretory Protein GL and Neurosecretory Protein GM in Chicks

We recently discovered novel cDNAs encoding the precursors of two small secretory proteins, neurosecretory protein GL (NPGL) and neurosecretory protein GM (NPGM), in the mediobasal hypothalamus (MBH) of chickens. In addition, we found colocalization of NPGL, NPGM, and histidine decarboxylase (HDC; histamine-producing enzyme) in same neurons of the medial mammillary nucleus of the hypothalamus. In this study, we elucidated the effect of several stresses, including food deprivation, environmental heat, inflammation, and social isolation, on the mRNA expression of NPGL, NPGM, and HDC in chicks using real-time PCR. Food deprivation for 24 h increased NPGM mRNA expression in the MBH. On the other hand, an environmental temperature of 37°C for 24 h did not affect their mRNA expression. Six hours after intraperitoneal injection of lipopolysaccharide, an inducer of inflammation, the mRNA expression of NPGM, but not that of NPGL and HDC increased. Social isolation for 3 h induced an increase in the mRNA expression of NPGL, NPGM, and HDC. These results indicate that NPGM, but not NPGL or HDC, may participate in several physiological responses to stress in chicks.

Identification of different critical embryonic periods to modify egg incubation temperature in mule ducks

Egg incubation of mule ducks, mainly used for fatty liver production, is one of the critical phases in this sector. Based on hatching rate, the best incubation parameters have already been well described for poultry, but the literature on ducks is lacking. In this study, we tested different incubation conditions by varying two important factors, temperature and relative humidity, in mule ducks. These variations were applied at different periods during embryogenesis in order to measure the impact of environmental disturbances on different zootechnical performances. The temperature was increased by 1.5 °C (16 h/24) and the relative humidity was set up to 65%, during 10 days. Six 10-day developmental windows were tested, from embryonic day 9 to embryonic day 14. Our results are in line with previous reports showing that increasing incubation temperature, even when relative humidity is adjusted, can have a negative impact on duck embryonic mortality up to 24.5% for the condition E10-E20 (P < 10^-5). However, the hatchability can be maintained at the level of the control groups when these modifications are applied on the latest windows (from the 11th embryonic day). Sex ratio, hatching BW, and internal temperature are also sensitive to these incubation changes, and their modification could have a major impact on later zootechnical performance. These results should contribute to the development or embryonic temperature programming approaches, especially for the fatty liver production industry.

Telencephalic regulation of the HPA axis in birds

The hypothalamo-pituitary-adrenal (HPA) axis is one of the major output systems of the vertebrate stress response. It controls the release of cortisol or corticosterone from the adrenal gland. These hormones regulate a range of processes throughout the brain and body, with the main function of mobilizing energy reserves to improve coping with a stressful situation. This axis is regulated in response to both physical (e.g., osmotic) and psychological (e.g., social) stressors. In mammals, the telencephalon plays an important role in the regulation of the HPA axis response in particular to psychological stressors, with the amygdala and part of prefrontal cortex stimulating the stress response, and the hippocampus and another part of prefrontal cortex inhibiting the response to return it to baseline. Birds also mount HPA axis responses to psychological stressors, but much less is known about the telencephalic areas that control this response. This review summarizes which telencephalic areas in birds are connected to the HPA axis and are known to respond to stressful situations. The conclusion is that the telencephalic control of the HPA axis is probably an ancient system that dates from before the split between sauropsid and synapsid reptiles, but more research is needed into the functional relationships between the brain areas reviewed in birds if we want to understand the level of this conservation.

The morphological and histological study of chicken left ovary during growth and development among Hy-line brown layers of different ages

Chicken ovaries are known to develop asymmetrically and only the left ovary fully develops. Although both have been greatly investigated, a gap in scientific reports is still felt between 2-mo-old and sexual maturity. In this study, we aimed at investigating the changes in components that occur during growth to analyze the morphohistological correlation between the left ovary and the follicle development at different age stages in Gallus domesticus. The ovaries were harvested from 60 chickens aged 1 and 3-wk-old, 1, 2, 3, and 4-mo-old (n = 10 per age group), then fixed in AAF solution. Hematoxylin-and Eosin protocol was used to stain the tissue for microscopic observations. Results revealed that the left ovary exhibited an ovarian tissue, a site of follicular growth that displayed various shapes from smooth to greatly indented as the follicles differentiated. Atretic follicles at various regression stages were noticed frequently as the chicks grew in age from 3-wk-old onward along with their differentiation. Rete ovarii, remnants from the male homologs were observed throughout the whole study showing epoöphoron, connecting rete, and gland-like structures that tend to diminish with age. The feature of the left ovary is closely related to the follicular developmental stage, and the bigger and differentiated the follicles are, the more indented and irregular its epithelium appears. Atresia is a normal physiological process that we observed throughout the whole study. Also that, rete ovarii do not spontaneously arise in the ovary but it develops and grows in juvenile chicken as well as in adult ones.

