Convection requirement is established by total metabolic rate in the newborn tammar wallaby

Development of the pulmonary vasculature in the gray short-tailed opossum (Monodelphis domestica)-3D reconstruction by microcomputed tomography

In the marsupial gray short-tailed opossum (Monodelphis domestica), the majority of lung development, including the maturation of pulmonary vasculature, takes place in ventilated functioning state during the postnatal period. The current study uses X-ray computed tomography (μCT) to three-dimensionally reconstruct the vascular trees of the pulmonary artery and pulmonary vein in 15 animals from neonate to postnatal day 57. The final 3D reconstructions of the pulmonary artery and pulmonary vein in the neonate and at 21, 35, and 57 dpn were transformed into a centerline model of the vascular trees. Based on the reconstructions, the generation of end-branching vessels, the median and maximum generation, and the number of vessels were calculated for the lungs. The pulmonary vasculature follows the lung anatomy with six pulmonary lobes indicated by the bronchial tree. The pulmonary arteries follow the bronchial tree closely, in contrast to the pulmonary veins, which run between the pulmonary segments. At birth the pulmonary vasculature has a simple branching pattern with a few vessel generations. Compared with the bronchial tree, the pulmonary vasculature appears to be more developed and extends to the large terminal air spaces. The pulmonary vasculature shows a marked gain in volume and a progressive increase in vascular complexity and density. The gray short-tailed opossum resembles the assumed mammalian ancestor and is suitable to inform on the evolution of the mammalian lung. Vascular genesis in the marsupial bears resemblance to developmental patterns described in eutherians. Lung development in general seems to be highly conservative within mammalian evolution.

Development of the terminal air spaces in the gray short-tailed opossum (Monodelphis domestica)- 3D reconstruction by microcomputed tomography

Marsupials are born with structurally immature lungs when compared to eutherian mammals. The gray short-tailed opossum (Monodelphis domestica) is born at the late canalicular stage of lung development. Despite the high degree of immaturity, the lung is functioning as respiratory organ, however supported by the skin for gas exchange during the first postnatal days. Consequently, the majority of lung development takes place in ventilated functioning state during the postnatal period. Microcomputed tomography (μCT) was used to three-dimensionally reconstruct the terminal air spaces in order to reveal the timeline of lung morphogenesis. In addition, lung and air space volume as well as surface area were determined to assess the functional relevance of the structural changes in the developing lung. The development of the terminal air spaces was examined in 35 animals from embryonic day 13, during the postnatal period (neonate to 57 days) and in adults. At birth, the lung of Monodelphis domestica consists of few large terminal air spaces, which are poorly subdivided and open directly from short lobar bronchioles. During the first postnatal week the number of smaller terminal air spaces increases and numerous septal ridges indicate a process of subdivision, attaining the saccular stage by 7 postnatal days. The 3D reconstructions of the terminal air spaces demonstrated massive increases in air sac number and architectural complexity during the postnatal period. Between 28 and 35 postnatal days alveolarization started. Respiratory bronchioles, alveolar ducts and a typical acinus developed. The volume of the air spaces and the surface area for gas exchange increased markedly with alveolarization. The structural transformation from large terminal sacs to the final alveolar lung in the gray short-tailed opossum follows similar patterns as described in other marsupial and placental mammals. The processes involved in sacculation and alveolarization during lung development seem to be highly conservative within mammalian evolution.

Respiratory characteristics of the tammar wallaby pouch young and functional limitations in a newborn with skin gas exchange

