Cardiovascular development in embryos of the American alligator Alligator mississippiensis: effects of chronic and acute hypoxia

Responses to Hypoxia and Hyperoxia in Embryonic Tiger Keelbacks (Rhabdophis tigrinus lateralis; Colubridae)

Studies examining the oxygen dependency of embryonic survival, growth, and differentiation have been conducted for decades in a diverse array of animal taxa but including only one oviparous snake, the viperine water snake Natrix maura. Here, we describe a study incubating eggs of the tiger keelback Rhabdophis tigrinus lateralis (Colubridae) under four oxygen conditions, hypoxia (7% and 11% O2), normoxia (21% O2), and hyperoxia (31% O2), for different lengths or at different stages of incubation. The length of hypoxic exposure is important in affecting embryonic development in R. t. lateralis, with prolonged hypoxic exposure retarding embryonic growth and differentiation, increasing embryonic mortality and deformity, reducing hatchling size and mass, and altering hatchling body shape relative to normoxic controls. Embryonic tiger keelbacks are most susceptible to hypoxia late in development, as revealed by the fact that a 5-day exposure of eggs to hypoxia of 7% O2 reduced embryo mass and hatchling mass if it occurred at late stages of incubation. Hyperoxia of 31% O2 did not enhance development of R. t. lateralis embryos, only affecting hatchling head width, which slightly differed between hyperoxic hatchlings and their normoxic siblings. This study demonstrates the importance of avoiding hypoxic exposure at late stages of embryonic development in snakes.

Strain-Specific developmental cardiovascular physiology landmarks of embryonic red jungle fowl and black Sumatran bantam chickens (Gallus gallus)

Model organisms are commonly used to investigate and characterize the physiology of embryonic and fetal vertebrates. The embryonic chicken has been used extensively to study the physiology of the developing cardiovascular system in vertebrates. Using this model general patterns of cardiovascular development have been thoroughly characterized in the avian embryo. However, given the differences in morphology, total incubation length and neonatal maturity found across avian species, the specific patterns of cardiovascular development established in a chicken strain may not apply universally to avian embryos. In fact, recent studies have demonstrated that aspects of cardiovascular regulation in embryonic chickens may be strain specific. Thus, we predicted that two chicken strains, the red jungle fowl and the black Sumatran bantam chicken, would exhibit different patterns of cardiovascular function and regulation during development. The red jungle fowl was selected as is represents the strain that domestic chickens are derived from and the Sumatran chicken was selected as one of the strains that were derived from the red jungle fowl. When measured on the same days of incubation, arterial pressure was lower in the Sumatran embryo compared to the red jungle fowl, while heart rate was similar between the strains. Cholinergic receptor tone on heart rate increased in strength over the study period and was present on day 13 of incubation in the Sumatran embryos while its function was delayed until day 17 of red jungle fowl incubation. Finally, both strains showed β adrenergic tone on heart rate on all days of incubation studied and α adrenergic tone on arterial pressure on day 19. Collectively our data suggest that timing of functional cholinergic tone on heart rate is strain dependent, while beta adrenergic tone on heart rate was functional on similar days of incubation.

Cardiovascular responses of embryonic alligator (Alligator mississippiensis) exposed to 10% O2 and sodium cyanide (NaCN), a chemoreflex-inducing compound

The possibly interactive effects of changes in atmospheric respiratory gases (hypoxia or hypercapnia) and pharmacological chemoreceptor stimulation have not been assessed previously. We present a series of experimental protocols investigating embryonic alligators' capacity to modulate a cardiovascular neural chemoreflex response to a known chemoreceptor stimulant, sodium cyanide (NaCN). We incubated alligator embryos in 21 % (normoxia) and 10 % O2 (hypoxia) beginning at 20 % of embryonic incubation, and at 70 % and 90 % of incubation we measured heart rate and blood pressure responses to NaCN. These NaCN responses also included examining the effects of NaCN after 1-h exposure to 10 % O2., ganglionic blockade with hexamethonium chloride and α-adrenergic blockade with phentolamine. Injections of NaCN into the chorioallantoic artery caused a rapid bradycardia followed by a secondary hypertension, which can be attributed to an autonomic nervous system mediated reflex loop. We compared the heart rate response to injections of 1 mg kg-1 NaCN before and after a 1-h 10 % O2 exposure, and it was clear that embryonic alligators lacked capacity to change the intensity of cardiovascular responses to this compound. Hexamethonium greatly lessened the rapid bradycardia, and at 90 % of incubation, the secondary hypertensive response to NaCN appeared due to α-adrenergic stimulation, as phentolamine lessened the response. Collectively, data indicate that while a cardiovascular chemoreflex can be induced by NaCN, the heart rate response lacks plasticity and is not modulated by hypoxic incubation in embryonic alligators.

Temperature effects on blood gases in embryonic American alligators (Alligator mississippiensis)

Numerous studies report on the influence of temperature on blood gases in ectothermic vertebrates, but there is merely a cursory understanding of these effects in developing animals. Animals that develop in eggs are at the mercy of environmental temperature and are expected to lack the capacity to regulate gas exchange and may regulate blood gases by means of altered conductance for gas exchange. We, therefore, devised a series of studies to characterize the developmental changes in blood gases when embryonic alligators were exposed to 25, 30 and 35 °C. To determine how blood parameters were impacted by changes in embryonic temperature, blood was sampled from the chorioallantoic membrane artery. The blood in the chorioallantoic membrane artery is a mixture of oxygen-poor and oxygen-rich blood, which based on the embryonic vascular anatomy may reflect blood that perfuses the chemoreceptors of the developing animal. Our findings indicate that following a 48 h exposure to 25 °C or 35 °C, there was a positive relationship between CAM artery blood PO2, PCO2 and glucose. However, blood pH suggests embryonic alligators lack an acute regulatory mechanism for adjusting blood pH.