Effect of contact incubation on stress, behavior and body composition in the precocial Red jungle fowl

Birds use contact incubation to warm their eggs above ambient temperature required for embryonic development. In contrast, birds in the industry as well as many birds in breeding programs and scientific studies are incubated in conventional incubators that warm eggs via circulating warm air. This means that contact incubated eggs have different thermal properties than eggs incubated in a conventional incubator. In light of previous studies showing that small differences in incubation temperature can affect chicks post-hatching phenotype, we investigated the consequences of incubating Red jungle fowl eggs at the same temperature (37 °C) either via contact incubation or warm air incubation. We found that contact incubated chicks had a more robust body composition, were more explorative and had a higher temperature preference early in life, as well as a sex dependent difference in plasma Corticosterone levels pre-hatch (measured in down-feathers) and post-hatch (measured in plasma) compared to chicks incubated in a conventional warm air incubator. While previous studies have demonstrated that embryonic development and post-hatch phenotype is sensitive to small variations in temperature, our study demonstrates for the first time that the way heat is distributed to the egg has a similar magnitude of effect on post-hatch phenotype and highlights the sensitivity of the incubation period in shaping birds post-hatch phenotype.

Corticosterone does not have a role in temperature sex reversal in the central bearded dragon (Pogona vitticeps)

Environmental sex determination (ESD) is common among ectothermic vertebrates. The stress axis and production of stress hormones (corticosteroids) regulates ESD in fish, but evidence of a similar influence in reptiles is sparse and conflicting. The central bearded dragon (Pogona vitticeps) has a system of sex determination involving the interplay between sex chromosomes (ZZ/ZW female heterogamety) and the thermal environment. High egg incubation temperatures induce sex reversal of the ZZ genotype, feminizing chromosomally male individuals. Here we show that corticosterone elevation is not associated with sex reversal in the central bearded dragon, either during embryonic development or adulthood. We also demonstrate experimentally that sex determination is not affected by corticosterone injection into the yolk. This strongly suggests that stress axis upregulation by high temperature during incubation does not cause sex reversal in P. vitticeps. Our work is in general agreement with other research in reptiles, which suggests that the stress axis does not mediate sex in reptiles with ESD. Alternative biological systems may be responsible for capturing environmental conditions during reptile development, such as cellular calcium and redox regulation or the action of temperature-sensitive splicing factors.

Effects of acute stressors experienced by five strains of layer breeders on measures of stress and fear in their offspring

Stressors experienced by layer breeders during egg production can lead to changes in the egg hormone content, potentially impacting their offspring, the commercial layers. Genetic differences might also affect the offspring's susceptibility to maternal experiences. In this study, we tested if maternal stress affects measures of stress and fear in five strains of layer breeders: commercial brown 1 & 2, commercial white 1 & 2 and a pure line White Leghorn. Each strain was equally separated into two groups: "Maternal Stress" (MS), where hens were subjected to a series of 8 consecutive days of acute psychological stressors, and "Control," which received routine husbandry. Additional eggs from Control were injected either with corticosterone diluted in a vehicle solution ("CORT") or just "Vehicle." Stress- and fear-responses of the offspring were measured in a plasma corticosterone test and a combined human approach and novel object test. While the stress treatments did not  affect the measured endpoints in the offspring, significant strain differences were found. The offspring of the white strains showed a higher physiological response compared to brown strains and the White 2 offspring was the least fearful strain in the human approach test. Our study found that neither the acute psychological stressors experienced by layer breeders nor the egg injections of corticosterone affected the parameters tested in their offspring. Post hoc power analyses suggest that the lack of treatment effects might be due to a small sample size (type II error). Although studies on larger flocks of layers are still needed, our results provide an initial understanding of an important subject, as in poultry production, layer breeders are often subjected to short-term stressors. In addition, our results suggest the dissociation between the physiological and behavioural parameters of stress response in laying hens, showing that increased concentrations of plasma corticosterone in response to stress might not be directly associated with high levels of fear.

Neither altered incubation temperature during fetal development nor preferred rearing temperature improves leg bone characteristics of broilers

The present study evaluated whether broiler femoral and tibiotarsal characteristics (as assessed at slaughter age) could be improved if birds were reared under their preferred temperature and whether continuous high or low incubation temperature during the fetal period improves bone characteristics of broilers reared under heat stress or thermal preference. Broiler breeder eggs were incubated from day 13 until hatching under cold (36 °C), control (37.5 °C), or hot (39 °C) temperatures. Under these conditions, the eggshell temperatures were 37.4 ± 0.1°C, 37.8 ± 0.15°C, and 38.8 ± 0.3°C, respectively. Then, broiler chicks were reared under control, preferred (determined previously in thermal preference test), or high temperatures. At day 42 of age, the broilers were weighed and euthanized, and femora and tibiotarsi collected to measure weight, length, diaphysis perimeter, breaking strength, maximum flexion, rigidity, ash, phosphorus, and calcium. Rearing under the preferred temperature did not affect broiler body weight or femoral and tibiotarsal characteristics (P > 0.05). In contrast, high rearing temperature, decreased the body weight, mineral contents of both bones, femoral breaking strength, and tibiotarsal rigidity (P < 0.05). Regarding incubation temperature effects, egg exposure to cold and hot temperatures during the fetal period minimized or avoided a few effects of high rearing temperature, such as those on femoral and tibiotarsal morphological characteristics, mineral composition, and mechanical properties at slaughter age (P < 0.05), but not all. In conclusion, rearing under the preferred broiler temperature did not improve the bone characteristics, and the negative effects of high rearing temperature on bone development were minimized but not completely prevented by high or low temperature incubation during the fetal period.