A short gestation, low birth weight and presence of cutaneous exchange of O₂ and CO₂ comprise altricial features of newborn marsupials and that collectively implies a highly immature respiratory system. In the present study, we investigated various respiratory characteristics of the neonatal/postnatal tammar wallaby, a species of marsupial in which > 30% of the newborn's total O₂ demands are supported by cutaneous rather than pulmonary gas exchange. The ventilatory response (HVR) to acute hypoxia (10% inspired O₂) was absent in the newborn (1 day old) pouch young; a hypoxic hypometabolism contributed entirely to the hyperventilation (increased pulmonary convection requirement). A high (compared to older animals) resting metabolic cost to breathe and an inefficient respiratory system suggest the lack of a HVR might be due to an energetic constraint that impinges on their ability to sustain an increase in ventilation. The latter was supported by the inability of the newborn to tolerate metabolic-ventilatory stimulation following administration of the metabolic uncoupler, 2,4-dinitrophenol (2,4-DNP). At 1 week of age, the cost of breathing was reduced, which coincided with the expression of a significant ventilatory response to hypoxia, a more energetically efficient respiratory system, and tolerance to 2,4-DNP. These data suggest this species of marsupial is born with major respiratory insufficiency, and that their pronounced dependence on the skin for metabolic gas exchange is of critical importance for survival.

Early postnatal lung development in the eastern quoll (Dasyurus viverrinus)

Early postnatal lung development (1-25 days) in the eastern quoll (Dasyurus viverrinus) was investigated to assess the morphofunctional status of one of the most immature marsupial neonates. Lung volume, surface density, surface area, and parenchymal and nonparenchymal volume proportions were determined using light microscopic morphometry. The lungs of the neonate were at the canalicular stage and consisted of two "balloon-like" airways with few septal ridges. The absolute volume of the lung was only 0.0009 cm³ with an air space surface density of 108.83 cm^-1 and a surface area of 0.082 cm² . The increase in lung volume in the first three postnatal days was mainly due to airspace expansion. The rapid postnatal development of the lung was indicated by an increase in the septal proportion of the parenchyma around day 4, which was reflected by an increase in the airspace surface density and surface area. By day 5, the lung entered the saccular stage of development with a reduction in septal thickness, expansion of the tubules into saccules and development of a double capillary system. The subsequent saccular period was characterized by repetitive septation steps, which increased the number of airway generations. The lungs of the newborn Dasyurus viverrinus must be considered as structurally and quantitatively insufficient to meet the respiratory requirements at birth. Hence, cutaneous gas exchange might be crucial for the first three postnatal days. The lung has to mature rapidly in the early postnatal period to support the increased metabolic requirements of the developing young.

Development of the skin in the eastern quoll (Dasyurus viverrinus) with focus on cutaneous gas exchange in the early postnatal period

A morphological and morphometric study of the skin development in the eastern quoll (Dasyurus viverrinus) was conducted to follow the transition from cutaneous to pulmonary gas exchange in this extremely immature marsupial species. Additionally, the development of the cardiac and respiratory system was followed, to evaluate the systemic prerequisites allowing for cutaneous respiration. The skin in the newborn D. viverrinus was very thin (36 ± 3 µm) and undifferentiated (no hair follicles, no sebaceous and perspiratory glands). Numerous superficial cutaneous capillaries were encountered, closely associated with the epidermis, allowing for gaseous exchange. The capillary volume density was highest in the neonate (0.33 ± 0.04) and decreased markedly during the first 4 days (0.06 ± 0.01). In the same time period, the skin diffusion barrier increased from 9 ± 1 µm to 44 ± 6 µm. From this age on the skin development was characterized by thickening of the different cutaneous layers, formation of hair follicles (day 55) and the occurrence of subcutaneous fat (day 19). The heart of the neonate D. viverrinus had incomplete interatrial, inter‐ventricular, and aortico‐pulmonary septa, allowing for the possibility that oxygenated blood from the skin mixes with that of the systemic circulation. The fast‐structural changes in the systemic circulations (closing all shunts) in the early postnatal period (3 days) necessitate the transition from cutaneous to pulmonary respiration despite the immaturity of the lungs. At this time, the lung was still at the canalicular stage of lung development, but had to be mature enough to meet the respiratory needs of the growing organism. The morphometric results for the skin development of D. viverrinus suggest that cutaneous respiration is most pronounced in neonates and decreases rapidly during the first 3 days of postnatal life. After this time a functional transition of the skin from cutaneous respiration to insulation and protection of the body takes place.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Skin structure in newborn marsupials with focus on cutaneous gas exchange

