Acid–base regulation during embryonic development in amniotes, with particular reference to birds

Acid-base and hematological regulation in chicken embryos during internal progressive hypercapnic hypoxia

Development of the capacity to mitigate potential disturbances to blood physiology in bird embryos is incompletely understood. We investigated regulation of acid-base and hematology in day 15 chicken embryos exposed to graded intrinsic hypercapnic hypoxia created by varying degrees of water submersion. Metabolic acidosis with additional respiratory or metabolic acidosis occurred at 2 h according to magnitude of submersion. Acid-base disturbance was partially compensated by metabolic alkalosis at 6 h, but compensatory metabolic alkalosis was absent at 24 h. Following submersion with only air cell exposed to air, both hypercapnic respiratory acidosis and metabolic acidosis occurred within 10 min. Subsequently, both forms of acidosis created lethal levels of [HCO₃^-] at ∼120 min. Blood hematology showed small but significant effects associated with induced acid-base disturbance. Increased Hct occurring during partial egg submersion lasting 24 h was attributed to an increase in MCV. By day 15 of development chicken embryos are able to partially compensate for and withstand all but severe induced internal hypoxic hypercapnia.

Proteomic Analysis of Chicken Chorioallantoic Membrane (CAM) during Embryonic Development Provides Functional Insight

In oviparous animals, the egg contains all resources required for embryonic development. The chorioallantoic membrane (CAM) is a placenta-like structure produced by the embryo for acid-base balance, respiration, and calcium solubilization from the eggshell for bone mineralization. The CAM is a valuable in vivo model in cancer research for development of drug delivery systems and has been used to study tissue grafts, tumor metastasis, toxicology, angiogenesis, and assessment of bacterial invasion. However, the protein constituents involved in different CAM functions are poorly understood. Therefore, we have characterized the CAM proteome at two stages of development (ED12 and ED19) and assessed the contribution of the embryonic blood serum (EBS) proteome to identify CAM-unique proteins. LC/MS/MS-based proteomics allowed the identification of 1470, 1445, and 791 proteins in CAM (ED12), CAM (ED19), and EBS, respectively. In total, 1796 unique proteins were identified. Of these, 175 (ED12), 177 (ED19), and 105 (EBS) were specific to these stages/compartments. This study attributed specific CAM protein constituents to functions such as calcium ion transport, gas exchange, vasculature development, and chemical protection against invading pathogens. Defining the complex nature of the CAM proteome provides a crucial basis to expand its biomedical applications for pharmaceutical and cancer research.

Developmental changes of free amino acids in amniotic, allantoic fluids and yolk of broiler embryo

1. A study was conducted to evaluate the developmental changes of protein and free amino acid concentrations in amniotic, allantoic fluids and yolk during the incubation period of broiler eggs.2. A total of 120 Cobb 500 fertile eggs were individually weighed and then placed in incubator. On incubation day: 8, 11, 13, 14, 16, and/or 18, amniotic, allantoic fluids and yolk were collected from 20 eggs for analysis of protein content and free α-amino acid concentration in allantoic and amniotic fluids and yolk.3. The total protein concentration in amniotic fluid increased from d 11 of incubation, and reached a peak at d 16 (69.85 g/l; P<0.01), then declined at d 18 (P<0.05). The total protein concentration in allantoic fluid increased with age of the embryo (P<0.01). Crude protein concentration in yolk decreased (P<0.05) from d 0 to 8, then increased gradually from d 8, and reached a peak at d 16 (P<0.05). The concentration of most free amino acids in amniotic and allantoic fluids and yolk was related to embryo weight. Amniotic fluid amino acids gradually increased from d 13 to 18, with arginine being the most abundant at d 11 and 14. Glutamate was the most predominant amniotic fluid amino acid at d 16 and 18. From d 13 to 18, the concentrations of most α-amino acids in allantoic fluid increased, and reached a peak at d 18 (aspartate, 373 μmol/l; asparagine, 519 μmol/l; glutamine, 1230 μmol/l; threonine, 537 μmol/l; citrulline, 112 μmol/l; arginine, 2747 μmol/l; alanine, 276 μmol/l; tyrosine, 330 μmol/l; tryptophan, 212 μmol/l; valine, 140 μmol/l; phenylalanine, 102 μmol/l; isoleucine, 92.39 μmol/l; lysine, 1088 μmol/l; P<0.05). Glutamine was the second most abundant amino acid in allantoic fluid at d 13 and 18. Glutamate was the most abundant α-amino acids at d 8, and 13 in the yolk.4. These results demonstrated that the concentration of free α-amino acids in chicken embryo fluid was related to embryo weight. Arginine, glutamine and glutamate were abundant free α-amino acid in chicken embryo fluid, to support the higher rates of tissue protein synthesis and growth for the embryo.