Hatching phase influences thermal preference of broilers throughout rearing

Here we aimed for the first time to analyse whether opposite hatching patterns associated or not to high incubation temperature from day 13 to hatching interferes with the thermal preference and response of broilers to heat stress throughout the rearing period. Fertile eggs from 56-week-old broiler breeders (Cobb-500^®) were used in a completely randomized trial with a 2x2 factorial arrangement (Short-Long and Long-Short hatching patterns: short time interval between internal and external pipping followed by long time interval between external pipping and hatching, and long time interval between internal and external pipping followed by short time interval between external pipping and hatching, respectively; and control and high incubation temperatures: 37.5°C and 39°C from the 13^rd day, respectively). Thermal manipulation from day 13 was chosen because it is known endocrine axes are already established at this time. At hatching, male chicks were reared in climatical chamber with 16 boxes, maintained at the temperature recommended for this strain, with 4 replicates of 18 chicks per treatment. Broilers with Long-Short hatching pattern and from eggs incubation at 37.5°C preferred the lowest ambient temperature at all analyzed ages, whereas broilers with Short-Long hatching pattern and from eggs incubated at 39°C preferred the highest temperatures from 21 days of age. Heat-exposed broilers showed increased respiratory frequency in all ages analyzed, which should have to contributed to maintainance of their rectal (body) temperature. The hatching patterns did not influence the feed intake, but broilers with Short-Long hatching pattern had better feed conversion, weight gain, and body weight. High incubation temperature reduced the feed consumption, as well as the weight gain and body weight by worsening the feed conversion. The results of this study reveal that hatching patterns associated or not to high incubation temperature influence the broiler thermal preference and heat response throughout the rearing period. Chicks with Long-Short and Short-Long hatching patterns should be reared separately, although this is not practical within a hatcher.

Effects of Maternal Stress on Measures of Anxiety and Fearfulness in Different Strains of Laying Hens

Maternal stress can affect the offspring of birds, possibly due to hormone deposition in the egg. Additionally, phenotypic diversity resulting from domestication and selection for productivity has created a variety of poultry lines that may cope with stress differently. In this study, we investigated the effects of maternal stress on the behavior of different strains of laying hens and the role of corticosterone as its mediator. For this, fertilized eggs of five genetic lines-two brown (Brown 1 and 2), two white (White 1 and 2), and one pure line White Leghorn-were reared identically as four flocks of 27 birds (24F: 3M) per strain. Each strain was equally separated into two groups: Maternal Stress ("MS"), where hens were subjected to a series of daily acute psychological stressors for 8 days before egg collection, and "Control," which received routine husbandry. Fertile eggs from both treatments were collected at three different ages forming different offspring groups that were treated as replicates; additional eggs from Control were injected either with corticosterone diluted in a vehicle solution ("CORT") or just "Vehicle." Eggs from each replicate were incubated and hatched, and offspring (N = 1,919) were brooded under identical conditions. To measure the effects of maternal stress on anxiety and fear-like behavior, offspring were subjected to a social isolation test (SI) between 5 and 10 days of age and a tonic immobility test (TI) at 9 weeks of age. Compared to Control, MS decreased the number of distress vocalizations emitted by White 2 in SI. No effects of MS were observed in TI, and no effects of CORT were observed in any tests. Overall, brown lines vocalized more in SI and remained in TI for a longer duration than white strains, suggesting genetic differences in fear behavior. Females vocalized more than males in TI and showed a trend toward significance for the same trait in SI. Overall, results suggest that the effects of maternal stress on fearfulness are not directly mediated by corticosterone. Moreover, it highlights behavioral differences across various strains of laying hens, suggesting that fear responses are highly dependent on genotype.

Effects of prenatal stress and genetics on embryonic survival and offspring growth of laying hens

Early-life exposure to stressors can shape the phenotype of the offspring resulting in changes that may affect their prehatch and posthatch development. This can be modeled indirectly through maternal exposure to stressors (natural model) or by offspring exposure to stress hormones (pharmacological model). In this study, both models were used to investigate the effects of genetic line on hatchability, late embryonic mortality, sex ratio, and body weight until 17 wk of age. To form the parent stock, fertilized eggs of 4 commercial genetic lines — two brown (brown 1 and 2), two white (white 1 and 2), and a pure line White Leghorn — were incubated, hatched, and housed identically in 4 flocks of 27 birds (24 females and 3 males) per strain. Each strain was equally separated into 2 groups: “maternal stress,” where hens were subjected to a series of acute psychological stressors (e.g., physical restraint, transportation) for 8 D before egg collection, and “control,” where hens received routine husbandry. At 3 maternal ages, fertile eggs from both treatments were collected, and additional eggs from the control group were injected with corticosterone (10 ng/mL egg content) (“CORT”). A “vehicle” treatment was included to account for effects of egg manipulation. Each maternal age comprised a replicate over time. Eggs were incubated and hatched, and the offspring (N = 1,919) were brooded until 17 wk under identical conditions. The results show that prenatal stress interacted with strain to decrease embryonic survival and growth. Among all strains, brown 2 was consistently the most affected line in both prehatch and posthatch development. Our study shows that embryonic survival and offspring growth are mostly affected by the pharmacological model and that strain differences may increase susceptibility to prenatal stress. Moreover, it suggests that the natural stressor model may be useful for quantifying the response of the mother to stressors, whereas the pharmacological model may be useful for quantifying the response of the embryo to increased levels of corticosterone.

