Perinatal changes in avian muscle: implications from ultrastructure for the development of endothermy

Ontogeny of skeletal and cardiac muscle mitochondria oxygen fluxes in two breeds of chicken

From its earliest days of domestication, the domestic chicken (Gallus gallus domesticus) has been selectively bred for specific traits. Decades of genetic selection have resulted in significant dissimilarities in metabolism and growth between breeds, in particular fast-growing broilers and highly productive layers. A chicken develops the capacity to elevate metabolism in response to decreases in ambient temperature upon hatching, including well-developed methods of regulating thermogenesis. However, a differential timing between incipient endothermic capacities of broiler and layer strains exists. Although both broiler and layer chicks show the hallmark rapid attainment of endothermic capacity of precocial birds, endothermic capacity of broilers matures faster than that of layers. Here we characterized changes in morphology and mitochondria physiology during the developmental transition as the animals become endothermic. Changes in body mass occurred at a faster rate in broilers, with hatching embryos showing significant increases over embryonic body mass, while layers did not exhibit significant differences in mass until after hatch. Heart and liver both exhibited rapid growth upon hatching that occurred with little change in body mass in both breeds. Skeletal and cardiac mitochondrial respiration capacity in broilers increased from the embryonic stage through hatching. Oxidative phosphorylation was more tightly coupled to ATP production in broilers than layer muscles during external pipping. By selecting for faster growth and higher meat yield, the physiological transition from ectothermy to endothermy was also affected: differences in whole-animal, tissue, and organelle responses are evident in these two divergent breeds of chicken.

Post-hatching development of mitochondrial function, organ mass and metabolic rate in two ectotherms, the American alligator (Alligator mississippiensis) and the common snapping turtle (Chelydra serpentina)

The ontogeny of endothermy in birds is associated with disproportionate growth of thermogenic organs and increased mitochondrial oxidative capacity. However, no similar study has been made of the development of these traits in ectotherms. For comparison, we therefore investigated the metabolism, growth and muscle mitochondrial function in hatchlings of a turtle and a crocodilian, two ectotherms that never develop endothermy. Metabolic rate did not increase substantially in either species by 30 days post-hatching. Yolk-free body mass and heart mass did not change through 30 days in alligators and heart mass was a constant proportion of body mass, even after 1 year. Yolk-free body mass and liver mass grew 36% and 27%, respectively, in turtles during the first 30 days post-hatch. The mass-specific oxidative phosphorylation capacity of mitochondria, assessed using permeabilized muscle fibers, increased by a non-significant 47% in alligator thigh and a non-significant 50% in turtle thigh over 30 days, but did not increase in the heart. This developmental trajectory of mitochondrial function is slower and shallower than that previously observed in ducks, which demonstrate a 90% increase in mass-specific oxidative phosphorylation capacity in thigh muscles over just a few days, a 60% increase in mass-specific oxidative phosphorylation capacity of the heart over a few days, and disproportionate growth of the heart and other organs. Our data thus support the hypothesis that these developmental changes in ducks represent mechanistic drivers for attaining endothermy.

Summary: Unlike endothermic species, there is little change in skeletal and cardiac muscle mitochondrial function from the embryonic stage through the first year of life in ectothermic alligators and snapping turtles.

Development of endothermy and concomitant increases in cardiac and skeletal muscle mitochondrial respiration in the precocial Pekin duck (Anas platyrhynchos domestica)

Attaining endothermic homeothermy occurs at different times post-hatching in birds and is associated with maturation of metabolic and aerobic capacity. Simultaneous measurements at the organism, organ and cellular levels during the transition to endothermy reveal means by which this change in phenotype occurs. We examined development of endothermy in precocial Pekin ducks ( ITALIC! Anas platyrhynchos domestica) by measuring whole-animal O2consumption ( ITALIC! V̇O2 ) as animals cooled from 35 to 15°C. We measured heart ventricle mass, an indicator of O2delivery capacity, and mitochondrial respiration in permeabilized skeletal and cardiac muscle to elucidate associated changes in mitochondrial capacities at the cellular level. We examined animals on day 24 of incubation through 7 days post-hatching. ITALIC! V̇O2  of embryos decreased when cooling from 35 to 15°C; ITALIC! V̇O2  of hatchlings, beginning on day 0 post-hatching, increased during cooling with a lower critical temperature of 32°C. Yolk-free body mass did not change between internal pipping and hatching, but the heart and thigh skeletal muscle grew at faster rates than the rest of the body as the animals transitioned from an externally pipped paranate to a hatchling. Large changes in oxidative phosphorylation capacity occurred during ontogeny in both thigh muscles, the primary site of shivering, and cardiac ventricles. Thus, increased metabolic capacity necessary to attain endothermy was associated with augmented metabolic capacity of the tissue and augmented increasing O2delivery capacity, both of which were attained rapidly at hatching.