Nutrient sources differ in the fertilised eggs of two divergent broiler lines selected for meat ultimate pH

The pHu+ and pHu− lines, which were selected based on the ultimate pH (pHu) of the breast muscle, represent a unique model to study the genetic and physiological controls of muscle energy store in relation with meat quality in chicken. Indeed, pHu+ and pHu− chicks show differences in protein and energy metabolism soon after hatching, associated with a different ability to use energy sources in the muscle. The present study aimed to assess the extent to which the nutritional environment of the embryo might contribute to the metabolic differences observed between the two lines at hatching. Just before incubation (E0), the egg yolk of pHu+ exhibited a higher lipid percentage compared to the pHu− line (32.9% vs. 27.7%). Although ¹H-NMR spectroscopy showed clear changes in egg yolk composition between E0 and E10, there was no line effect. In contrast, ¹H-NMR analysis performed on amniotic fluid at embryonic day 10 (E10) clearly discriminated the two lines. The amniotic fluid of pHu+ was richer in leucine, isoleucine, 2-oxoisocaproate, citrate and glucose, while choline and inosine were more abundant in the pHu− line. Our results highlight quantitative and qualitative differences in metabolites and nutrients potentially available to developing embryos, which could contribute to metabolic and developmental differences observed after hatching between the pHu+ and pHu− lines.

Abnormal persistence of the chorioallantoic membrane is associated with severe developmental abnormalities in freshwater turtles

Development in oviparous reptiles requires the correct formation and function of extra-embryonic membranes in the egg. In 2017, we incubated 2583 eggs from five species of freshwater turtle during a long-term ecological study and opened eggs that failed to hatch. We described a previously unreported developmental anomaly: the retention of an extra-embryonic membrane around 7 turtles (1 Spiny Softshell Turtle (Apalone spinifera (Le Sueur, 1827)), 1 Snapping Turtle (Chelydra serpentina (Linnaeus, 1758)), and 5 Northern Map Turtles (Graptemys geographica (Le Sueur, 1817))) that were alive but unhatched >14 days after their clutch mates had emerged. We investigated the association between retention of this membrane and the exhibition of other developmental deformities of varying severity, and we tested whether this novel abnormality was associated with reduced fertility or hatching success in affected clutches. Consultation of ∼150 years of literature suggests that we observed persistence of the chorioallantoic membrane (CAM; also called the chorioallantois). Our data suggest that clutches where at least one turtle exhibits a persistent CAM may also exhibit slightly reduced fertility or hatch success in the rest of the clutch compared with conspecific clutches that do not contain this anomaly. Future research should investigate the factors predicting CAM retention and other developmental abnormalities.

American alligator (Alligator mississippiensis) embryos tightly regulate intracellular pH during a severe acidosis

Crocodilian nests naturally experience high CO2 (hypercarbia), which leads to increased blood Pco2 and reduced blood pH (pHe) in embryos; their response to acid–base challenges is not known. During acute hypercarbia, snapping turtle embryos preferentially regulate tissue pH (pHi) against pHe reductions. This is proposed to be associated with CO2 tolerance in reptilian embryos and is not found in adults. In the present study, we investigated pH regulation in American alligator (Alligator mississippiensis (Daudin, 1802)) embryos exposed to 1 h of hypercarbia hypoxia (13 kPa Pco2, 9 kPa Po2). Hypercarbia hypoxia reduced pHe by 0.42 pH unit, while heart and brain pHi increased, with no change in the pHi of other tissues. The results indicate that American alligator embryos preferentially regulate pHi, similar to snapping turtle embryos, which represents a markedly different strategy of acid–base regulation than what is observed in adult reptiles. These findings suggest that preferential pHi regulation may be a strategy of acid–base regulation used by embryonic reptiles.