Ontogeny of the adrenocortical response in an extremely altricial bird

Life history theory predicts that physiological and behavioral responsiveness to stress should be delayed in development until the benefits of heightened reactivity outweigh the costs of potentially chronic glucocorticoid levels. Birds often acquire stress-responsiveness at locomotor independence, however, both stress-responsiveness and locomotor ability are delayed in birds with altricial developmental strategies. Parrots (Psittacidae) are extremely altricial, but it is not known whether they also postpone physiological responsiveness to stress until locomotor independence. We quantified individual variation in baseline and stress-induced plasma corticosterone (CORT) concentrations, the main avian glucocorticoid, in wild green-rumped parrotlets (Forpus passerinus) of Venezuela at four stages of nestling development. Parrotlet neonates are very underdeveloped and compete for parental care among extreme sibling size hierarchies, a competitive scenario that might benefit from early hypothalamic-pituitary-adrenal (HPA) functionality. Nestlings that underwent a standardized restraint stress-treatment showed higher average CORT concentrations compared to baseline in all age groups sampled, and exhibited no evidence of age-related changes in the stress response. This is 2 weeks before locomotor independence and earlier than previously documented for altricial species. Results suggest that precocity of HPA function may be advantageous to growth and survivorship in extremely altricial birds.

The development of the serotonergic and dopaminergic systems during chicken mid-late embryogenesis

Serotonin (5-HT) acts as a morphogen influencing embryonic brain development, and as a neurotransmitter regulating multiple biological functions with lifelong effects on animal physical, physiological and mental health, especially during the rapid growth phase prior to birth when embryos face many challenges to reach structural and functional completion. In this study, the development of the serotoninergic (5-HTergic) system and its modulatory effect on the dopaminergic (DAergic) system and related neural circuits were investigated during the mid-late embryogenesis, embryonic day (E)12-E20, in the chicken's brain. During 5-HTergic neuronal maturation, a growth-related anatomical and functional remodeling was highlighted: the 5-HT neurons continuously grew during E12-E20 except for a remarkable regression during E14-E16. Correspondingly, there was a time-dependent change in the 5-HT synthetic capacity. Specifically, 5-HT concentrations in the raphe nuclei increased from E12 to E14, reaching a first plateau during E14-E16, then continuously increased up to E19, and reaching a second plateau between E19-E20. The second plateau of the 5-HT concentration was in correspondence with the establishment of the 5-HTergic autoregulatory loop during E19-E20 and the development of the DAergic system. The DA concentrations remained unchanged from E12 to E16, then started to increase at E16, reaching a maximum at E19, and diminished before hatching. The unique developing time sequence between the 5-HTergic and DAergic systems suggests that the 5-HTergic system may play a critical role in forming the 5-HT - DA neural circuit during chicken embryogenesis. These results provide new insights for understanding the functional organization of the 5-HTergic system during embryonic development and raise the possibility that prenatally modulating the 5-HTergic system may lead to long-lasting brain structural and functional alterations.

In ovo metabolism and yolk glucocorticoid concentration interact to influence embryonic glucocorticoid exposure patterns

Vertebrates release glucocorticoids during stressful events. If stress occurs during reproduction, the resulting offspring can show altered phenotypes that are thought to arise from increased exposure to maternal glucocorticoids. Developing offspring can metabolize maternal glucocorticoids, which can alter the pattern of exposure they encounter. For egg laying vertebrates, we are just beginning to understand how embryonic steroid metabolism impacts embryonic exposure to maternal glucocorticoids. Here we injected three doses of radioactive corticosterone into Japanese quail (Coturnix japonica) eggs to determine the degree of embryonic exposure at days six and nine of development. We found that increasing injection dose increased the amount of radioactivity found in the embryo at day six but by day nine the effect of injection dose disappeared as the amount of radioactivity within the embryo dropped to equivalent levels for all three doses. Interestingly, when examined as a percentage of initial dose, there were no differences between treatment groups at any time points. Importantly, using thin-layer chromatography we characterized that some free steroid, putatively identified as corticosterone, does reach the developing embryo. Together, our data suggest that the in ovo metabolism of maternal corticosterone can eventually eliminate it from the egg, but before this happens, embryos developing in eggs with elevated amounts of maternal corticosterone are exposed to higher levels early in development. This has important implications for how we understand the developmental steroid environment and the mechanisms underlying maternal stress effects.

