Descriptive study of the diaphragm and lungs in the short-nosed echidna, Tachyglossus aculeatus (Mammalia: monotremata)

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.

Tachyglossus aculeatus (Monotremata: Tachyglossidae)

Tachyglossus aculeatus (Shaw, 1792) is a monotreme commonly called the short-beaked echidna. Although considered Australia’s most common native mammal because of its continent-wide distribution, its population numbers everywhere are low. It is easily distinguished from all other native Australian mammals because of its spine-covered body, hairless beak, and unique “rolling” gait. The five subspecies, one of which is found in Papua New Guinea, show variations in fur density, spine diameter, length, and number of grooming claws. The Kangaroo Island short-beaked echidna Tachyglossus aculeatus multiaculeatus is listed as “Endangered” but all other Tachyglossus are listed as “Least Concern” in the 2016 International Union for Conservation of Nature and Natural Resources Red List.

A new scenario of the evolutionary derivation of the mammalian diaphragm from shoulder muscles

The evolutionary origin of the diaphragm remains unclear, due to the lack of a comparable structure in other extant taxa. However, recent researches into the developmental mechanism of this structure have yielded new insights into its origin. Here we summarize current understanding regarding the development of the diaphragm, and present a possible scenario for the evolutionary acquisition of this uniquely mammalian structure. Recent developmental analyses indicate that the diaphragm and forelimb muscles are derived from a shared cell population during embryonic development. Therefore, the embryonic positions of forelimb muscle progenitors, which correspond to the position of the brachial plexus, likely played an important role in the evolution of the diaphragm. We surveyed the literature to reexamine the position of the brachial plexus among living amniotes and confirmed that the cervico-thoracic transition in ribs reflects the brachial plexus position. Using this osteological correlate, we concluded that the anterior borders of the brachial plexuses in the stem synapsids were positioned at the level of the fourth spinal nerve, suggesting that the forelimb buds were laid in close proximity of the infrahyoid muscles. The topology of the phrenic and suprascapular nerves of mammals is similar to that of subscapular and supracoracoid nerves, respectively, of the other amniotes, suggesting that the diaphragm evolved from a muscle positioned medial to the pectoral girdle (cf. subscapular muscle). We hypothesize that the diaphragm was acquired in two steps: first, forelimb muscle cells were incorporated into tissues to form a primitive diaphragm in the stem synapsid grade, and second, the diaphragm in cynodonts became entrapped in the region controlled by pulmonary development.

Assessment of goblet cell orifice distribution across the rabbit bulbar conjunctiva based on numerical density and nearest neighbors analysis

PURPOSE

To assess density and spatial distribution of the goblet cell orifices at the surface of the rabbit bulbar conjunctiva as an indicator of functional activity.

METHODS

Specimens of the superior or inferior bulbar conjunctiva from six healthy young adult (2 kg) pigmented rabbits were obtained using a special preparation technique by which the conjunctiva was carefully stretched out during fixation with buffered glutaraldehyde. The apical surface of the specimens was examined by scanning electron microscopy. From high magnification prints, the areas and dimensions of 32-49 orifices/image were measured. In addition, the centre-to-centre spacing and spatial distribution of the orifices were assessed using a nearest neighbors principle.

RESULTS

The bulbar conjunctival surface is composed of polygonal cells decorated with surface microplicae and in between which are individual goblet cell orifices. The goblet cell orifices are characterized by tending to be oval in shape (long:short dimensions ratio of 1.57 +/- 0.23) and usually having a distinct line of microvilli around the perimeter. The orifices had a wide range of areas (from 13 to 188 μm(2); group mean +/- SD of 54 +/- 36 μm(2)), and distribution of orifice areas was skewed or even bimodal. The overall orifice density was 387 +/- 68/mm(2), with the group-averaged nearest neighbors distance being 34 +/- 3 μm. Comparisons of the measured nearest neighbors distances to that for an optimum spacing based on numerical density reveals the goblet cell orifices to be slightly further apart and that they were not obviously in groups or clustered.

CONCLUSIONS

Goblet cell orifices at the bulbar conjunctival surface, a presumed indicator of functional secretory activity, appear to have reproducible density and a discrete and reasonably predictable spatial distribution.