A morphological and morphometric study of the skin of a variety of newborn marsupials (Dasyurus viverrinus, Monodelphis domestica, Trichosurus vulpecula, Isoodon obesulus, Perameles nasuta, Phascolarctos cinereus, Potorous tridactylus, Petrogale penicillata, Thylogale thetidi, Macropus dorsalis) and of a monotreme hatchling (Ornithorhynchus anatinus) was undertaken to assess the possibility of cutaneous gas exchange. Additionally, the lungs of some of these species were investigated to assess its structural degree at birth. The skin in the different newborn marsupials and the monotreme hatchling had a similar structure (no hair follicles and no sebaceous or perspiratory glands) and was in all cases less developed than the skin of altricial eutherians. The thickness of the entire skin (36-186 μm) and its different layers, epidermis (6-29 μm) and dermis (29-171 μm) varied among the marsupial species and reflected the differences in size and developmental degree of the neonates. In the skin of all marsupial neonates and the monotreme hatchling, numerous superficial cutaneous capillaries were encountered, some closely associated with the epidermis, indicating the possibility that the skin participated in gaseous exchange. The skin of the newborn D. viverrinus had the highest capillary volume density and shortest skin diffusion barrier of all marsupial neonates, suggesting that skin gas exchange in the dasyurid neonate might be the most pronounced. A graduation of the skin capillary density among the marsupial neonates inversely followed the respective lung structure and general developmental degree of the neonates.

Comparative anatomy of neonates of the three major mammalian groups (monotremes, marsupials, placentals) and implications for the ancestral mammalian neonate morphotype

The existing different modes of reproduction in monotremes, marsupials and placentals are the main source for our current understanding of the origin and evolution of the mammalian reproduction. The reproductive strategies and, in particular, the maturity states of the neonates differ remarkably between the three groups. Monotremes, for example, are the only extant mammals that lay eggs and incubate them for the last third of their embryonic development. In contrast, marsupials and placentals are viviparous and rely on intra-uterine development of the neonates via choriovitelline (mainly marsupials) and chorioallantoic (mainly placentals) placentae. The maturity of a newborn is closely linked to the parental care strategy once the neonate is born. The varying developmental degrees of neonates are the main focus of this study. Monotremes and marsupials produce highly altricial and nearly embryonic offspring. Placental mammals always give birth to more developed newborns with the widest range from altricial to precocial. The ability of a newborn to survive and grow in the environment it was born in depends highly on the degree of maturation of vital organs at the time of birth. Here, the anatomy of four neonates of the three major extant mammalian groups is compared. The basis for this study is histological and ultrastructural serial sections of a hatchling of Ornithorhynchus anatinus (Monotremata), and neonates of Monodelphis domestica (Marsupialia), Mesocricetus auratus (altricial Placentalia) and Macroscelides proboscideus (precocial Placentalia). Special attention was given to the developmental stages of the organs skin, lung, liver and kidney, which are considered crucial for the maintenance of vital functions. The state of the organs of newborn monotremes and marsupials are found to be able to support a minimum of vital functions outside the uterus. They are sufficient to survive, but without capacities for additional energetic challenges. The organs of the altricial placental neonate are further developed, able to support the maintenance of vital functions and short-term metabolic increase. The precocial placental newborn shows the most advanced state of organ development, to allow the maintenance of vital functions, stable thermoregulation and high energetic performance. The ancestral condition of a mammalian neonate is interpreted to be similar to the state of organ development found in the newborns of marsupials and monotremes. In comparison, the newborns of altricial and precocial placentals are derived from the ancestral state to a more mature developmental degree associated with advanced organ systems.

Phase contrast imaging reveals low lung volumes and surface areas in the developing marsupial