Early-life adversity programs long-term cytokine and microglia expression within the HPA axis in female Japanese quail

Stress exposure during pre and post-natal development can have persistent and often dysfunctional effects on several physiological systems, including immune function, affecting the ability to combat infection. The neuro-immune response is inextricably linked to the action of the Hypothalamic Pituitary Adrenal (HPA) axis. Cytokines released from neuro-immune cells, including microglia, activate the HPA axis while glucocorticoids in turn regulate cytokine release from microglia. Because of the close links between these two physiological systems, coupled with potential for persistent changes to HPA axis activity following developmental stress, components of the neuro-immune system could be targets for developmental programming. However, little is known of any programming effects of developmental stress on neuro-immune function. We investigated whether developmental stress exposure via elevated pre-natal corticosterone (CORT) or post-natal unpredictable food availability, had long-term effects on pro (IL-1β) and anti-inflammatory (IL-10) cytokine and microglia-dependent gene (CSF1R) expression within HPA axis tissues in a precocial bird, the Japanese quail (Coturnix japonica). Following post-natal stress, we observed increased IL-1β expression in the pituitary gland, reduced IL-10 expression in the amygdala and hypothalamus and reduced CSF1R expression within the hypothalamus and pituitary gland. Post-natal stress disrupted the ratio of IL-1β:IL-10 expression within the hippocampus and hypothalamus. Pre-natal stress only increased IL-1β expression in the pituitary gland. We found no evidence for interactive or cumulative effects across life stages on basal cytokine and glia expression in adulthood. We show that post-natal stress may have a larger impact than elevated pre-natal CORT on basal immunity in HPA axis specific brain regions, with changes in cytokine homeostasis and microglia abundance. These results provide evidence for post-natal programming of a pro-inflammatory neuro-immune phenotype at the expense of reduced microglia, which could have implications for CNS health and subsequent neuro-immune responses.

Interactions of body temperature and nutritional status on hypothalamo-pituitary-adrenal axis activity in pre-thermoregulatory eastern bluebird chicks (Sialia sialis)

Early life experiences can affect the function of the hypothalamo-pituitary-adrenal (HPA) axis of vertebrates, with potential fitness consequences later in life. In altricial species, for example, variation in parental behavior, e.g. brooding or feeding, can modify the activity of the HPA axis of the young by altering their exposure to noxious stimuli as the young develop in the nest. We have shown that a drop in the body temperature of eastern bluebird (Sialia sialis) chicks, such as occurs when females are away from the nest, elevates their blood corticosterone levels. If repeated during the early days of their development, cooling bouts also reduce the chicks' later corticosterone secretion in response to handling. Thus, variation in maternal behavior has the capacity to shape the function of the chicks' HPA axis. To better understand how maternal absence from the nest activates the HPA axis of bluebird chicks, we experimentally mimicked the cooling that occurs when the female is away from the nest, and investigated a) the age at which the HPA axis becomes capable of responding to cooling by increasing corticosterone secretion, b) whether corticosterone secretion remains elevated throughout long periods of cooling, and c) whether fasting (also potentially associated with maternal absence) interacts with cooling to affect corticosterone secretion. Cooling for 18 min significantly elevated circulating corticosterone levels of chicks as young as 4 days post-hatch, indicating that their HPA axis is sensitive to cooling very early in life. Corticosterone levels remained elevated throughout longer bouts of cooling. However, a 1-hr period of fasting had no effect on corticosterone secretion, regardless of whether chicks were cooled or not. Collectively, these data demonstrate that variation in maternal brooding behavior can substantially modify the corticosterone profiles of chicks during early postnatal development, and that chick temperature is likely the main driver of this.

In-ovo green light photostimulation during different embryonic stages affect somatotropic axis

Previous studies demonstrated that in-ovo photostimulation with monochromatic green light increased the somatotropic axis expression in broilers embryos. The objective of the current study was to detect the critical period for in-ovo GL photostimulation, in order to find the optimal targeted photostimulation period during the incubation process. Three hundred thirty-six fertile broiler eggs were divided into 4 groups. The first group was incubated under dark conditions as a negative control. The second incubated under intermittent monochromatic green light using light-emitting diode (LED) lamps with an intensity of 0.1 W\m2 at shell level from d 0 of the incubation as a positive control. The third group incubated under intermittent monochromatic green light from d 10 of the incubation. The last group incubated under intermittent monochromatic green light from d 15 of the incubation. In-ovo green light photostimulation from embryonic d 0 (ED0) increased plasma growth hormone (GH), as well as hypothalamic growth hormone releasing hormone (GHRH) and liver growth hormone receptor (GHR) and insulin-like growth factor-1 (IGF-1) mRNA levels. In-ovo green light photostimulation from ED10 increased the GH plasma levels compared to the negative control group, without affecting somatotropic axis mRNA genes expressions of GHRH, GHR, and IGF-1. In-ovo green light photostimulation from ED15 caused an increase in both the plasma GH levels and the somatotropic axis mRNA genes expressions of GHRH, GHR, and IGF-1, compared to the negative control group. These results suggest that the critical period of somatotropic axis acceleration by GL photostimulation start at 15 d of incubation.