Developmental plasticity of the cardiovascular system in oviparous vertebrates: effects of chronic hypoxia and interactive stressors in the context of climate change

Animals at early life stages are generally more sensitive to environmental stress than adults. This is especially true of oviparous vertebrates that develop in variable environments with little or no parental care. These organisms regularly experience environmental fluctuations as part of their natural development, but climate change is increasing the frequency and intensity of these events. The developmental plasticity of oviparous vertebrates will therefore play a critical role in determining their future fitness and survival. In this Review, we discuss and compare the phenotypic consequences of chronic developmental hypoxia on the cardiovascular system of oviparous vertebrates. In particular, we focus on species-specific responses, critical windows, thresholds for responses and the interactive effects of other stressors, such as temperature and hypercapnia. Although important progress has been made, our Review identifies knowledge gaps that need to be addressed if we are to fully understand the impact of climate change on the developmental plasticity of the oviparous vertebrate cardiovascular system.

Developmental programming of sarcoplasmic reticulum function improves cardiac anoxia tolerance in turtles

Oxygen deprivation during embryonic development can permanently remodel the vertebrate heart, often causing cardiovascular abnormalities in adulthood. While this phenomenon is mostly damaging, recent evidence suggests developmental hypoxia produces stress-tolerant phenotypes in some ectothermic vertebrates. Embryonic common snapping turtles (Chelydra serpentina) subjected to chronic hypoxia display improved cardiac anoxia tolerance after hatching, which is associated with altered Ca2+ homeostasis in heart cells (cardiomyocytes). Here, we examined the possibility that changes in Ca2+ cycling, through the sarcoplasmic reticulum (SR), underlie the developmentally programmed cardiac phenotype of snapping turtles. We investigated this hypothesis by isolating cardiomyocytes from juvenile turtles that developed in either normoxia (21% O2; 'N21') or chronic hypoxia (10% O2; 'H10') and subjected the cells to anoxia/reoxygenation, in either the presence or absence of SR Ca2+-cycling inhibitors. We simultaneously measured cellular shortening, intracellular Ca2+ concentration ([Ca2+]i), and intracellular pH (pHi). Under normoxic conditions, N21 and H10 cardiomyocytes shortened equally, but H10 Ca2+ transients (Δ[Ca2+]i) were twofold smaller than those of N21 cells, and SR inhibition only decreased N21 shortening and Δ[Ca2+]i. Anoxia subsequently depressed shortening, Δ[Ca2+]i and pHi in control N21 and H10 cardiomyocytes, yet H10 shortening and Δ[Ca2+]i recovered to pre-anoxic levels, partly due to enhanced myofilament Ca2+ sensitivity. SR blockade abolished the recovery of anoxic H10 cardiomyocytes and potentiated decreases in shortening, Δ[Ca2+]i and pHi. Our novel results provide the first evidence of developmental programming of SR function and demonstrate that developmental hypoxia confers a long-lasting, superior anoxia-tolerant cardiac phenotype in snapping turtles, by modifying SR function and enhancing myofilament Ca2+ sensitivity.

Short communication: Characterizing arterial and venous blood gases over the gas exchange surface, the chorioallantoic membrane, of embryonic American alligators (Alligator mississippiensis) at two points of development

Assessments of arterial and venous blood gases are required to understand the function of respiratory organs in animals at different stages of development. We measured blood gases in the arteries entering and veins leaving the chorioallantoic membrane (CAM) in embryonic alligators (Alligator mississippiensis). The CAM accounts for virtually all gas exchange in these animals, and we hypothesized that the CAM vasculature would be larger in eggs incubated in hypoxia (10% O2 for 50% or 70% of incubation), which would be reflected in a lower partial pressure of CO2 (PCO2). Contrary to this hypothesis, our measurements revealed no effects of hypoxic incubation on PCO2, and seemingly no increase in vascularization of the CAM in response to incubation in 10% O2. PCO2 was lower on the venous side, but only significantly different from arterial blood at 70% of incubation. The calculated blood flow to the CAM increased with development and was lower in both groups of alligators that had been incubated in hypoxia. Future studies should include measurements of blood parameters taken from embryos held in conditions that mirror incubation O2 levels, in combination with direct measurements of CAM artery blood flow.

Embryonic and juvenile snakes (Natrix maura, Linnaeus 1758) compensate for high elevation hypoxia via shifts in cardiovascular physiology and metabolism

The colonization of novel environments requires a favorable response to conditions never, or rarely, encountered in recent evolutionary history. For example, populations colonizing upslope habitats must cope with lower atmospheric pressure at elevation, and thus reduced oxygen availability. The embryo stage in oviparous organisms is particularly susceptible, given its lack of mobility and limited gas exchange via diffusion through the eggshell and membranes. Especially little is known about responses of Lepidosaurian reptiles to reduced oxygen availability. To test the role of physiological plasticity during early development in response to high elevation hypoxia, we performed a transplant experiment with the viperine snake (Natrix maura, Linnaeus 1758). We maintained gravid females originating from low elevation populations (432 m above sea level [ASL]-normoxia) at both the elevation of origin and high elevation (2877 m ASL-extreme high elevation hypoxia; approximately 72% oxygen availability relative to sea level), then incubated egg clutches at both low and high elevation. Regardless of maternal exposure to hypoxia during gestation, embryos incubated at extreme high elevation exhibited altered developmental trajectories of cardiovascular function and metabolism across the incubation period, including a reduction in late-development egg mass. This physiological response may have contributed to the maintenance of similar incubation duration, hatching success, and hatchling body size compared to embryos incubated at low elevation. Nevertheless, after being maintained in hypoxia, juveniles exhibit reduced carbon dioxide production relative to oxygen consumption, suggesting altered energy pathways compared to juveniles maintained in normoxia. These findings highlight the role of physiological plasticity in maintaining rates of survival and fitness-relevant phenotypes in novel environments.