Marsupials are born with immature lungs when compared to eutherian mammals and rely, to various extents, on cutaneous gas exchange in order to meet metabolic requirements. Indeed, the fat-tailed dunnart is born with lungs in the canalicular stage of development and relies almost entirely on the skin for gas exchange at birth; consequently undergoing the majority of lung development in air. Plane radiographs and computed tomography data sets were acquired using phase contrast imaging with a synchrotron radiation source for two marsupial species, the fat-tailed dunnart and the larger tammar wallaby, during the first weeks of postnatal life. Phase contrast imaging revealed that only two lung sacs contain air after the first hour of life in the fat-tailed dunnart. While the lung of the tammar wallaby was comparatively more developed, both species demonstrated massive increases in air sac number and architectural complexity during the postnatal period. In addition, both the tammar wallaby and fat-tailed dunnart had lower lung volumes and parenchymal surface areas than were expected from morphometrically determined allometric equations relating these variables to body mass during the neonatal period. However, lung volume is predicted to scale with mass as expected after the neonatal marsupial reaches a body mass of ∼1 g and no longer relies on the skin for gas exchange. Decreased lung volume in the marsupial neonate further supports the maxim that cutaneous gas exchange occurs in the marsupial neonate because the respiratory apparatus is not yet capable of meeting the gas exchange requirements of the newborn.

The ventilatory response to hypoxia and hypercapnia is absent in the neonatal fat-tailed dunnart

At birth, the newborn fat-tailed dunnart relies on cutaneous gas exchange to meet metabolic demands, with continuous lung ventilation emerging several days later. We hypothesized that the delayed expression of lung ventilation (VE) in these animals is in part owing to a low responsiveness of the respiratory control system to blood gas perturbations. To address this hypothesis we assessed the ventilatory and metabolic response to hypoxia (10% O2) and hypercapnia (5% CO2) using closed-system respirometry from birth to 23 days postpartum (P). Neonatal fat-tailed dunnarts displayed no significant hypoxic or hypercapnic ventilatory responses at any age. Regardless, significant hyperventilation through a suppression of metabolic rate (Vo2) was observed at birth in response to hypercapnia and in response to hypoxia at all ages, except P12. Therefore, reliance on cutaneous gas exchange during early life may be partially attributed to reduced chemosensitivity or a lack of central integration of chemosensitive afferent information. This may be in part due to the relative immaturity of this species at birth, compared to other mammals.

Structural and functional development of the respiratory system in a newborn marsupial with cutaneous gas exchange

Marsupials are born with structurally immature lungs and rely, to varying degrees, on cutaneous gas exchange. With a gestation of 13 d and a birth weight of 13 mg, the fat-tailed dunnart (Sminthopsis crassicaudata) is one of the smallest and most immature marsupial newborns. We determined that the skin is almost solely responsible for gas exchange in the early neonatal period. Indeed, fewer than 35% of newborn dunnarts were observed to make any respiratory effort on the day of birth, with pulmonary ventilation alone not meeting the demand for oxygen until approximately 35 d postpartum. Despite the lack of pulmonary ventilation, the phrenic nerve had made contact with the diaphragm, and the respiratory epithelium was sufficiently developed to support gas exchange on the day of birth. Both type I and type II (surfactant-producing) alveolar epithelial cells were present, with fewer than 7% of the cells resembling undifferentiated alveolar epithelial precursor cells. The type I epithelial cells did, however, display thickened cytoplasmic extensions, leading to a high diffusion distance for oxygen. In addition, the architecture of the lung was immature, resembling the early canalicular stage, with alveolarization not commencing until 45 d postpartum. The pulmonary vasculature was also immature, with a centrally positioned single-capillary layer not evident until 100 d postbirth. These structural limitations may impede efficient pulmonary gas exchange, forcing the neonatal fat-tailed dunnart to rely predominately on its skin, a phenomenon supported by a low metabolic rate and small size.