Corticosterone induces growth hormone expression in pituitary somatotrophs during goose embryonic development

Treatment of fetal rat and embryonic chicken with exogenous glucocorticoids induces premature differentiation of growth hormone (GH) secreting cells. The effect of corticosterone (CORT) on somatotroph differentiation was mostly studied in pituitary cells in vitro. Currently, there is no evidence for glucocorticoid-mediated induction of somatotroph differentiation during pituitary development in bird species other than chicken. In this study, we sought to find out if in ovo injection of corticosterone into developing goose embryos could induce premature increase of GH in somatotrophs. On embryonic day (e) 15, the albumen of fertile goose eggs was injected with 300 μl of 0.9% saline, 300 μl 5 × 10^-8M CORT, or 300 μl 5 × 10^-6 M CORT. Embryos were allowed to develop until e20 and e28 and isolated pituitaries were subjected to quantitative real-time PCR and immunocytochemistry to detect GH mRNA and protein, respectively. At e20 and e28, blood from chorioallantoic vessels was subjected to radioimmunoassay for analysis of plasma GH protein. In ovo administration of exogenous corticosterone brought about a 2.5-fold increase in the expression of GH mRNA and increased the in situ expression of GH protein in goose pituitary cells, and enhanced plasma GH levels compared to that of the respective controls at e20. These findings prove that administration of glucocorticoid could stimulate the expression of GH in somatotrophs during goose embryonic development. Our results suggest the probable involvement of membrane glucocorticoid receptor in the corticosterone mediated expression of GH. Together with previously published data, our results suggest that corticosterone mediated induction of GH expression during embryonic development is relatively conserved among different vertebrates.

Corticosterone induces growth hormone expression in pituitary somatotrophs during goose embryonic development

Treatment of fetal rat and embryonic chicken with exogenous glucocorticoids induces premature differentiation of growth hormone (GH) secreting cells. The effect of corticosterone (CORT) on somatotroph differentiation was mostly studied in pituitary cells in vitro. Currently, there is no evidence for glucocorticoid-mediated induction of somatotroph differentiation during pituitary development in bird species other than chicken. In this study, we sought to find out if in ovo injection of corticosterone into developing goose embryos could induce premature increase of GH in somatotrophs. On embryonic day (e) 15, the albumen of fertile goose eggs was injected with 300 μl of 0.9% saline, 300 μl 5 × 10^-8M CORT, or 300 μl 5 × 10^-6 M CORT. Embryos were allowed to develop until e20 and e28 and isolated pituitaries were subjected to quantitative real-time PCR and immunocytochemistry to detect GH mRNA and protein, respectively. At e20 and e28, blood from chorioallantoic vessels was subjected to radioimmunoassay for analysis of plasma GH protein. In ovo administration of exogenous corticosterone brought about a 2.5-fold increase in the expression of GH mRNA and increased the in situ expression of GH protein in goose pituitary cells, and enhanced plasma GH levels compared to that of the respective controls at e20. These findings prove that administration of glucocorticoid could stimulate the expression of GH in somatotrophs during goose embryonic development. Our results suggest the probable involvement of membrane glucocorticoid receptor in the corticosterone mediated expression of GH. Together with previously published data, our results suggest that corticosterone mediated induction of GH expression during embryonic development is relatively conserved among different vertebrates.

Dissecting the role of regulators of thyroid hormone availability in early brain development: Merits and potential of the chicken embryo model

Thyroid hormones (THs) are important mediators of vertebrate central nervous system (CNS) development, thereby regulating the expression of a wide variety of genes by binding to nuclear TH receptors. TH transporters and deiodinases are both needed to ensure appropriate intracellular TH availability, but the precise function of each of these regulators and their coaction during brain development is only partially understood. Rodent knockout models already provided some crucial insights, but their in utero development severely hampers research regarding the role of TH regulators during early embryonic stages. The establishment of novel gain- and loss-of-function techniques has boosted the position of externally developing non-mammalian vertebrates as research models in developmental endocrinology. Here, we elaborate on the chicken as a model organism to elucidate the function of TH regulators during embryonic CNS development. The fast-developing, relatively big and accessible embryo allows easy experimental manipulation, especially at early stages of brain development. Recent data on the characterisation and spatiotemporal expression pattern of different TH regulators in embryonic chicken CNS have provided the necessary background to dissect the function of each of them in more detail. We highlight some recent advances and important strategies to investigate the role of TH transporters and deiodinases in various CNS structures like the brain barriers, the cerebellum, the retina and the hypothalamus. Exploiting the advantages of this non-classical model can greatly contribute to complete our understanding of the regulation of TH bioavailability throughout embryonic CNS development.

The highs and lows of programmed cardiovascular disease by developmental hypoxia: studies in the chicken embryo

It is now established that adverse conditions during pregnancy can trigger a fetal origin of cardiovascular dysfunction and/or increase the risk of heart disease in later life. Suboptimal environmental conditions during early life that may promote the development of cardiovascular dysfunction in the offspring include alterations in fetal oxygenation and nutrition as well as fetal exposure to stress hormones, such as glucocorticoids. There has been growing interest in identifying the partial contributions of each of these stressors to programming of cardiovascular dysfunction. However, in humans and in many animal models this is difficult, as the challenges cannot be disentangled. By using the chicken embryo as an animal model, science has been able to circumvent a number of problems. In contrast to mammals, in the chicken embryo the effects on the developing cardiovascular system of changes in oxygenation, nutrition or stress hormones can be isolated and determined directly, independent of changes in the maternal or placental physiology. In this review, we summarise studies that have exploited the chicken embryo model to determine the effects on prenatal growth, cardiovascular development and pituitary-adrenal function of isolated chronic developmental hypoxia.