Hypoxic incubation at 50% of atmospheric levels shifts the cardiovascular response to acute hypoxia in American alligators, Alligator mississippiensis

We designed a series of studies to investigate whether hypoxia (10% O2) from 20% of incubation to hatching, or from 20 to 50% of incubation, affects cardiovascular function when juvenile American alligators reached an age of 4-5 years compared to juveniles that were incubated in 21% O2. At this age, we measured blood flows in all the major arteries as well as heart rate, blood pressure, and blood gases in animals in normoxia and acute hypoxia (10% O2 and 5% O2). In all three groups, exposure to acute hypoxia of 10% O2 caused a decrease in blood O2 concentration and an increase in heart rate in 4-5-year-old animals, with limited effects on blood flow in the major outflow vessels of the heart. In response to more acute hypoxia (5% O2), where blood O2 concentration decreased even further, we measured increased heart rate and blood flow in the right aorta, subclavian artery, carotid artery, and pulmonary artery; however, blood flow in the left aorta either decreased or did not change. Embryonic exposure to hypoxia increased the threshold for eliciting an increase in heart rate indicative of a decrease in sensitivity. Alligators that had been incubated in hypoxia also had higher arterial PCO2 values in normoxia, suggesting a reduction in ventilation relative to metabolism.

Exposure to hypoxia during embryonic development affects blood flow patterns and heart rate in juvenile American alligators during digestion

The developmental environment can alter an organism's phenotype through epigenetic mechanisms. We incubated eggs from American alligators in 10% O₂ (hypoxia) to investigate the functional plasticity of blood flow patterns in response to feeding later in life. Digestion is associated with marked elevations of metabolism, and we therefore used the feeding-induced stimulation of tissue O₂ demand to determine whether there are lasting effects of developmental hypoxia on the cardiovascular response to digestion later in life. In all animals studied, digestion elicited tachycardia and an elevation of blood flow in the right aorta, left aorta, and the pulmonary artery, whereas flows in the carotid and subclavian artery did not change. We found that heart rate and systemic blood flow remained elevated for a longer time period in juvenile alligators that had been incubated in hypoxia; we also found that the pulmonary blood flow was elevated at 24, 36, and 48 h. Collectively, our findings demonstrate that exposure to hypoxia during incubation has lasting effects on the hemodynamics of juvenile alligators 4 years after hatching.

Effects of Eggshell Thickness, Calcium Content, and Number of Pores in Erosion Craters on Hatching Rate of Chinese Alligator Eggs

The Chinese alligator (Alligator sinensis), found only in a small region in southeastern Anhui Province, is listed as critically endangered (CR) by the International Union for Conservation of Nature (IUCN) due to its current declining population trend. Any abnormalities in the physical properties of an egg can decrease the hatching rate. In particular, eggshells play an essential role in embryo development, motivating us to analyze the microstructures of the eggshells of Chinese alligators. In this study, we categorized the eggshells into two groups, based on the hatching rates, and analyzed the relationship between the eggshell parameters (eggshell thickness, calcium content, and number of pores in erosion craters) and the hatching rate, as well as the relationships between the eggshell parameters. We found that the shells of the eggs with high hatching rates were thicker than those of the eggs with low hatching rates. There were also fewer erosion-crater pores on the surfaces of the eggs with high hatching rates than on the surfaces of the eggs with low hatching rates. Moreover, the shell Ca content was significantly higher in the eggs with high hatching rates than in the eggs with low hatching rates. Cluster modeling indicated that the highest hatching rate occurred when the eggshell thickness was 200-380 µm and there were 1-12 pores. These results suggest that eggs with adequate Ca contents, thicker shells, and less air permeability are more likely to hatch. Furthermore, our findings can inform future studies, which will be vital for the survival of the critically endangered Chinese alligator species.

Effects of Developmental Hypoxia on the Vertebrate Cardiovascular System

Developmental hypoxia has profound and persistent effects on the vertebrate cardiovascular system, but the nature, magnitude, and long-term outcome of the hypoxic consequences are species specific. Here we aim to identify common and novel cardiovascular responses among vertebrates that encounter developmental hypoxia, and we discuss the possible medical and ecological implications.

Regulation of heart rate in vertebrates during hypoxia: A comparative overview

Acute exposure to low oxygen (hypoxia) places conflicting demands on the heart. Whilst an increase in heart rate (tachycardia) may compensate systemic oxygen delivery as arterial oxygenation falls, the heart itself is an energetically expensive organ that may benefit from slowing (bradycardia) to reduce work when oxygen is limited. Both strategies are apparent in vertebrates, with tetrapods (mammals, birds, reptiles, and amphibians) classically exhibiting hypoxic tachycardia and fishes displaying characteristic hypoxic bradycardia. With a richer understanding of the ontogeny and evolution of the responses, however, we see similarities in the underlying mechanisms between vertebrate groups. For example, in adult mammals, primary bradycardia results from the hypoxic stimulation of carotid body chemoreceptors that are overwhelmed by mechano-sensory feedback from the lung associated with hyperpnoea. Fish-like bradycardia prevails in the mammalian foetus (which, at this stage, is incapable of pulmonary ventilation), and in fish and foetus alike, the bradycardia ensues despite an elevation of circulating catecholamines. In both cases, the reduced heart rate may primarily serve to protect the heart. Thus, the comparative perspective offers fundamental insight into how and why different vertebrates regulate heart rate in different ways during periods of hypoxia.

Changes in hemoglobin function and isoform expression during embryonic development in the American alligator, Alligator mississippiensis

In the developing embryos of egg-laying vertebrates, O₂ flux takes place across a fixed surface area of the eggshell and the chorioallantoic membrane. In the case of crocodilians, the developing embryo may experience a decrease in O₂ flux when the nest becomes hypoxic, which may cause compensatory adjustments in blood O₂ transport. However, whether the switch from embryonic to adult hemoglobin isoforms (isoHbs) plays some role in these adjustments is unknown. Here, we provide a detailed characterization of the developmental switch of isoHb synthesis in the American alligator, Alligator mississippiensis. We examined the in vitro functional properties and subunit composition of purified alligator isoHbs expressed during embryonic developmental stages in normoxia and hypoxia (10% O₂). We found distinct patterns of isoHb expression in alligator embryos at different stages of development, but these patterns were not affected by hypoxia. Specifically, alligator embryos expressed two main isoHbs: HbI, prevalent at early developmental stages, with a high O₂ affinity and high ATP sensitivity, and HbII, prevalent at later stages and identical to the adult protein, with a low O₂ affinity and high CO₂ sensitivity. These results indicate that whole blood O₂ affinity is mainly regulated by ATP in the early embryo and by CO₂ and bicarbonate from the late embryo until adult life, but the developmental regulation of isoHb expression is not affected by hypoxia exposure.

