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

Abstract

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.

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

“…Marsupialia consists of nearly 350 extant species divided into 4 Australasian and 3 South American orders [Nilsson et al, 2010;Gallus et al, 2015;May-Collado et al, 2015]. While the neuroanatomy, connectivity, neocortical development, and physiology of the brain in some representative marsupial species have been studied [Saunders et al, 1989;Rosa et al, 1999;Ashwell et al, 2008;Wong and Kaas, 2009;Watson et al, 2012], little is known about the cellular composition of marsupial brains and how it compares with other clades, besides a report of a low neuronal density in the neocortex of a single species, the opossum (Didelphis virginiana) , in comparison to other mammals [Haug, 1987]. Recently, two more detailed studies were conducted on the cellular composition of another species, the gray short-tailed opossum Monodelphis domestica, during development [Seelke et al, 2013] and across the primary sensory fields of its neocortex [Seelke et al, 2014].…”

The marmoset monkey as a model for visual neuroscience

“…This difference in anatomical projections is obvious following the effects of area 17 lesions, which in primates causes a condition that approximates complete blindness but in other mammals is much less severe (Killackey et al, 1971, 1972; Funk and Rosa, 1998). Also, the subdivisions within the input layers, and presumed segregation of innervation to sublayers containing different cell classes, varies in other mammals from prominent (Freund et al, 1985; Wong and Kaas, 2008) to weak or absent (Wong and Kaas, 2009). It is notable that in at least one species, the tree shrew, the geniculostriate terminations in layer 4 convey not transient and sustained responses, but rather “on” and “off” responses, which are similarly segregated in the retina and in the LGN (Conley et al, 1984; Van Hooser et al, 2013).…”

Active vibrissal sensing in rodents and marsupials

“…Fossil evidence suggests the presence of opossum-like animals in the early Cretaceous period (greater than 100 mya) [36]; hence M. domestica may have many of the traits possessed by a common ancestor of both modern marsupial and placental mammals. In particular, the opossum brain has a smaller number of cortical areas than most mammals [37], lacks a distinct motor cortex, and has relatively few corticospinal neurons [38]. Thus, it may be reasonably representative of an ancestral mammalian brain (motor cortex and the corticospinal tract are both new in mammals).…”