Effect of a photoperiodic green light programme during incubation on embryo development and hatch process

This study was conducted to evaluate the effect of a 12-h light, 12-h dark (12L : 12D) photoperiod of green light during day 1 to day 18 of incubation time, on embryo growth, hormone concentration and the hatch process. In the test group, monochromatic light was provided by a total of 204 green light-emitting diodes (522 nm) mounted in a frame which was placed above the top tray of eggs to give even spread of illumination. No light-dark cycle was used in the control group. Four batches of eggs (n=300/group per batch) from fertile Ross 308 broiler breeders were used in this experiment. The beak length and crown-rump length of embryos incubated under green light were significantly longer than that of control embryos at day 10 and day 12, respectively (P<0.01). Furthermore, green light-exposed embryos had a longer third toe length compared with control embryos at day 10, day 14 and day 17 (P=0.02). At group level (n=4 batches), light stimulation had no effect on chick weight and quality at take-off, the initiation of hatch and hatch window. However, the individual hatching time of the light exposure focal chicks (n=33) was 3.4 h earlier (P=0.49) than the control focal chicks (n=36) probably due to the change in melatonin rhythm of the light group. The results of this study indicate that green light accelerates embryo development and alters hatch-related hormones (thyroid and corticosterone), which may result in earlier hatching.

In ovo leptin administration modulates glucocorticoid receptor mRNA expression specifically in the hypothalamus of broiler chickens

The glucocorticoid receptor (GR) is well documented to play a crucial role in the central control of energy homeostasis in mammals. However, the distribution and function of the GR in the chicken brain are less clear. Leptin is a key hormone regulating energy homeostasis in mammals, yet its action in the chicken is still under debate. In this study, the distribution of GR mRNA in the chicken brain and the effects of in ovo administration of leptin and its antagonist on early post-hatch growth and GR mRNA expression in different hypothalamic nuclei were investigated via in situ hybridization (ISH) and quantitative PCR. GR mRNA was widely expressed in the chicken brain, mainly in the corpus striatum, nucleus rotundus, dorsolateral nucleus, nucleus ovoidalis, nucleus reticularis superior and the hippocampus (Hp) and in the preoptic area of the hypothalamus. High doses of leptin (5.0μg) significantly promoted post-hatch growth, resulting in a significant high body weight increased by 24.64% at day (D) 21 of life. Meanwhile, hypothalamic expression of GR mRNA in the LL and HL groups was down-regulated significantly by 7.02% and 13.65% respectively (P<0.05). ISH revealed region-specific changes: GR mRNA was found to be significantly decreased (P<0.05) in the paraventricular nucleus, periventricular nucleus and ventromedial nucleus but not in the Hp, infundibular nucleus or lateral hypothalamus of D21 broiler chickens. The leptin antagonist was able to reverse the effect of leptin on the growth rate and hypothalamic GR mRNA expression. These results provide evidence that in ovo administration of leptin influences early post-hatch growth and the hypothalamic expression of GR mRNA in broiler chickens.

Domestication and ontogeny effects on the stress response in young chickens (Gallus gallus)

Domestication is thought to increase stress tolerance. The connection between stressor exposure, glucocorticoids and behavioural responses has been studied in adults, where domestication effects are evident. Early stress exposure may induce detrimental effects both in short-and long term. Previous research has reported a lack of glucocorticoid response in newly hatched chickens (Gallus gallus), whereas others have found opposite results. Hence it remains unclear whether the HPA-axis is functional from hatch, and if domestication has affected the early post-hatch ontogeny of the stress response. Our aims were to investigate the early ontogeny of the HPA-axis and characterize behavioural and hormonal stress responses in ancestral Red Junglefowl and in two domestic layer strains. Plasma corticosteone and behavioural responses before and after physical restraint was measured on day one, nine, 16 and 23 post hatch. The results showed significant increases of corticosterone after stress in all three breeds at all the different ages. The HPA-response decreased with age and was lower in Red Junglefowl. Behavioural responses also decreased with age, and tended to be stronger in Red Junglefowl. In summary, the HPA-axis is reactive from day one, and domestication may have affected its development and reactivity, alongside with related behaviour responses.

Incubation Temperature during Fetal Development Influences Morphophysiological Characteristics and Preferred Ambient Temperature of Chicken Hatchlings

Skin and feather characteristics, which play a critical role in body temperature maintenance, can be affected by incubation circumstances, such as incubation temperature. However, no study to date has assessed the influence of incubation temperature during the fetal stage on morphometric characteristics and vascular development of the skin, feather characteristics, and their relationship to hormone levels and preferred temperature in later life in chickens. Broiler breeder eggs were exposed to low (36°C), control (37.5°C), or high (39°C) temperatures (treatments LT, CK, and HT, respectively) from day 13 of incubation onward, because it is known that the endocrine axes are already established at this time. During this period, eggshell temperature of HT eggs (38.8±0.33°C) was higher than of LT (37.4±0.08°C) and CK eggs (37.8 ±0.15°C). The difference between eggshell and incubator air temperature diminished with the increasing incubation temperature, and was approximately zero for HT. HT hatchlings had higher surface temperature on the head, neck, and back, and thinner and more vascularized skin than did CK and LT hatchlings. No differences were found among treatments for body weight, total feather weight, number and length of barbs, barbule length, and plasma T4 concentration. LT hatchlings showed lower plasma T3 and GH, as well as lower T3/T4 ratio and decreased vascularity in the neck, back, and thigh skin compared to CK hatchlings. On the other hand, HT hatchlings had decreased skin thickness and increased vascularity, and preferred a higher ambient temperature compared to CK and HT hatchlings. In addition, for all treatments, surface temperature on the head was higher than of the other body regions. We conclude that changes in skin thickness and vascularity, as well as changes in thyroid and growth hormone levels, are the result of embryonic strategies to cope with higher or lower than normal incubation temperatures. Additionally exposure to increased temperature during incubation is an environmental factor that can exert early-life influence on ambient temperature preference of broiler hatchlings in later life.

