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

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.

Strategies for Hypothermia Compensation in Altricial and Precocial Newborn Mammals and Their Monitoring by Infrared Thermography

Thermoregulation in newborn mammals is an essential species-specific mechanism of the nervous system that contributes to their survival during the first hours and days of their life. When exposed to cold weather, which is a risk factor associated with mortality in neonates, pathways such as the hypothalamic–pituitary–adrenal axis (HPA) are activated to achieve temperature control, increasing the circulating levels of catecholamine and cortisol. Consequently, alterations in blood circulation and mechanisms to produce or to retain heat (e.g., vasoconstriction, piloerection, shivering, brown adipocyte tissue activation, and huddling) begin to prevent hypothermia. This study aimed to discuss the mechanisms of thermoregulation in newborn domestic mammals, highlighting the differences between altricial and precocial species. The processes that employ brown adipocyte tissue, shivering, thermoregulatory behaviors, and dermal vasomotor control will be analyzed to understand the physiology and the importance of implementing techniques to promote thermoregulation and survival in the critical post-birth period of mammals. Also, infrared thermography as a helpful method to perform thermal measurements without animal interactions does not affect these parameters.

PERINATAL CENTRILOBULAR HEPATIC NECROSIS IN GIANT PANDAS (AILUROPODA MELANOLEUCA): A RETROSPECTIVE STUDY

Between 1983 and 2012, six giant panda cubs (Ailuropoda melanoleuca) born at a zoological institution were stillborn or died between the ages of 3 and 200 h. Two of the six cubs had panhepatic centrilobular hepatic necrosis (CHN), granulocytic extramedullary hematopoiesis (GEM), positive liver culture for Staphylococcus species, and terminal liver failure. Another low-weight cub was administered oxygen therapy immediately after birth and developed hyaline membranes in air spaces and hepatic necrosis restricted to the hilar region. A retrospective analysis of liver and lung lesions, pulmonary microanatomy, blood-gas barrier ultrastructure, and hepatic myofibroblast proliferation was conducted on the six cubs. Neonates with CHN had concurrent severe periportal GEM accompanied by severe myofibroblast proliferation. The pulmonary blood-gas barrier was markedly increased in one cub with CHN. Developmentally, the lungs of all but one cub were at the late saccular stage, and the lowest-weight cub was in early saccular stage, consistent with immaturity, and had pneumonia comparable to neonatal respiratory distress syndrome (RDS). Stage of lung development was eliminated as the primary factor leading to CHN. The pathogenesis of CHN in these neonates is proposed to be transformation of hepatic stellate cells to myofibroblasts initiating blockage and microvascular constriction of hepatic sinusoids, resulting in insufficient perfusion and cellular hypoxia of hepatocytes surrounding central veins in acinar zone 3.

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.

Vascular Niche in Lung Alveolar Development, Homeostasis, and Regeneration

Endothelial cells (ECs) constitute small capillary blood vessels and contribute to delivery of nutrients, oxygen and cellular components to the local tissues, as well as to removal of carbon dioxide and waste products from the tissues. Besides these fundamental functions, accumulating evidence indicates that capillary ECs form the vascular niche. In the vascular niche, ECs reciprocally crosstalk with resident cells such as epithelial cells, mesenchymal cells, and immune cells to regulate development, homeostasis, and regeneration in various organs. Capillary ECs supply paracrine factors, called angiocrine factors, to the adjacent cells in the niche and orchestrate these processes. Although the vascular niche is anatomically and functionally well-characterized in several organs such as bone marrow and neurons, the effects of endothelial signals on other resident cells and anatomy of the vascular niche in the lung have not been well-explored. This review discusses the role of alveolar capillary ECs in the vascular niche during development, homeostasis and regeneration.

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.