A Case for Hydrogen Sulfide Metabolism as an Oxygen Sensing Mechanism

The ability to detect oxygen availability is a ubiquitous attribute of aerobic organisms. However, the mechanism(s) that transduce oxygen concentration or availability into appropriate physiological responses is less clear and often controversial. This review will make the case for oxygen-dependent metabolism of hydrogen sulfide (H₂S) and polysulfides, collectively referred to as reactive sulfur species (RSS) as a physiologically relevant O₂ sensing mechanism. This hypothesis is based on observations that H₂S and RSS metabolism is inversely correlated with O₂ tension, exogenous H₂S elicits physiological responses identical to those produced by hypoxia, factors that affect H₂S production or catabolism also affect tissue responses to hypoxia, and that RSS efficiently regulate downstream effectors of the hypoxic response in a manner consistent with a decrease in O₂. H₂S-mediated O₂ sensing is then compared to the more generally accepted reactive oxygen species (ROS) mediated O₂ sensing mechanism and a number of reasons are offered to resolve some of the confusion between the two.

Developmental programming of DNA methylation and gene expression patterns is associated with extreme cardiovascular tolerance to anoxia in the common snapping turtle

Background

Environmental fluctuation during embryonic and fetal development can permanently alter an organism’s morphology, physiology, and behaviour. This phenomenon, known as developmental plasticity, is particularly relevant to reptiles that develop in subterranean nests with variable oxygen tensions. Previous work has shown hypoxia permanently alters the cardiovascular system of snapping turtles and may improve cardiac anoxia tolerance later in life. The mechanisms driving this process are unknown but may involve epigenetic regulation of gene expression via DNA methylation. To test this hypothesis, we assessed in situ cardiac performance during 2 h of acute anoxia in juvenile turtles previously exposed to normoxia (21% oxygen) or hypoxia (10% oxygen) during embryogenesis. Next, we analysed DNA methylation and gene expression patterns in turtles from the same cohorts using whole genome bisulfite sequencing, which represents the first high-resolution investigation of DNA methylation patterns in any reptilian species.

Results

Genome-wide correlations between CpG and CpG island methylation and gene expression patterns in the snapping turtle were consistent with patterns observed in mammals. As hypothesized, developmental hypoxia increased juvenile turtle cardiac anoxia tolerance and programmed DNA methylation and gene expression patterns. Programmed differences in expression of genes such as SCN5A may account for differences in heart rate, while genes such as TNNT2 and TPM3 may underlie differences in calcium sensitivity and contractility of cardiomyocytes and cardiac inotropy. Finally, we identified putative transcription factor-binding sites in promoters and in differentially methylated CpG islands that suggest a model linking programming of DNA methylation during embryogenesis to differential gene expression and cardiovascular physiology later in life. Binding sites for hypoxia inducible factors (HIF1A, ARNT, and EPAS1) and key transcription factors activated by MAPK and BMP signaling (RREB1 and SMAD4) are implicated.

Conclusions

Our data strongly suggests that DNA methylation plays a conserved role in the regulation of gene expression in reptiles. We also show that embryonic hypoxia programs DNA methylation and gene expression patterns and that these changes are associated with enhanced cardiac anoxia tolerance later in life. Programming of cardiac anoxia tolerance has major ecological implications for snapping turtles, because these animals regularly exploit anoxic environments throughout their lifespan.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-021-00414-7.

Elevation, oxygen, and the origins of viviparity

Research focused on understanding the evolutionary factors that shape parity mode evolution among vertebrates have long focused on squamate reptiles (snakes and lizards), which contain all but one of the evolutionary transitions from oviparity to viviparity among extant amniotes. While most hypotheses have focused on the role of cool temperatures in favoring viviparity in thermoregulating snakes and lizards, there is a growing appreciation in the biogeographic literature for the importance of lower oxygen concentrations at high elevations for the evolution of parity mode. However, the physiological mechanisms underlying how hypoxia might reduce fitness, and how viviparity can alleviate this fitness decrement, has not been systematically evaluated. We qualitatively evaluated previous research on reproductive and developmental physiology, and found that (1) hypoxia can negatively affect fitness of squamate embryos, (2) oxygen availability in the circulatory system of adult lizards can be similar or greater than an egg, and (3) gravid females can possess adaptive phenotypic plasticity in response to hypoxia. These findings suggest that the impact of hypoxia on the development and physiology of oviparous and viviparous squamates would be a fruitful area of research for understanding the evolution of viviparity. To that end, we propose an integrative research program for studying hypoxia and the evolution of viviparity in squamates.

The Long-Term Effects of Developmental Hypoxia on Cardiac Mitochondrial Function in Snapping Turtles

It is well established that adult vertebrates acclimatizing to hypoxic environments undergo mitochondrial remodeling to enhance oxygen delivery, maintain ATP, and limit oxidative stress. However, many vertebrates also encounter oxygen deprivation during embryonic development. The effects of developmental hypoxia on mitochondrial function are likely to be more profound, because environmental stress during early life can permanently alter cellular physiology and morphology. To this end, we investigated the long-term effects of developmental hypoxia on mitochondrial function in a species that regularly encounters hypoxia during development-the common snapping turtle (Chelydra serpentina). Turtle eggs were incubated in 21% or 10% oxygen from 20% of embryonic development until hatching, and both cohorts were subsequently reared in 21% oxygen for 8 months. Ventricular mitochondria were isolated, and mitochondrial respiration and reactive oxygen species (ROS) production were measured with a microrespirometer. Compared to normoxic controls, juvenile turtles from hypoxic incubations had lower Leak respiration, higher P:O ratios, and reduced rates of ROS production. Interestingly, these same attributes occur in adult vertebrates that acclimatize to hypoxia. We speculate that these adjustments might improve mitochondrial hypoxia tolerance, which would be beneficial for turtles during breath-hold diving and overwintering in anoxic environments.