The structural design of the bat wing web and its possible role in gas exchange

The structure of the skin in the epauletted fruit bat (Epomophorus wahlbergi) wing and body trunk was studied with a view to understanding possible adaptations for gas metabolism and thermoregulation. In addition, gas exchange measurements were performed using a respirometer designed for the purpose. The body skin had an epidermis, a dermis with hair follicles and sweat glands and a fat-laden hypodermis. In contrast, the wing web skin was made up of a thin bilayered epidermis separated by a connective tissue core with collagen and elastic fibres and was devoid of hair follicles and sweat glands. The wings spanned 18-24 cm each, with about 753 cm2 of surface exposed to air. The body skin epidermis was thick (61 +/- 3 microm, SEM), the stratum corneum alone taking a third of it (21 +/- 3 microm). In contrast, the wing web skin epidermis was thinner at 9.8 +/- 0.7 microm, with a stratum corneum measuring 4.1 +/- 0.3 microm (41%). The wing capillaries in the wing web skin ran in the middle of the connective tissue core, with a resultant surface-capillary diffusion distance of 26.8 +/- 3.2 microm. The rate of oxygen consumption (VO2) of the wings alone and of the whole animal measured under light anaesthesia at ambient temperatures of 24 masculineC and 33 masculineC, averaged 6% and 10% of the total, respectively. Rate of carbon dioxide production had similar values. The membrane diffusing capacity for the wing web was estimated to be 0.019 ml O2 min(-1) mmHg(-1). We conclude that in Epomophorus wahlbergi, the wing web has structural modifications that permit a substantial contribution to the total gas exchange.

Functional respiratory morphology in the newborn quokka wallaby (Setonix brachyurus)

A morphological and morphometric study of the lung of the newborn quokka wallaby (Setonix brachyurus) was undertaken to assess its morphofunctional status at birth. Additionally, skin structure and morphometry were investigated to assess the possibility of cutaneous gas exchange. The lung was at canalicular stage and comprised a few conducting airways and a parenchyma of thick-walled tubules lined by stretches of cuboidal pneumocytes alternating with squamous epithelium, with occasional portions of thin blood-gas barrier. The tubules were separated by abundant intertubular mesenchyme, aggregations of developing capillaries and mesenchymal cells. Conversion of the cuboidal pneumocytes to type I cells occurred through cell broadening and lamellar body extrusion. Superfluous cuboidal cells were lost through apoptosis and subsequent clearance by alveolar macrophages. The establishment of the thin blood-gas barrier was established through apposition of the incipient capillaries to the formative thin squamous epithelium. The absolute volume of the lung was 0.02 +/- 0.001 cm(3) with an air space surface area of 4.85 +/- 0.43 cm(2). Differentiated type I pneumocytes covered 78% of the tubular surface, the rest 22% going to long stretches of type II cells, their precursors or low cuboidal transitory cells with sparse lamellar bodies. The body weight-related diffusion capacity was 2.52 +/- 0.56 mL O(2) min(-1) kg(-1). The epidermis was poorly developed, and measured 29.97 +/- 4.88 microm in thickness, 13% of which was taken by a thin layer of stratum corneum, measuring 4.87 +/- 0.98 microm thick. Superficial capillaries were closely associated with the epidermis, showing the possibility that the skin also participated in some gaseous exchange. Qualitatively, the neonate quokka lung had the basic constituents for gas exchange but was quantitatively inadequate, implying the significance of percutaneous gas exchange.

Development of the respiratory system in marsupials

Marsupials at birth are small and relatively undeveloped. At birth, the lung in some species is at the canalicular stage of development and though lung diffusion and metabolic rate are strongly correlated, the allometric exponent suggests that smaller newborns have relatively smaller diffusing capacity with respect to their demand for oxygen. Without improvement in functional or structural parameters newborn marsupials are reliant to varying degrees on skin gas exchange to compensate for the immaturity of the lung. Indeed, in some species there is complete reliance on the skin for gas exchange at birth. Nevertheless, with an early dependence on ventilation, the CNS would appear already to contain neurons with properties and connections that permit rhythmic motor output at birth and pulmonary reflexes mature soon after. Despite appropriate neural control and the presence of surfactant, the highly compliant nature of the newborn chest wall results in substantial chest wall distortion during inspiratory effort which reduce the efficacy of the lung for ventilation. This review explores the morpho-functional development of the respiratory system, including oxygen transport and cardiac shunts, and the establishment of convective requirement in marsupials, a group that places emphasis on extended postnatal development.

