Morphology and evolution of the oral shield in marsupial neonates including the newborn monito del monte (Dromiciops gliroides, Marsupialia Microbiotheria) pouch young

Translating the Timing of Developmental Benchmarks in Short-Tailed Opossums (Monodelphisdomestica) to Facilitate Comparisons with Commonly Used Rodent Models

INTRODUCTION

The gray short-tailed opossum, Monodelhis domestica (M. domestica), is a widely used marsupial model species that presents unique advantages for neurodevelopmental studies. Notably their extremely altricial birth allows manipulation of postnatal pups at timepoints equivalent to embryonic stages of placental mammals. A robust literature exists on the development of short-tailed opossums, but many researchers working in the more conventional model species of mice and rats may find it daunting to identify the appropriate age at which to conduct experiments.

METHODS

Here, we present detailed staging diagrams taken from photographic observations of 40 individual pups, in 6 litters, over 25 timepoints across postnatal development. We also present a comparative neurodevelopmental timeline of short-tailed opossums (M. domestica), the house mouse (Mus musculus), and the laboratory rat (Rattus norvegicus) during embryonic as well as postnatal development, using timepoints taken from this study and a review of existing literature, and use this dataset to present statistical models comparing the opossum to the rat and mouse.

RESULTS

One aim of this research was to aid in testing the generalizability of results found in rodents to other mammalian brains, such as the more distantly related metatherians. However, this broad dataset also allows the identification of potential heterochronies in opossum development compared to rats and mice. In contrast to previous work, we found broad similarity between the pace of opossum neural development with that of rats and mice. We also found that development of some systems was accelerated in the opossum, such as the forelimb motor plant, oral motor control, and some aspects of the olfactory system, while the development of the cortex, some aspects of the retina, and other aspects of the olfactory system are delayed compared to the rat and mouse.

DISCUSSION

The pace of opossum development is broadly similar to that of mice and rats, which underscores the usefulness of this species as a compliment to the more commonly used rodents. Many features that differ the most between opossums and rats and mice were either clustered around the day of birth and were features that have functional importance for the pup immediately after or during birth, or were features that have reduced functional importance for the pup until later in postnatal development, given that it is initially attached to the mother.

Functional constraints during development limit jaw shape evolution in marsupials

Differences in jaw function experienced through ontogeny can have striking consequences for evolutionary outcomes, as has been suggested for the major clades of mammals. By contrast to placentals, marsupial newborns have an accelerated development of the head and forelimbs, allowing them to crawl to the mother's teats to suckle within just a few weeks of conception. The different functional requirements that marsupial newborns experience in early postnatal development have been hypothesized to have constrained their morphological diversification relative to placentals. Here, we test whether marsupials have a lower ecomorphological diversity and rate of evolution in comparison with placentals, focusing specifically on their jaws. To do so, a geometric morphometric approach was used to characterize jaw shape for 151 living and extinct species of mammals spanning a wide phylogenetic, developmental and functional diversity. Our results demonstrate that jaw shape is significantly influenced by both reproductive mode and diet, with substantial ecomorphological convergence between metatherians and eutherians. However, metatherians have markedly lower disparity and rate of mandible shape evolution than observed for eutherians. Thus, despite their ecomorphological diversity and numerous convergences with eutherians, the evolution of the jaw in metatherians appears to be strongly constrained by their specialized reproductive biology.

Culture of Marsupial Oocytes and Conceptuses

Marsupial oocytes and conceptuses provide special challenges to scientists wanting to develop reliable in vitro techniques. Yet these techniques are essential to the study of development. Such techniques also provide tools to help prevent further decline in marsupial biodiversity using assisted reproductive technology. Specific marsupial characters have made development of in vitro techniques difficult. Some of these are the high-energy requirements of cleavage and blastocyst formation and maintenance because of cell-zona adhesion; the essential nature of the shell coat for most of development; the prevalence of embryonic arrests in vivo and in vitro during cleavage and unilaminar blastocyst stages; and the fragility of blastocysts leading to precocious shell loss and developmental failure in vitro. The advantages of marsupial culture during development are that the gestation period is very short, that the implantation is superficial in many, and that the neonates are altricial. This chapter outlines solutions to some of these problems in a representative, polytocous, dasyurid marsupial, Sminthopsis macroura. It is a natural superovulator with the shortest gestation period of any mammal, which has been cultured to within 5 h of birth. The other representative marsupial, the monovular phalangerid Trichosurus vulpecula, has a very superficial implantation that allows vesicles to readily detach from the endometrium.

The embryo of the silky shrew opossum, Caenolestes fuliginosus (Tomes, 1863): First description of the embryo of Paucituberculata

The development of caenolestid marsupials (order Paucituberculata) is virtually unknown. We provide here the first description of Caenolestes fuliginosus embryos collected in the Colombian Central Andes. Our sample of four embryos comes from a single female caught during a fieldtrip at Río Blanco (Manizales, Caldas), in 2014. The sample was processed for macroscopic description using a Standard Event System and for histological descriptions (sectioning and staining). The grade of development of the lumbar flexure and coelomic closure differed between embryos, two of them being more advanced than the others (similar to McCrady's stages 30 and 29, respectively). The pericardial and peritoneal cavities were present, the hepatic anlage was organized in hepatic cords, the heart was in its final position, and the mesonephros was functional. Compared to other Neotropical marsupials, an early appearance of the frontonasal-maxillary fusion and the cervical growth (thickness) was observed; however, absorption of the pharyngeal arches into the body and lung development was delayed. Besides these differences, embryos were similar to equivalent stages in Didelphis virginiana and Monodelphis domestica. Previous proposals of litter size of four for C. fuliginosus are supported.