Cardiovascular responses to putative chemoreceptor stimulation of embryonic common snapping turtles (Chelydra serpentina) chronically incubated in hypoxia (10% O2)

Developmental hypoxia has been shown to result in significant changes in cardiovascular development of American alligators and common snapping turtles. These include similar effects on cardiac mass and aspects of cardiovascular function. However, given the distant phylogenetic relationship between crocodilians and chelonians, we hypothesized that snapping turtles would also exhibit differences in the effects of developmental hypoxia on cardiovascular regulation. This hypothesis was based in part on prior studies that documented differences in plasticity of vagal tone on the heart between alligators and snapping turtles incubated in hypoxic conditions. To test this hypothesis, we investigated how 10% O₂ exposure over final 80% of incubation altered the heart rate and blood pressure response to two chemical manipulations of the "chemoreflex" in common snapping turtles at 70% and 90% of incubation. NaCN injections produced a dose dependent bradycardia that was mediated by cholinergic receptor stimulation. This reflex was relatively unaffected by hypoxic incubation conditions in snapping turtle embryos. Injections of the 5-HT₃ agonist phenylbiguanide (PBG) caused a pronounced bradycardia that decreased in intensity at 90% of incubation in embryos from the normoxic group while the heart rate response was unchanged in the hypoxic group. This differs from the previously reported diminished heart rate response of embryonic alligators incubated in 10% O₂, suggesting plasticity in this chemoreflex response differs between the species. Our data also indicate the cardiovascular response is mediated by a secondary cholinergic receptor stimulation however the inability of ganglionic blockade to inhibit the PBG response leaves the location of the receptors antagonized by PBG in question in embryonic snapping turtles. Primarily, our findings refute the hypothesis that hypoxic incubation decreases the "chemoreflex' response of snapping turtle embryos.

High temperatures limit developmental resilience to high-elevation hypoxia in the snake Natrix maura (Squamata: Colubridae)

Climate change is generating range shifts in many organisms, notably along the altitudinal gradient. However, moving up in altitude exposes organisms to lower oxygen availability, which may negatively affect development and fitness, especially at high temperatures. To test this possibility in a potentially upward-colonizing species, we artificially incubated developing embryos of the viperine snake Natrix maura Linnaeus 1758, using a split-clutch design, in conditions of extreme high elevation or low elevation at two ecologically-relevant incubation temperatures (24 and 32 °C). Embryos at low and extreme high elevations incubated at cool temperatures did not differ in development time, hatchling phenotype or locomotor performance. However, at the warmer incubation temperature and at extreme high elevation, hatching success was reduced. Further, embryonic heart rates were lower, incubation duration longer and juveniles born smaller. Nonetheless, snakes in this treatment were faster swimmers than siblings in other treatment groups, suggesting a developmental trade-off between size and performance. Constraints on development may be offset by the maintenance of important performance metrics, thus suggesting that early life-history stages will not prevent the successful colonization of high-elevation habitat even under the dual limitations of reduced oxygen and increased temperature.

High-elevation hypoxia impacts perinatal physiology and performance in a potential montane colonizer

Climate change is generating range shifts in many organisms, notably along the elevational gradient in mountainous environments. However, moving up in elevation exposes organisms to lower oxygen availability, which may reduce the successful reproduction and development of oviparous organisms. To test this possibility in an upward‐colonizing species, we artificially incubated developing embryos of the viperine snake (Natrix maura) using a split‐clutch design, in conditions of extreme high elevation (hypoxia at 2877 m above sea level; 72% sea‐level equivalent O₂ availability) or low elevation (control group; i.e. normoxia at 436 m above sea level). Hatching success did not differ between the two treatments. Embryos developing at extreme high elevation had higher heart rates and hatched earlier, resulting in hatchlings that were smaller in body size and slower swimmers compared to their siblings incubated at lower elevation. Furthermore, post‐hatching reciprocal transplant of juveniles showed that snakes which developed at extreme high elevation, when transferred back to low elevation, did not recover full performance compared to their siblings from the low elevation incubation treatment. These results suggest that incubation at extreme high elevation, including the effects of hypoxia, will not prevent oviparous ectotherms from producing viable young, but may pose significant physiological challenges on developing offspring in ovo. These early‐life performance limitations imposed by extreme high elevation could have negative consequences on adult phenotypes, including on fitness‐related traits.

Embryonic developmental oxygen preconditions cardiovascular functional response to acute hypoxic exposure and maximal β-adrenergic stimulation of anesthetized juvenile American alligators (Alligator mississippiensis)

The effects of the embryonic environment on juvenile phenotypes are widely recognized. We investigated the effect of embryonic hypoxia on the cardiovascular phenotype of 4-year-old American alligators (Alligator mississippiensis). We hypothesized that embryonic 10% O₂ preconditions cardiac function, decreasing the reduction in cardiac contractility associated with acute 5% O₂ exposure in juvenile alligators. Our findings indicate that dobutamine injections caused a 90% increase in systolic pressure in juveniles that were incubated in 21% and 10% O₂, with the 10% O₂ group responding with a greater rate of ventricular relaxation and greater left ventricle output compared with the 21% O₂ group. Further, our findings indicate that juvenile alligators that experienced embryonic hypoxia have a faster rate of ventricular relaxation, greater left ventricle stroke volume and greater cardiac power following β-adrenergic stimulation, compared with juvenile alligators that did not experience embryonic hypoxia. When juveniles were exposed to 5% O₂ for 20 min, normoxic-incubated juveniles had a 50% decline in left ventricle maximal rate of pressure development and maximal pressure; however, these parameters were unaffected and decreased less in the hypoxic-incubated juveniles. These data indicate that embryonic hypoxia in crocodilians alters the cardiovascular phenotype, changing the juvenile response to acute hypoxia and β-adrenergic stimulation.