Investigating proteins and proteases composing amniotic and allantoic fluids during chicken embryonic development

In amniotes, the amniotic fluid is a significant contributor to fetal development and health. While numerous studies have been conducted in mammalian amniotic fluid, the composition of amniotic and other extraembryonic fluids in avian egg along with their physiological functions remain largely unexplored. In such a context, our objective was to characterize the chicken amniotic fluid (AmF) and allantoic fluid (AlF) properties, protein composition, and some associated functions from day 8 to day 16 of incubation. SDS-PAGE combined to mass spectrometry analysis revealed common and specific proteins to each fluid, suggesting distinct properties and functions. Indeed, major AlF proteins are mostly "egg yolk" proteins involved in lipid, vitamin metabolisms, and metal ion transport, while major AmF proteins resemble those of albumen. Drastic changes in the AmF protein profiles were observed during incubation, when the albumen transfers from day 12 onwards, while few changes were detected for the AlF protein profile. The decreases in osmolality (from 231 to 183 mOsm/kg) and pH (from 8.26 to 7.26) observed in the AlF during incubation are associated with water and electrolytes reallocation for the embryo needs. In contrast, AmF pH value remained stable (≈7.5). Active proteolytic enzymes have been identified in the 2 fluids using gelatin zymography, followed by mass spectrometry analysis for protease identification. A total of 12 proteases was detected in the AlF, compared to 5 in the AmF. We have shown that AlF concentrates proteolytic enzymes assumed to participate in digestive processes: aminopeptidase N, dipeptidyl peptidase-4, meprin A, and 72 kDa type IV collagenase preproprotein. The other proteases identified in both fluids also could have a role in morphogenesis (hepatocyte growth factor activator, suppressor of tumorigenicity 14, astacin-like metalloendopeptidase) and hemostasis (prothrombin and coagulation factor X). Altogether, these data suggest that the roles of chicken AlF and AmF are not merely associated with protection of the embryo and regulation of metabolic disposable wastes, but also they could have more sophisticated roles during embryonic development.

Preferential intracellular pH regulation: hypotheses and perspectives

The regulation of vertebrate acid-base balance during acute episodes of elevated internal PCO2  is typically characterized by extracellular pH (pHe) regulation. Changes in pHe are associated with qualitatively similar changes in intracellular tissue pH (pHi) as the two are typically coupled, referred to as 'coupled pH regulation'. However, not all vertebrates rely on coupled pH regulation; instead, some preferentially regulate pHi against severe and maintained reductions in pHe Preferential pHi regulation has been identified in several adult fish species and an aquatic amphibian, but never in adult amniotes. Recently, common snapping turtles were observed to preferentially regulate pHi during development; the pattern of acid-base regulation in these species shifts from preferential pHi regulation in embryos to coupled pH regulation in adults. In this Commentary, we discuss the hypothesis that preferential pHi regulation may be a general strategy employed by vertebrate embryos in order to maintain acid-base homeostasis during severe acute acid-base disturbances. In adult vertebrates, the retention or loss of preferential pHi regulation may depend on selection pressures associated with the environment inhabited and/or the severity of acid-base regulatory challenges to which they are exposed. We also consider the idea that the retention of preferential pHi regulation into adulthood may have been a key event in vertebrate evolution, with implications for the invasion of freshwater habitats, the evolution of air breathing and the transition of vertebrates from water to land.

Embryonic common snapping turtles (Chelydra serpentina) preferentially regulate intracellular tissue pH during acid-base challenges

The nests of embryonic turtles naturally experience elevated CO2 (hypercarbia), which leads to increased blood PCO2 and a respiratory acidosis resulting in reduced blood pH [extracellular pH (pHe)]. Some fishes preferentially regulate tissue pH [intracellular pH (pHi)] against changes in pHe; this has been proposed to be associated with exceptional CO2 tolerance and has never been identified in amniotes. As embryonic turtles may be CO2 tolerant based on nesting strategy, we hypothesized that they preferentially regulate pHi, conferring tolerance to severe acute acid-base challenges. This hypothesis was tested by investigating pH regulation in common snapping turtles (Chelydra serpentina) reared in normoxia then exposed to hypercarbia (13kPa PCO2) for 1h at three developmental ages, 70 and 90% of incubation, and in yearlings. Hypercarbia reduced pHe but not pHi, at all developmental ages. At 70% of incubation, pHe was depressed by 0.324 pH units while pHi of brain, white muscle, and lung increased; heart, liver, and kidney pHi remained unchanged. At 90% of incubation, pHe was depressed by 0.352 pH units but heart pHi increased with no change in pHi of other tissues. Yearling exhibited a pHe reduction of 0.235 pH units but had no changes in pHi of any tissues. The results indicate common snapping turtles preferentially regulate pHi during development, but the degree of the response is reduced throughout development. This is the first time preferential pHi regulation has been identified in an amniote. These findings may provide insight into the evolution of acid-base homeostasis during development of amniotes, and vertebrates in general.