Transition from ectothermy to endothermy: the development of metabolic capacity in a bird (Gallus gallus)

The evolution of endothermy is one of the most significant events in vertebrate evolution. Adult mammals and birds are delineated from their early ontogenetic stages, as well as from other vertebrates, by high resting metabolic rates and consequent internal heat production. We used the embryonic development of a bird (Gallus gallus) as a model to investigate the metabolic transition between ectothermy and endothermy. Increases in aerobic capacity occur at two functional levels that are regulated independently from each other: (i) upregulation of gene expression; and (ii) significant increases in the catalytic activity of the main oxidative control enzymes. Anaerobic capacity, measured as lactate dehydrogenase activity, is extremely high during early development, but diminishes at the same time as aerobic capacity increases. Changes in lactate dehydrogenase activity are independent from its gene expression. The regulatory mechanisms that lead to endothermic metabolic capacity are similar to those of ectotherms in their response to environmental change. We suggest that the phylogenetic occurrence of endothermy is restricted by its limited selective advantages rather than by evolutionary innovation.

A molecular model for the evolution of endothermy in the theropod-bird lineage

Ectothermy is a primitive state; therefore, a shared common ancestor of crocodiles, dinosaurs, and birds was at some point ectothermic. Birds, the extant descendants of the dinosaurs, are endothermic. Neither the metabolic transition within this lineage nor the place the dinosaurs held along the ectothermic-endothermic continuum is defined. This paper presents a conceptual model for the evolution of endothermy in the theropod-bird lineage. It is recognized that other animals (some fish, insects, etc.) are functionally endothermic. However, endothermy in other clades is beyond the scope of this paper, and we address the onset of endothermy in only the theropod/bird clade. The model begins with simple changes in a single gene of a common ancestor, and it includes a series of concomitant physiological and morphological changes, beginning perhaps as early as the first archosaurian common ancestor of dinosaurs and crocodiles. These changes continued to accumulate within the theropod-avian lineage, were maintained and refined through selective forces, and culminated in extant birds. Metabolic convergence or homoplasy is evident in the inherent differences between the endothermy of mammals and the endothermy of extant birds. The strength and usefulness of this model lie in the phylogenetic, genetic, evolutionary, and adaptive plausibility of each of the suggested developmental steps toward endothermy. The model, although conceptual in nature, relies on an extensive knowledge base developed by numerous workers in each of these areas. In addition, the model integrates known genetic, metabolic, and developmental aspects of extant taxa that phylogenetically bracket theropod dinosaurs for comparison with information derived from the fossil record of related extinct taxa.

Growth and Development of Homeothermy in Nestling European Shags (Phalacrocorax Aristotelis)

European Shag (Phalacrocorax aristotelis) nestlings were studied on a small island off the coast of central Norway. Increase in body mass (BM) with age (t, days) was described by the logistic equation: BM = 1,622 g/[1 + e−0.172(t−19.9)]. All growth parameters measured (body mass, and length of tarsus, wing and head) showed highest relative growth rate when the nestlings were 5–10 days old, that is, before the nestlings had achieved homeothermy. An incipient endothermic response was noted when nestlings were 9 days old, and they became homeothermic at ages of 15–18 days. Respective mass-specific resting metabolic rates for nestlings 0, 15, and 45 days old were 47, 261, and 147% of the predicted value for adult nonpasserine birds of similar body masses. Mass-specific minimal thermal conductance decreased from 366% of predicted adult value at hatching, to 220% of that predicted when nestlings were 21 days old. For nestlings 15 days old, the factorial metabolic scope (resting metabolic rate/peak metabolic rate) was only 1.5, but that increased rapidly thereafter. Rapid increase in the mass-specific RMR and decrease in minimal thermal conductance is suggested to contribute importantly to improve homeothermic ability during the first two weeks of the developmental period. At hatching, leg and pectoral muscles constituted 5.3 and 2.2%, respectively, of total wet body mass. Relative leg-muscle mass increased rapidly and had almost reached adult proportions when the nestlings were 25–30 days old. In contrast, pectoral-muscle mass increased in an almost direct proportion to the body mass during the first 30 days of the growth period, and increased rapidly thereafter. At hatching, the water fraction (water content/lipid-free wet mass) was significantly lower in the leg than in the pectoral muscles (0.920 vs. 0.931). The water fraction of leg muscles also remained lower during the entire growth period. Judging from the proportionately greater mass and higher degree of maturity of the leg compared to pectoral muscles, the former would seem to be the main site of cold-induced heat production during early development of homeothermy in European Shag nestlings.