Developmental plasticity of cardiac anoxia-tolerance in juvenile common snapping turtles ( Chelydra serpentina)

For some species of ectothermic vertebrates, early exposure to hypoxia during embryonic development improves hypoxia-tolerance later in life. However, the cellular mechanisms underlying this phenomenon are largely unknown. Given that hypoxic survival is critically dependent on the maintenance of cardiac function, we tested the hypothesis that developmental hypoxia alters cardiomyocyte physiology in a manner that protects the heart from hypoxic stress. To test this hypothesis, we studied the common snapping turtle, which routinely experiences chronic developmental hypoxia and exploits hypoxic environments in adulthood. We isolated cardiomyocytes from juvenile turtles that embryonically developed in either normoxia (21% O₂) or hypoxia (10% O₂), and subjected them to simulated anoxia and reoxygenation, while simultaneously measuring intracellular Ca²⁺, pH and reactive oxygen species (ROS) production. Our results suggest developmental hypoxia improves cardiomyocyte anoxia-tolerance of juvenile turtles, which is supported by enhanced myofilament Ca²⁺-sensitivity and a superior ability to suppress ROS production. Maintenance of low ROS levels during anoxia might limit oxidative damage and a greater sensitivity to Ca²⁺ could provide a mechanism to maintain contractile force. Our study suggests developmental hypoxia has long-lasting effects on turtle cardiomyocyte function, which might prime their physiology for exploiting hypoxic environments.

Hypoxia-induced reprogramming of the cardiac phenotype in American alligators (Alligator mississippiensis) revealed by quantitative proteomics

Hypoxic exposure during development can have a profound influence on offspring physiology, including cardiac dysfunction, yet many reptile embryos naturally experience periods of hypoxia in buried nests. American alligators experimentally exposed to developmental hypoxia demonstrate morphological and functional changes to the heart that persist into later life stages; however, the molecular bases of these changes remain unknown. We tested if targeted and persistent changes in steady-state protein expression underlie this hypoxic heart phenotype, using isobaric tags for relative and absolute quantitation (iTRAQ) proteomics. Alligator eggs were reared under normoxia or 10% hypoxia, then either sampled (embryo) or returned to normoxia for 2 years (juvenile). Three salient findings emerge from the integrated analysis of the 145 differentially expressed proteins in hypoxia-reared animals: (1) significant protein-protein interaction networks were identified only in up-regulated proteins, indicating that the effects of developmental hypoxia are stimulatory and directed; (2) the up-regulated proteins substantially enriched processes related to protein turnover, cellular organization, and metabolic pathways, supporting increased resource allocation towards building and maintaining a higher functioning heart; and (3) the juvenile cardiac proteome retained many of the signature changes observed in embryonic hearts, supporting long-term reprogramming of cardiac myocytes induced by hypoxia during critical periods of development.

Transplanting gravid lizards to high elevation alters maternal and embryonic oxygen physiology, but not reproductive success or hatchling phenotype

Increased global temperatures have opened previously inhospitable habitats, such as at higher elevations. However, the reduction of oxygen partial pressure with increase in elevation represents an important physiological constraint that may limit colonization of such habitats, even if the thermal niche is appropriate. To test the mechanisms underlying the response to ecologically-relevant levels of hypoxia, we performed a translocation experiment with the common wall lizard (Podarcis muralis), a widespread European lizard amenable to establishing populations outside its natural range. We investigated the impacts of hypoxia on the oxygen physiology and reproductive output of gravid common wall lizards and the subsequent development and morphology of their offspring. Lowland females transplanted to high elevations increased their haematocrit and haemoglobin concentration within days and maintained routine metabolism compared to lizards kept at native elevations. However, transplanted lizards suffered from increased reactive oxygen metabolite production near the oviposition date, suggesting a cost of reproduction at high elevation. Transplanted females and females native to different elevations did not differ in reproductive output (clutch size, egg mass, relative clutch mass, or embryonic stage at oviposition) or in post-oviposition body condition. Developing embryos reduced heart rates and prolonged incubation times at high elevations within the native range and at extreme high elevations beyond the current range, but this reduced oxygen availability did not affect metabolic rate, hatching success, or hatchling size. These results suggest that this opportunistic colonizer is capable of successfully responding to novel environmental constraints in these important life-history stages.

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.

Chronic captopril treatment reveals the role of ANG II in cardiovascular function of embryonic American alligators (Alligator mississippiensis)

Angiotensin II (ANG II) is a powerful vasoconstrictor of the renin-angiotensin system (RAS) that plays an important role in cardiovascular regulation in adult and developing vertebrates. Knowledge of ANG II's contribution to developmental cardiovascular function comes from studies in fetal mammals and embryonic chickens. This is the first study to examine the role of ANG II in cardiovascular control in an embryonic reptile, the American alligator (Alligator mississippiensis). Using chronic low (~ 5-mg kg embryo^-1), or high doses (~ 450-mg kg embryo^-1) of captopril, an angiotensin-converting enzyme (ACE) inhibitor, we disrupted the RAS and examined the influence of ANG II in cardiovascular function at 90% of embryonic development. Compared to embryos injected with saline, mean arterial pressure (MAP) was significantly reduced by 41 and 72% under low- and high-dose captopril treatments, respectively, a greater decrease in MAP than observed in other developing vertebrates following ACE inhibition. Acute exogenous ANG II injection produced a stronger hypertensive response in low-dose captopril-treated embryos compared to saline injection embryos. However, ACE inhibition with the low dose of captopril did not change adrenergic tone, and the ANG II response did not include an α-adrenergic component. Despite decreased MAP that caused a left shifted baroreflex curve for low-dose captopril embryos, ANG II did not influence baroreflex sensitivity. This study demonstrates that ANG II contributes to cardiovascular function in a developing reptile, and that the RAS contributes to arterial blood pressure maintenance during development across multiple vertebrate groups.

Cardiovascular adjustments with egg temperature at 90% incubation in embryonic American alligators, Alligator mississippiensis

American alligators (Alligator mississippiensis) deposit eggs in a mound nest, potentially subjecting embryos to daily variations in temperature. Whilst adult crocodilian cardiovascular responses to changes in temperature have been investigated, similar studies in alligator embryos are limited. We investigated cardiovascular function of embryonic alligators during heating and cooling as well as at different temperatures. We measured arterial blood pressure (P_m) and heart rate (f_H) in response to cooling (30-26 °C), heating (26-36 °C), followed by a reciprocal cooling event (36-26 °C) and assessed the cardiac baroreflex at 30 and 36 °C. Embryonic f_H increased during heating events and decreased during cooling events, while embryos were hypotensive at 26 and 36 °C, although P_m did not differ between heating or cooling events. There was a clear temperature-dependent heart rate hysteresis at a given embryo's temperature, depending on whether embryos were cooling or heating. Cardiovascular regulation through the cardiac limb of the baroreflex was not affected by temperature, despite previous studies suggesting that vagal tone is present at both low and high temperatures.