Development of mechanics and pulmonary reflexes

The mechanical properties of the respiratory system are paramount in converting neural output into ventilation. The highly compliant chest wall of the newborn results in chest distortion and volume loss during inspiration and, as the chest is also unable to resist the inward recoil of the lung, there is a reduction in lung volume at end expiration (functional residual capacity) and a tendency for alveoli to collapse. Vagal innervation of the lungs and airways is responsible for eliciting various reflexes that result in the dynamic modification of respiratory mechanics and an improvement in ventilation. From the first breath, the newborn increases the frequency of augmented breaths to improve lung compliance and prolongs the expiratory time constant in order to increase the amount of air remaining in the lung at end expiration and help prevent lung collapse. This review examines the respiratory mechanics of the mammalian neonate at birth and during early development together with the vagal reflexes that are responsible for the dynamic modification of respiratory mechanics in order to ensure that effective gas exchange occurs from birth.

Hypothermia and hypoxia inhibit the Hering-Breüer reflex in the marsupial newborn

The effects of lowering body temperature (Tb) on metabolic rate, ventilation, and the strength of the Hering-Breüer expiratory promoting reflex (HB reflex; determined from an inhibitory ratio calculated from volumetric measurements of the respiratory rhythm) were examined in 18-day-old ectothermic pouch young of the tammar wallaby during normoxia or hypoxia (10% O2). Hypoxia and hypothermia, either singularly or combined, depressed metabolic rate. At all Tb, the hypoxic hyperventilation was associated with a significant hyperpnea. At pouch Tb (36.5°C) during normoxia, inflation of the lungs with -5 or -10 cmH2O extrathoracic pressure induced a significant HB reflex. Exposure to cold reduced the strength of the reflex, almost abolishing it at 28°C. For Tb above 28°C, the reflex in hypoxia was always less than the corresponding normoxic value. Taken in context with the changes in metabolic state that occurred, these data in the ectothermic marsupial newborn suggest that the decline in the HB reflex during moderate hypothermia is the result of a direct effect of Tb on vagal mechanisms rather than a temperature-driven decline in metabolic rate that should have acted to strengthen the HB reflex. Therefore, it seems that inputs inhibitory to breathing are more negatively affected during cold than those inputs that are excitatory.

Morphometry and allometry of the postnatal marsupial lung development: an ultrastructural study

An utrastructural morphometric study of the postnatally remodelling lungs of the quokka wallaby (Setonix brachyurus) was undertaken. Allometric scaling of the volumes of the parenchymal components against body mass was performed. Most parameters showed a positive correlation with body mass in all the developmental stages, except the volume of type II pneumocytes during the alveolar stage. The interstitial tissue and type II cell volumes increased slightly faster than body mass in the saccular stage, their growth rates declining in the alveolar stage. Conversely, type I pneumocyte volumes increased markedly in both the saccular and alveolar stages. Both capillary and endothelial volumes as well as the capillary and airspace surface areas showed highest rates of increase during the alveolar stage, at which time the rate was notably higher than that of the body mass. The pulmonary diffusion capacity increased gradually, the rate being highest in the alveolar stage and the adult values attained were comparable to those of eutherians.

Mechanics of the respiratory system in the newborn tammar wallaby

We investigated whether the mechanical properties of the respiratory system represent a major constraint to spontaneous breathing in the newborn tammar wallaby Macropus eugenii, which is born after a very short gestation (approximately 28 days, birth mass approximately 380 mg). The rate of oxygen consumption (V̇O2) through the skin was approximately 33 % of the total V̇O2 at day 1 and approximately 14 % at day 6. The mass-specific resting minute ventilation (V̇e) and the ventilatory equivalent (V̇e/V̇O2) were approximately the same at the two ages, with a breathing pattern significantly deeper and slower at day 1. The mass-specific compliance of the respiratory system (Crs) did not differ significantly between the two age groups and was close to the values predicted from measurements in eutherian newborns. Mass-specific respiratory system resistance (Rrs) at day 1 was higher than at day 6, and also higher than in eutherian newborns. Chest distortion, quantified as the degree of abdominal motion during spontaneous breathing compared with that required to inflate the lungs passively, at day 1 was very large, whereas it was modest at day 6. We conclude that, in the tammar wallaby at birth, the high resistance of the respiratory system and the distortion of the chest wall greatly reduce the mechanical efficiency of breathing. At this age, gas exchange through the skin is therefore an important complement to pulmonary ventilation.