Cortical Cyto- and Chemoarchitecture in Three Small Australian Marsupial Carnivores: &lt;i&gt;Sminthopsis macroura&lt;/i&gt;, &lt;i&gt;Antechinus stuartii&lt;/i&gt; and &lt;i&gt;Phascogale calura&lt;/i&gt;

1,837

In the effort to understand the evolution of mammalian brains, we have found that common relationships between brain structure mass and numbers of nonneuronal (glial and vascular) cells apply across eutherian mammals, but brain structure mass scales differently with numbers of neurons across structures and across primate and nonprimate clades. This suggests that the ancestral scaling rules for mammalian brains are those shared by extant nonprimate eutherians - but do these scaling relationships apply to marsupials, a sister group to eutherians that diverged early in mammalian evolution? Here we examine the cellular composition of the brains of 10 species of marsupials. We show that brain structure mass scales with numbers of nonneuronal cells, and numbers of cerebellar neurons scale with numbers of cerebral cortical neurons, comparable to what we have found in eutherians. These shared scaling relationships are therefore indicative of mechanisms that have been conserved since the first therians. In contrast, while marsupials share with nonprimate eutherians the scaling of cerebral cortex mass with number of neurons, their cerebella have more neurons than nonprimate eutherian cerebella of a similar mass, and their rest of brain has fewer neurons than eutherian structures of a similar mass. Moreover, Australasian marsupials exhibit ratios of neurons in the cerebral cortex and cerebellum over the rest of the brain, comparable to artiodactyls and primates. Our results suggest that Australasian marsupials have diverged from the ancestral Theria neuronal scaling rules, and support the suggestion that the scaling of average neuronal cell size with increasing numbers of neurons varies in evolution independently of the allocation of neurons across structures.

Section: Introductionmentioning

confidence: 99%

Cellular Scaling Rules for the Brains of Marsupials: Not as “Primitive” as Expected

Santos

Porfírio²,

Cunha³

et al. 2017

1,837

“…In addition, area PS stains poorly for VGluT2 and PV immunopositive terminations, suggesting that it receives less input from the thalamus compared to the adjoining area 17, and for CO, which further suggests that it is not as metabolically active as the adjacent area 17. Area PS in the short-tailed opossum might also correspond to the rostromedial visual area of the brown antechinus, a small Australian marsupial carnivore, as both areas stain poorly for CB, PV and SMI-32 [Ashwell et al, 2008]. …”

Section: Discussionmentioning

confidence: 99%

An Architectonic Study of the Neocortex of the Short-Tailed Opossum <i>(Monodelphis domestica)</i>

Wong

Kaas²

2009

Short-tailed opossums (Monodelphis domestica) belong to the branch of marsupial mammals that diverged from eutherian mammals approximately 180 million years ago. They are small in size, lack a marsupial pouch, and may have retained more morphological characteristics of early marsupial neocortex than most other marsupials. In the present study, we used several different histochemical and immunochemical procedures to reveal the architectonic characteristics of cortical areas in short-tailed opossums. Subdivisions of cortex were identified in brain sections cut in the coronal, sagittal, horizontal or tangential planes and processed for a calcium-binding protein, parvalbumin (PV), neurofilament protein epitopes recognized by SMI-32, the vesicle glutamate transporter 2 (VGluT2), myelin, cytochrome oxidase (CO), and Nissl substance. These different procedures revealed similar boundaries among areas, suggesting that functionally relevant borders were detected. The results allowed a fuller description and more precise demarcation of previously identified sensory areas, and the delineation of additional subdivisions of cortex. Area 17 (V1) was especially prominent, with a densely populated layer 4, high myelination levels, and dark staining of PV and VGluT2 immunopositive terminations. These architectonic features were present, albeit less pronounced, in somatosensory and auditory cortex. The major findings support the conclusion that short-tailed opossums have fewer cortical areas and their neocortex is less distinctly laminated than most other mammals.

“…It was also surprising that CART immunoreactivity was weak in isocortical areas. In particular, the mystacial representations in both marsupials [Mayner, 1989;Ashwell et al, 2005;Ashwell et al, 2008] were free of immunoreactivity, whereas these regions in rodents contain both CART mRNA [Couceyro et al, 1997] and peptide [Koylu et al, 1998]. …”

Section: Differences and Similarities In Cart Immunoreactivity In Marmentioning

confidence: 98%

Distribution of CART (Cocaine- and Amphetamine-Regulated Transcript) Peptide in Mature and Developing Marsupial Brain

Ashwell

Mai²

2010

Self Cite

CART (cocaine- and amphetamine-regulated transcript) is a neuromodulator involved in feeding, drug reward, stress and cardiovascular function. We have immunohistochemically studied the distribution of the CART peptide in the brains of two adult marsupial species: the brown antechinus (Antechinus stuartii) as a representative of polyprotodont marsupials and the tammar wallaby (Macropus eugenii) as a representative of diprotodont marsupials. We have also examined the distribution of CART during postnatal development in the tammar wallaby. There were similarities and differences both between the two marsupial species and between the marsupials and eutherians in CART distribution. Both marsupials showed immunoreactivity to CART in the olfactory bulb, piriform cortex, extended amygdala, the supraoptic, paraventricular and arcuate nuclei of the hypothalamus, somatosensory and auditory nuclei of the brainstem, vagal/solitary complex, raphe obscurus and raphe pallidus and presumptive presympathetic neurons of the ventrolateral medulla, as has been seen in eutherians. On the other hand, immunoreactivity to CART was weak in or absent from isocortical areas, and immunoreactivity to CART was poor or minimal in the ventral tegmental area and nucleus accumbens of both species; regions where immunoreactivity to CART is very strong in the brains of eutherians. During development, CART was present at birth (P0) in the lateral trigeminal ganglion, spinal trigeminal tract and the vagal sensorimotor complex, but did not appear in mid- or forebrain regions until much later (from P37). These anatomical findings indicate that although CART is likely to serve very similar functions in both eutherians and marsupials, there are potentially functionally significant differences between the two mammalian groups.