Quantitative Analysis of the Timing of Development of the Cerebellum and Precerebellar Nuclei in Monotremes, Metatherians, Rodents, and Humans

We have used a quantitative statistical approach to compare the pace of development in the cerebellum and precerebellar systems relative to body size in monotremes and metatherians with that in eutherians (rodents and humans). Embryos, fetuses, and early postnatal mammals were scored on whether key structural events had been reached in the development of the cerebellum itself (CC-corpus cerebelli; 10 milestones), or the pontine and inferior olivary precerebellar nuclear groups (PC; 4 milestones). We found that many early cerebellar and precerebellar milestones (e.g., formation of Purkinje cell layer and deep cerebellar nuclei) were reached at a smaller absolute body length in both metatherians and eutherians together, compared to monotremes. Some later milestones (e.g., formation of the external granular layer and primary fissuration) were reached at a smaller body length in metatherians than eutherians. When the analysis was performed with proportional body length expressed as a natural log-transformed ratio of length at birth, milestones were reached at a much smaller proportional body length in rodents and humans than in the metatherians or monotremes. The findings are consistent with the slower pace of metabolic activity and embryonic development in monotremes. They also indicate slightly advanced maturation of some early features of the cerebellum in some metatherians (i.e., early cerebellar development in dasyurids relative to body size), but do not support the notion of an accelerated development of the cerebellum to cope with the demands of early birth. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1998-2013, 2020. © 2019 American Association for Anatomy.

Preterm birth disrupts the development of feeding and breathing coordination

All mammals must breathe and breathe continuously from birth. Similarly, all mammals, including infants, have high functional demands for feeding. However, the pathway that food takes through the pharynx interrupts respiration. The coordination between swallowing and breathing is therefore critical for all infant mammals. Clinically, this coordination differs between term and preterm infants. However, the neurological mechanisms underlying this coordination and how it matures as infants grow are poorly understood. Here, we integrate high-resolution data from multiple physiologic processes across a longitudinal time frame to study suck-swallow-breathe dynamics in a preterm animal model, the infant pig. In doing so, we test the hypothesis that preterm birth will have an impact on some, but not all, behaviors associated with suck-swallow-breath performance. We hypothesize that coordination will be disrupted, reflecting incomplete connections in the brainstem. We found that preterm pigs became rhythmic and mature in sucking and swallowing behaviors, suggesting substantial postnatal maturation in the coordination of these behaviors. However, their ability to coordinate swallowing and breathing never developed. These results have implications for the nature of clinical care of human infants, as well as for how feeding processes develop in mammals. Clinically, they provide a foundation for developing interventions for preterm infants. Additionally, these results suggest that the lack of coordination between swallowing and breathing may be a significant factor in determining the minimum gestation time across mammals. NEW & NOTEWORTHY Preterm infants face a variety of challenges associated with safe feeding, but obtaining high-resolution longitudinal data to understand these challenges in humans is challenging. We used a pig model to acquire high-speed videofluoroscopic and respiratory inductance plethysmograph data throughout the nursing period to show that preterm birth does not have substantial impacts on the ability of infants to perform isolated behaviors. However, it does decrease the ability of preterm infants to coordinate among behaviors during feeding.

Ontogeny and phylogeny of the mammalian chondrocranium: the cupula nasi anterior and associated structures of the anterior head region

BACKGROUND

The study of chondrocrania has a long tradition with a focus on single specimens and stages. It revealed great interspecific diversity and a notion of intraspecific variation. As an embryonic structure, the chondrocranium is subject to major changes in ontogeny with resorption and ossification of different cartilaginous structures. The cupula nasi anterior is the anteriormost portion of the cartilaginous nasal capsule and is expected to mirror much of the animal's life history and lifestyle. Its diversity in mammals is reflected in the external nasal anatomy of newborns. Marsupials and placentals show marked differences, likely related to breathing and suckling behavior.

RESULTS

We examined histological sections of five marsupial and three placentals species and traced the development of the cupula nasi anterior and the anterior nasal capsule. We found ontogenetic variation for nearly 50% of the 43 characters defined herein. By comparing to the literature and considering ontogenetic variation, we performed an analysis of character evolution in 70 mammalian species and reconstructed the nasal anatomy of the therian ancestor.

CONCLUSIONS

At birth, marsupials have a complete but simple cupula nasi anterior, whereas placentals display a more diverse morphology due to reductions and variations of chondrocranial elements. The more compact nasal capsule in marsupials is related to a long and strong fixation to the mother's teat after birth. Within marsupials and placentals, several derived characters distinguish major taxa, probably related to developmental and functional constraints. The reconstructed ancestral anatomy of the cupula nasi anterior supports the hypothesis that the therian ancestor was placental-like and that the marsupial lifestyle is more derived.