Evolutionary significance of myosin heavy chain heterogeneity in birds

This article reviews the complexity, expression, genetics, regulation, function, and evolution of the avian myosin heavy chain (MyHC). The majority of pertinent studies thus far published have focussed on domestic chicken and, to a much lesser extent, Japanese quail. Where possible, information available about wild species has also been incorporated into this review. While studies of additional species might modify current interpretations, existing data suggest that some fundamental properties of myosin proteins and genes in birds are unique among higher vertebrates. We compare the characteristics of myosins in birds to those of mammals, and discuss potential molecular mechanisms and evolutionary forces that may explain how avian MyHCs acquired these properties.

Oxygen diffusion coefficient in isolated chicken red and white skeletal muscle fibers in ontogenesis

Oxygen diffusion from medium to cultured isolated muscle fibers from red gastrocnemius muscle (deep part) (RGM) and white pectoralis muscle (WPM) of embryonic and postnatal chickens (about 6 months) was explored. The intracellular effective O(2) diffusion coefficient (D(i)) in muscle fiber was calculated from a model of a cylindrical fiber with a uniform distribution of an oxygen sink based on these experimentally measured parameters: critical tension of O(2) (PO(2)) on the surface of a fiber, specific rate of O(2) consumption by a weight unit of muscle fibers (;VO(2)), and average diameter of muscle fibers. The results document the rapid hypertrophic growth of RGM fibers when compared to WPM fibers in the second half of the embryonic period and the higher values of;VO(2) and critical PO(2) during the ontogenetic period under study. The oxygen D(i) in RGM fibers of embryos and 1-day chickens was two to three times higher than observed for WPM fibers. For senior chickens, the oxygen D(i) value in RGM and WPM fibers does not differ. The D(i) of O(2) in both RGM and WPM fibers increased from 1.4-2.7 x 10(-8) to 90-95 x 10(-8) cm(2)/s with an ontogenetic increase in fiber diameter from 7. 5 to 67.0 microm. At all stages the oxygen D(i) values in RGM and WPM fibers are significantly lower than the O(2) diffusion coefficient in water: for 11-day embryos they are 889 and 1714 times lower and for adult individuals 25 and 27 times lower, respectively. Why oxygen D(i) values in RGM and WPM fibers are so low and why they are gradually increasing during the course of hypertrophic ontogenetic growth are still unclear.

Postnatal changes in mitochondrial protein mass and respiration in skeletal muscle from the newborn pig

Quantitative and functional changes occurring in mitochondria were studied in pig skeletal muscle between birth and 5 days of life. Postnatal changes were followed separately on intermyofibrillar and subsarcolemmal mitochondria isolated from rhomboïdeus (RH) and longissimus dorsi (LD) muscles. The integrity and purity of the isolated mitochondria was checked by electron microscopic observations. The mass of mitochondrial protein was not different between muscles at birth. It increased tremendously during the first 5 days of life, by 49% in LD (P < 0.001) and 93% in RH (P < 0.001) muscle and was 30% higher in RH than in LD muscle at 5 days of life (P < 0.05). Mitochondria isolated from RH muscle exhibited 30% higher oxidative and phosphorylative capacities than those from LD muscle at 5 days of life (P < 0.05). Intermyofibrillar (IM) mitochondria had high respiration rate, enzyme activities and coupling parameters (respiratory control ratio, phosphorus-oxygen ratio) from birth. Subsarcolemmal (SS) mitochondria were less active than IM mitochondria; their respiration rate and enzyme activities were 60% lower (P < 0.01) and increased with age, particularly in LD muscle (P < 0.05). Short-term cold exposure had no effect on mitochondrial mass and activity. These results suggest that muscle mitochondria are functional from birth and are changing primarily quantitatively. SS and IM mitochondria exhibit specific changes that are probably involved in the postnatal acquisition of skeletal muscle oxidative metabolism.