Pre- and Post-Natal Stress Programming: Developmental Exposure to Glucocorticoids Causes Long-Term Brain-Region Specific Changes to Transcriptome in the Precocial Japanese Quail

Exposure to stress during early development can permanently influence an individual's physiology and behaviour, and affect its subsequent health. The extent to which elevated glucocorticoids cause such long-term 'programming' remains largely untested. In the present study, using the Japanese quail as our study species, we independently manipulated exposure to corticosterone during pre- and/or post-natal development and investigated the subsequent effects on global gene expression profiles within the hippocampus and hypothalamus upon achieving adulthood. Our results showed that the changes in transcriptome profiles in response to corticosterone exposure clearly differed between the hippocampus and the hypothalamus. We also showed that these effects depended on the developmental timing of exposure and identified brain-region specific gene expression patterns that were either: (i) similarly altered by corticosterone regardless of the developmental stage in which hormonal exposure occurred or (ii) specifically and uniquely altered by either pre-natal or post-natal exposure to corticosterone. Corticosterone-treated birds showed alterations in networks of genes that included known markers of the programming actions of early-life adversity (e.g. brain-derived neurotrophic factor and mineralocorticoid receptor within the hippocampus; corticotrophin-releasing hormone and serotonin receptors in the hypothalamus). Altogether, for the first time, these findings provide experimental support for the hypothesis that exposure to elevated glucocorticoids during development may be a key hormonal signalling pathway through which the long-term phenotypic effects associated with early-life adversity emerge and potentially persist throughout the lifespan. These data also highlight that stressors might have different long-lasting impacts on the brain transcriptome depending on the developmental stage in which they are experienced; more work is now required to relate these mechanisms to organismal phenotypic differences.

Incubation temperature alters thermal preference and response to heat stress of broiler chickens along the rearing phase

The current study aimed to investigate whether embryonic temperature manipulation may alter thermal preference throughout the rearing phase of broiler chickens and how this manipulation may affect response to thermal challenge, metabolism, growth rate and feed intake rate. Eggs were exposed to a constant incubation temperature [machine temperatures: 36°C (Low), 37.5°C (Control), and 39°C (High); eggshell temperature of 37.4 ± 0.08°C, 37.8 ± 0.15°C, and 38.8 ± 0.33°C, respectively] from d 13 till hatching. Low treatment chickens showed lower plasma T3 and GH levels at d 1 of age and lower T3 level at d 42 of age compared to the Control treatment. Preferred ambient, rectal temperature, T4 level, growth rate, food intake rate, and response to thermal challenge were not altered in these chickens. On the other hand, High-treatment chickens exhibited high preferred ambient temperature and rectal temperature during the first 2 wk post-hatch, lower plasma T3 level at d 21 and 42 and a delayed increase in respiratory movement in response to thermal challenge compared to the Control treatment. However, chickens subjected to the Control and High treatments did not differ in T4 and GH level and performance. We conclude that exposure to high temperature during late embryonic development has long-lasting effects on the thermoregulatory system of broiler chickens by affecting the heat tolerance of these chickens. Moreover, the preferred ambient temperature of the chickens from heat-treated eggs correspond to those recommended for the strain under study, whereas for the cold-treated and control-chickens it was 1°C below, indicating that incubation temperature might have consequences on the ambient temperature chickens require during the rearing phase.

The timing of embryonic exposure to elevated temperature alters stress endocrinology in domestic chickens (Gallus domesticus)

Patterns of glucocorticoid (GC) release in response to stimuli vary both among individuals and within individuals across their lifetime. While much work has focused on how the prenatal steroid environment can affect GC release, relatively little is known about how environmental parameters, such as incubation temperature affect GCs. We tested the hypothesis that variation and timing of elevated incubation temperature within the thermoneutral zone can alter the pattern of GC release. We incubated domestic chicken eggs (Gallus domesticus) at the optimal incubation temperature (37.5 °C) or at a slightly higher temperature (+1.1 °C) either early, late, or throughout incubation. At three weeks post-hatch, all birds were (i) exposed to a capture-restraint stress to measure stress-induced GC release (naïve). Three days following the naïve stressor, birds were (ii) exposed to a heat challenge, which was followed the next day by a second capture-restraint stress (post-heat challenge). Regardless of treatment, birds had similar patterns of GC release following the naïve stress series. However, during the post-heat challenge stress series, birds incubated at optimal temperatures increased their peak GC release. In contrast, birds exposed to slightly elevated temperatures for any period of development failed to increase peak GC release, and their specific response varied with timing of exposure to the elevated incubation temperature. Our results demonstrate that subtle variation in the embryonic environment, such as elevated incubation temperature within the thermoneutral zone, can impact the pattern of GC release of offspring. Further work is needed to understand the mechanisms underlying these changes and the relationship between fitness and environmentally-altered phenotypes.