Membrane mediated development of the vertebrate blood-gas-barrier

During embryonic lung development, establishment of the gas-exchanging units is guided by epithelial tubes lined by columnar cells. Ultimately, a thin blood-gas barrier (BGB) is established and forms the interface for efficient gas exchange. This thin BGB is achieved through processes, which entail lowering of tight junctions, stretching, and thinning in mammals. In birds the processes are termed peremerecytosis, if they involve cell squeezing and constriction, or secarecytosis, if they entail cutting cells to size. In peremerecytosis, cells constrict at a point below the protruding apical part, resulting in fusion of the opposing membranes and discharge of the aposome, or the cell may be squeezed by the more endowed cognate neighbors. Secarecytosis may entail formation of double membranes below the aposome, subsequent unzipping and discharge of the aposome, or vesicles form below the aposome, fuse in a bilateral manner, and release the aposome. These processes occur within limited developmental windows, and are mediated through cell membranes that appear to be of intracellular in origin. In addition, basement membranes (BM) play pivotal roles in differentiation of the epithelial and endothelial layers of the BGB. Laminins found in the BM are particularly important in the signaling pathways that result in formation of squamous pneumocytes and pulmonary capillaries, the two major components of the BGB. Some information exists on the contribution by BM to BGB formation, but little is known regarding the molecules that drive peremerecytosis, or even the origins and composition of the double and vesicular membranes involved in secarecytosis.

Effects of anticancer drug docetaxel on the structure and function of the rabbit olfactory mucosa

Docetaxel (DCT) is an anticancer drug which acts by disrupting microtubule dynamics in the highly mitotic cancer cells. Thus, this drug has a potential to affect function and organization of tissues exhibiting high cellular turnover. We investigated, in the rabbit, the effects of a single human equivalent dose (6.26 mg/kg, i.v.) of DCT on the olfactory mucosa (OM) through light and electron microscopy, morphometry, Ki-67 immunostaining, TUNEL assay and the buried food test for olfactory sensitivity. On post-exposure days (PED) 5 and 10, there was disarrangement of the normal cell layering in the olfactory epithelium (OE), apoptotic death of cells of the OE, Bowman's glands and axon bundles, and the presence (including on PED 3) of blood vessels in the bundle cores. A decrease in bundle diameters, olfactory cell densities and cilia numbers, which was most significant on PED 10 (49.3%, 63.4% and 50%, respectively), was also evident. Surprisingly by PED 15, the OM regained normal morphology. Furthermore, olfactory sensitivity decreased progressively until PED 10 when olfaction was markedly impaired, and with recovery from the impairment by PED 15. These observations show that DCT transiently alters the structure and function of the OM suggesting a high regenerative potential for this tissue.

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.

Development and remodeling of the vertebrate blood-gas barrier

During vertebrate development, the lung inaugurates as an endodermal bud from the primitive foregut. Dichotomous subdivision of the bud results in arborizing airways that form the prospective gas exchanging chambers, where a thin blood-gas barrier (BGB) is established. In the mammalian lung, this proceeds through conversion of type II cells to type I cells, thinning, and elongation of the cells as well as extrusion of the lamellar bodies. Subsequent diminution of interstitial tissue and apposition of capillaries to the alveolar epithelium establish a thin BGB. In the noncompliant avian lung, attenuation proceeds through cell-cutting processes that result in remarkable thinning of the epithelial layer. A host of morphoregulatory molecules, including transcription factors such as Nkx2.1, GATA, HNF-3, and WNT5a; signaling molecules including FGF, BMP-4, Shh, and TFG- β and extracellular proteins and their receptors have been implicated. During normal physiological function, the BGB may be remodeled in response to alterations in transmural pressures in both blood capillaries and airspaces. Such changes are mitigated through rapid expression of the relevant genes for extracellular matrix proteins and growth factors. While an appreciable amount of information regarding molecular control has been documented in the mammalian lung, very little is available on the avian lung.

Pre-hatch lung development in the ostrich

We studied development of the ostrich lung using light microscopy as well as electron microscopy techniques. At E24, the lung comprised a few epithelial tubes, interspersed with abundant mesenchyme with scattered profiles of incipient blood vessels. Between E24 and E39, the epithelial thickness was reduced by 90% from 13.5 ± 0.41 μm to 1.33 ± 0.014 μm (mean ± SD, respectively). Atria were evident at E32, and by E35, the first portions of the blood-gas barrier (BGB) measuring 3.41 ± 1.12 μm were encountered. Gas exchange tissue was well formed by E39 with atria, infundibulae, air capillaries and a mature blood-gas barrier (BGB). BGB formation proceeded through the complex processes of secarecytosis and peremerecytosis, which entailed decapitation of epithelial cells by cutting or pinching off respectively and by E39, the BGB was thin at 2.21 ± 1.21 μm. Vascular remodeling by intussusceptive angiogenesis was a late stage process mediated by intraluminal pillars in the pulmonary vasculature.