Fixed cervical count and the origin of the mammalian diaphragm

Why is mammalian cervical count fixed across the historically long and ecologically broad mammalian radiation? Multiple lines of evidence, considered together, suggest a link between fixed cervical count and the muscularization of the diaphragm, a key innovation in mammalian history. We test this hypothesis by documenting the anteroposterior (AP) movement of the diaphragm, a lateral plate derivative, relative to that of the somitic thoracolumbar transition in unusually patterned mammals, by comparing the temporal occurrence of an osteological proxy for the diaphragm and fixed cervical counts in the fossil record, and by quantifying morphological differentiation within the mammalian cervical series. We then integrate these anatomical observations with details of diaphragm function and development to propose a sequence of innovations in mammalian evolution that could have led to fixed cervical count. We argue that the novel commitment of migratory muscle precursor cells (MMPs) of mid-cervical somites to a fate in the abaxial diaphragm defined these somites as a new and uniquely mammalian modular subunit. We further argue that the coordination of primaxial abaxial patterning constrained subsequent AP migration of the forelimb, thereby secondarily fixing cervical count.

The evolutionary origin of the mammalian diaphragm

The comparatively low compliance of the mammalian lung results in an evolutionary dilemma: the origin and evolution of this bronchoalveolar lung into a high-performance gas-exchange organ results in a high work of breathing that cannot be achieved without the coupled evolution of a muscular diaphragm. However, despite over 400 years of research into respiratory biology, the origin of this exclusively mammalian structure remains elusive. Here we examine the basic structure of the body wall muscles in vertebrates and discuss the mechanics of costal breathing and functional significance of accessory breathing muscles in non-mammalian amniotes. We then critically examine the mammalian diaphragm and compare hypotheses on its ontogenetic and phylogenetic origin. A closer look at the structure and function across various mammalian groups reveals the evolutionary significance of collateral functions of the diaphragm as a visceral organizer and its role in producing high intra-abdominal pressure.

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.

Function of intracoelomic septa in lung ventilation of amniotes: lessons from lizards

Aspiration breathing is the dominant mechanism of lung inflation among extant amniotes. However, aspiration has two fundamental problems associated with it: paradoxical visceral translation and partial lung collapse. These can constrain the inspiratory tidal volume, reduce the effective lung ventilation, and ultimately curtail the aerobic capacity of an animal. Separation of the pleural and peritoneal cavities by an intracoelomic septum can restrict the cranial shift of abdominal viscera and provide structural support to the caudal lung surface. A muscular septum, such as the diaphragm of mammals or the diaphragmaticus of crocodilians, can exert active control over visceral translation and the degree of lung inflation. To a lesser degree, a nonmuscular septum can also function as a passive barrier when stretched taut by rib rotation. Studies of the posthepatic septum in teiid lizards and the postpulmonary septum in varanid lizards underscore the importance of nonmuscular septa in aspiration. These septa provide plausible functional models that help us infer the evolution of mammalian and avian lung ventilatory systems, respectively.

Reconstructing the evolution of the respiratory apparatus in tetrapods

The structural type of a lung for animals that are derived from a single ancestral group can be characterized using extant phylogenetic bracketing. Functional morphological approximation can then be used to provide further information on the functional attributes. Combining information from diverse sources, plausible explanations are deduced for the respiratory apparatus of extinct species. The air-breathing apparatus of tetrapods has its origin in gill breathing. The lungs of the first tetrapods were probably long and consisted of a single series of parenchyma-filled chambers, arranged along an intrapulmonary duct. The duct gave rise to a broad central lumen in anurans. In amniotes a cartilaginous reinforcement evolved. The septate nature of the gas-exchange tissue (parenchyma) is recognizable in all tetrapods except birds. Active expiration began with the origin of transverse body wall musculature in amphibians, whereas active, negative-pressure inspiration is seen only in amniotes. The functional transition of trunk musculature from locomotor to respiratory is most complete in birds.

Control of breathing in the echidna (Tachyglossus aculeatus) during hibernation

Resting non-hibernating echidnas are characterised by low metabolic rates, but also have a very low respiratory frequency and a variable respiratory minute volume, often resulting in low levels of arterial O(2) and high CO(2). As the echidna lies at one physiological extreme among the hibernators, in terms of its large size and low metabolism and ventilatory requirement when not hibernating, a study of control of breathing during hibernation in echidnas should provide a useful test of the generality of various models. We used non-invasive techniques to study breathing patterns and the control of ventilation in 6 echidnas. Hibernating echidnas (T(b) range 7-10 degrees C) showed episodic breathing with bursts of breaths (average 36+/-16 breaths in 24+/-5 min) followed by a period of apnea (76+/-17 min) then a series (8+/-4) of slow breaths at 14+/-1 min intervals leading up to the next burst. Increasing CO(2) levels in the inspired air increased the number of breaths in a burst, eventually leading to continuous breathing. Inter burst breaths were controlled by O(2): hypoxia increased inter burst breaths, and decreased burst length, while hyperoxia abolished inter burst breaths and increased the apneic period. Overall, while CO(2) was a strong respiratory stimulus in hibernating echidnas, O(2) had little effect on total ventilation, but did have a strong effect on the breathing pattern.