Convective oxygen transport during development in embryos of the snapping turtle Chelydra serpentina

This study investigated the maturation of convective oxygen transport in embryos of the snapping turtle (Chelydra serpentina). Measurements included: mass, oxygen consumption (VO2), heart rate (fH), blood oxygen content and affinity and blood flow distribution at 50%, 70% and 90% of the incubation period. Body mass increased exponentially, paralleled by increased cardiac mass and metabolic rate. Heart rate was constant from 50% to 70% of incubation but was significantly reduced at 90%. Hematocrit (Hct) and hemoglobin concentration (Hb) were constant at the three points of development studied but arteriovenous difference (A-V diff) doubled from 50 to 90% of incubation. Oxygen affinity was lower early in 50% of incubation compared to all other age groups. Blood flow was directed predominantly to the embryo but highest to the CAM at 70% incubation and was directed away from the yolk as it was depleted at 90% incubation. The findings indicate that the plateau or reduction in egg VO2 characteristic of the late incubation period of turtle embryos may be related to an overall reduction in mass-specific VO2 that is correlated with decreasing relative heart mass and plateaued CAM blood flow. Importantly, if the blood properties remain unchanged prior to hatching, as they did during the incubation period studied in the current investigation, this could account for the pattern of VO2 previously reported for embryonic snapping turtles prior to hatching.

Developmental cardiovascular physiology of the olive ridley sea turtle (Lepidochelys olivacea)

Our understanding of reptilian cardiovascular development and regulation has increased substantially for two species the American alligator (Alligator mississippiensis) and the common snapping turtle (Chelydra serpentina) during the past two decades. However, what we know about cardiovascular maturation in many other species remains poorly understood or unknown. Embryonic sea turtles have been studied to understand the maturation of metabolic function, but these studies have not addressed the cardiovascular system. Although prior studies have been pivotal in characterizing development, and factors that influence it, the development of cardiovascular function, which supplies metabolic function, is unknown in sea turtles. During our investigation we focused on quantifying how cardiovascular morphological and functional parameters change, to provide basic knowledge of development in the olive ridley sea turtle (Lepidochelys olivacea). Embryonic mass, as well as mass of the heart, lungs, liver, kidney, and brain increased during turtle embryo development. Although heart rate was constant during this developmental period, arterial pressure approximately doubled. Further, while embryonic olive ridley sea turtles lacked cholinergic tone on heart rate, there was a pronounced beta adrenergic tone on heart rate that decreased in strength at 90% of incubation. This beta adrenergic tone may be partially originating from the sympathetic nervous system at 90% of incubation, with the majority originating from circulating catecholamines. Data indicates that olive ridley sea turtles share traits of embryonic functional cardiovascular maturation with the American alligator (Alligator mississippiensis) but not the common snapping turtle (Chelydra serpentina).

Reduced egg shell permeability affects embryonic development and hatchling traits in Lycodon rufozonatum and Pelodiscus sinensis

The response of embryos to unpredictable hypoxia is critical for successful embryonic development, yet there remain significant gaps in our understanding of such responses in reptiles with different types of egg shell. We experimentally generated external regional hypoxia by sealing either the upper half or bottom half of the surface area of eggs in 2 species of reptiles (snake [Lycodon rufozonatum] with parchment egg shell and Chinese soft-shelled turtle [Pelodiscus sinensis] with rigid egg shell), then monitored the growth pattern of the opaque white patch in turtle eggs (a membrane that attaches the embryo to the egg shell and plays an important role in gas exchange), the embryonic heart rate, the developmental rate and the hatchling traits in turtle and snake eggs in response to external regional hypoxia. The snake embryos from the hypoxia treatments facultatively increased their heart rate during incubation, and turtle embryos from the upper-half hypoxia treatment enhanced their growth of the opaque white patch. Furthermore, the incubation period and hatching success of embryos were not affected by the hypoxia treatment in these 2 species. External regional hypoxia significantly affected embryonic yolk utilization and offspring size in the snake and turtle. Compared to sham controls, embryos from the upper-half hypoxia treatment used less energy from yolk and, therefore, developed into smaller hatchlings, but embryos from the bottom-half hypoxia treatment did not.

Embryonic hypoxia programmes postprandial cardiovascular function in adult common snapping turtles (Chelydra serpentina)

Reduced oxygen availability (hypoxia) is a potent stressor during embryonic development, altering the trajectory of trait maturation and organismal phenotype. We previously documented that chronic embryonic hypoxia has a lasting impact on the metabolic response to feeding in juvenile snapping turtles (Chelydra serpentina). Turtles exposed to hypoxia as embryos (10% O2, H10) exhibited an earlier and increased peak postprandial oxygen consumption rate, compared to control turtles (21% O2, N21). In the current study, we measured central blood flow patterns to determine whether the elevated postprandial metabolic response in H10 turtles is linked to lasting impacts on convective transport. Five years after hatching, turtles were instrumented to quantify systemic (Q̇sys) and pulmonary (Q̇pul) blood flows and heart rate (fH) before and after a ∼5% body mass meal. In adult N21 and H10 turtles, fH was increased significantly by feeding. While total stroke volume (Vstot) remained at fasted values, this tachycardia contributed to an elevation in total cardiac output (Q̇tot). However, there was a postprandial reduction in a net left-right (L-R) shunt in N21 snapping turtles only. Relative to N21 turtles, H10 animals exhibited higher Q̇sys due to increased blood flow through the right systemic outflow vessels of the heart. This effect of hypoxic embryonic development, reducing a net L-R cardiac shunt, may support the increased postprandial metabolic rate we previously reported in H10 turtles, and is further demonstration of adult reptile cardiovascular physiology being programmed by embryonic hypoxia.