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 pulmonary blood-gas barrier in the avian embryo: inauguration, development and refinement

In vertebrates, efficient gas exchange depends primarily on establishment of a thin blood-gas barrier (BGB). The primordial air conduits of the developing avian lung are lined with a cuboidal epithelium that is ultimately converted to a squamous one that participates in the formation of the BGB. In the early stages, cells form intraluminal protrusions (aposomes) then transcellular double membranes separating the aposome from the basal part of the cell establish, unzip and sever the aposome from the cell. Additionally, better endowed cells squeeze out adjacent cells or such cells constrict spontaneously thus extruding the squeezed out aposome. Formation of vesicles or vacuoles below the aposome and fusion of such cavities with their neighboring cognates results in severing of the aposome. Augmentation of cavities and their subsequent fusion with the apical plasma membranes results in formation of numerous microfolds separating concavities on the apical part of the cell. Abscission of such microfolds results in a smooth squamous epithelium just before hatching.

Comparative functional structure of the olfactory mucosa in the domestic dog and sheep

Olfactory acuity differs among animal species depending on age and dependence on smell. However, the attendant functional anatomy has not been elucidated. We sought to determine the functional structure of the olfactory mucosa in suckling and adult dog and sheep. Mucosal samples harvested from ethmoturbinates were analyzed qualitatively and quantitatively. In both species, the olfactory mucosa comprised olfactory, supporting and basal cells, and a lamina propria containing bundles of olfactory cell axons, Bowman's glands and vascular elements. The olfactory cells terminated apically with an expanded knob, from which cilia projected in a radial fashion from its base and in form of a tuft from its apex in the dog and the sheep respectively. Olfactory cilia per knob were more numerous in the dog (19 ± 3) compared to the sheep (7 ± 2) (p<0.05). In the dog, axonal bundles exhibited one to two centrally located capillaries and the bundles were of greater diameters (73.3 ± 10.3 μm) than those of the sheep (50.6 ± 6.8 μm), which had no capillaries. From suckling to adulthood in the dog, the packing density of the olfactory and supporting cells increased by 22.5% and 12.6% respectively. Surprisingly in the sheep, the density of the olfactory cells decreased by 26.2% while that of the supportive cells showed no change. Overall epithelial thickness reached 72.5 ± 2.9 μm in the dog and 56.8 ± 3.1 μm in the sheep. These observations suggest that the mucosa is better structurally refined during maturation in the dog than in the sheep.

Lung development of monotremes: evidence for the mammalian morphotype

The reproductive strategies and the extent of development of neonates differ markedly between the three extant mammalian groups: the Monotremata, Marsupialia, and Eutheria. Monotremes and marsupials produce highly altricial offspring whereas the neonates of eutherian mammals range from altricial to precocial. The ability of the newborn mammal to leave the environment in which it developed depends highly on the degree of maturation of the cardio-respiratory system at the time of birth. The lung structure is thus a reflection of the metabolic capacity of neonates. The lung development in monotremes (Ornithorhynchus anatinus, Tachyglossus aculeatus), in one marsupial (Monodelphis domestica), and one altricial eutherian (Suncus murinus) species was examined. The results and additional data from the literature were integrated into a morphotype reconstruction of the lung structure of the mammalian neonate. The lung parenchyma of monotremes and marsupials was at the early terminal air sac stage at birth, with large terminal air sacs. The lung developed slowly. In contrast, altricial eutherian neonates had more advanced lungs at the late terminal air sac stage and postnatally, lung maturation proceeded rapidly. The mammalian lung is highly conserved in many respects between monotreme, marsupial, and eutherian species and the structural differences in the neonatal lungs can be explained mainly by different developmental rates. The lung structure of newborn marsupials and monotremes thus resembles the ancestral condition of the mammalian lung at birth, whereas the eutherian newborns have a more mature lung structure.