Developmental plasticity of mitochondrial function in American alligators, Alligator mississippiensis

The effect of hypoxia on cellular metabolism is well documented in adult vertebrates, but information is entirely lacking for embryonic organisms. The effect of hypoxia on embryonic physiology is particularly interesting, as metabolic responses during development may have life-long consequences, due to developmental plasticity. To this end, we investigated the effects of chronic developmental hypoxia on cardiac mitochondrial function in embryonic and juvenile American alligators (Alligator mississippiensis). Alligator eggs were incubated in 21% or 10% oxygen from 20 to 90% of embryonic development. Embryos were either harvested at 90% development or allowed to hatch and then reared in 21% oxygen for 3 yr. Ventricular mitochondria were isolated from embryonic/juvenile alligator hearts. Mitochondrial respiration and enzymatic activities of electron transport chain complexes were measured with a microrespirometer and spectrophotometer, respectively. Developmental hypoxia induced growth restriction and increased relative heart mass, and this phenotype persisted into juvenile life. Embryonic mitochondrial function was not affected by developmental hypoxia, but at the juvenile life stage, animals from hypoxic incubations had lower levels of Leak respiration and higher respiratory control ratios, which is indicative of enhanced mitochondrial efficiency. Our results suggest developmental hypoxia can have life-long consequences for alligator morphology and metabolic function. Further investigations are necessary to reveal the adaptive significance of the enhanced mitochondrial efficiency in the hypoxic phenotype.

Periods of cardiovascular susceptibility to hypoxia in embryonic american alligators (Alligator mississippiensis)

During embryonic development, environmental perturbations can affect organisms' developing phenotype, a process known as developmental plasticity. Resulting phenotypic changes can occur during discrete, critical windows of development. Critical windows are periods when developing embryos are most susceptible to these perturbations. We have previously documented that hypoxia reduces embryo size and increases relative heart mass in American alligator, and this study identified critical windows when hypoxia altered morphological, cardiovascular function and cardiac gene expression of alligator embryos. We hypothesized that incubation in hypoxia (10% O2) would increase relative cardiac size due to cardiac enlargement rather than suppression of somatic growth. We exposed alligator embryos to hypoxia during discrete incubation periods to target windows where the embryonic phenotype is altered. Hypoxia affected heart growth between 20 and 40% of embryonic incubation, whereas somatic growth was affected between 70 and 90% of incubation. Arterial pressure was depressed by hypoxic exposure during 50-70% of incubation, whereas heart rate was depressed in embryos exposed to hypoxia during a period spanning 70-90% of incubation. Expression of Vegf and PdgfB was increased in certain hypoxia-exposed embryo treatment groups, and hypoxia toward the end of incubation altered β-adrenergic tone for arterial pressure and heart rate. It is well known that hypoxia exposure can alter embryonic development, and in the present study, we have identified brief, discrete windows that alter the morphology, cardiovascular physiology, and gene expression in embryonic American alligator.

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.

Phenotypic plasticity in the common snapping turtle (Chelydra serpentina): long-term physiological effects of chronic hypoxia during embryonic development

Studies of embryonic and hatchling reptiles have revealed marked plasticity in morphology, metabolism, and cardiovascular function following chronic hypoxic incubation. However, the long-term effects of chronic hypoxia have not yet been investigated in these animals. The aim of this study was to determine growth and postprandial O2 consumption (V̇o2), heart rate (fH), and mean arterial pressure (Pm, in kPa) of common snapping turtles (Chelydra serpentina) that were incubated as embryos in chronic hypoxia (10% O2, H10) or normoxia (21% O2, N21). We hypothesized that hypoxic development would modify posthatching body mass, metabolic rate, and cardiovascular physiology in juvenile snapping turtles. Yearling H10 turtles were significantly smaller than yearling N21 turtles, both of which were raised posthatching in normoxic, common garden conditions. Measurement of postprandial cardiovascular parameters and O2 consumption were conducted in size-matched three-year-old H10 and N21 turtles. Both before and 12 h after feeding, H10 turtles had a significantly lower fH compared with N21 turtles. In addition, V̇o2 was significantly elevated in H10 animals compared with N21 animals 12 h after feeding, and peak postprandial V̇o2 occurred earlier in H10 animals. Pm of three-year-old turtles was not affected by feeding or hypoxic embryonic incubation. Our findings demonstrate that physiological impacts of developmental hypoxia on embryonic reptiles continue into juvenile life.

Challenges and opportunities in developmental integrative physiology

This review explores challenges and opportunities in developmental physiology outlined by a symposium at the 2014 American Physiological Society Intersociety Meeting: Comparative Approaches to Grand Challenges in Physiology. Across animal taxa, adverse embryonic/fetal environmental conditions can alter morphological and physiological phenotypes in juveniles or adults, and capacities for developmental plasticity are common phenomena. Human neonates with body sizes at the extremes of perinatal growth are at an increased risk of adult disease, particularly hypertension and cardiovascular disease. There are many rewarding areas of current and future research in comparative developmental physiology. We present key mechanisms, models, and experimental designs that can be used across taxa to investigate patterns in, and implications of, the development of animal phenotypes. Intraspecific variation in the timing of developmental events can be increased through developmental plasticity (heterokairy), and could provide the raw material for selection to produce heterochrony--an evolutionary change in the timing of developmental events. Epigenetics and critical windows research recognizes that in ovo or fetal development represent a vulnerable period in the life history of an animal, when the developing organism may be unable to actively mitigate environmental perturbations. 'Critical windows' are periods of susceptibility or vulnerability to environmental or maternal challenges, periods when recovery from challenge is possible, and periods when the phenotype or epigenome has been altered. Developmental plasticity may allow survival in an altered environment, but it also has possible long-term consequences for the animal. "Catch-up growth" in humans after the critical perinatal window has closed elicits adult obesity and exacerbates a programmed hypertensive phenotype (one of many examples of "fetal programing"). Grand challenges for developmental physiology include integrating variation in developmental timing within and across generations, applying multiple stressor dosages and stressor exposure at different developmental timepoints, assessment of epigenetic and parental influences, developing new animal models and techniques, and assessing and implementing these designs and models in human health and development.