Cerebral Cortex of the Koala (Phascolarctos cinereus)
Introduction
The koala (Phascolarctos cinereus) is an arboreal, diprotodontid marsupial, which leads a solitary and mostly nocturnal life. The species is found down the eastern and southern side of mainland Australia from Queensland to South Australia.
The eucalyptus diet of the koala is low in nutrients and high in turpenes, phenols and fibre, so the mammal leads a low-energy lifestyle and is highly sedentary, sleeping as much as 20 hours per day.
Material and methods
The adult specimen depicted here (0505_354) is from the Nelson Collection held at the Australian National Wildlife Collection in Australian Capital Territory, Australia. The brain had been embedded, sectioned at a thickness of 30 µm and stained with thionin.
The sections were photographed with an Olympus BHS2 microscope using a 4x objective and the images stitched with Adobe Photoshop CS5.1 to create montages of the isocortex, and limbic and olfactory allocortex. Section numbers refer to the sequence from the rostral tip of the olfactory bulb. Multiplying the section number by 0.03 mm will give the distance in mm from the rostral end of the olfactory bulb.
The section images have been placed in Adobe Illustrator 2021 and labelled using a modified Paxinos/Watson system of brain nomenclature. Boundaries between putative cortical regions have been indicated by filled wedges.
The cartoon in each plate shows the position of the section in the rostrocaudal extent of the cerebral hemisphere.
Observations
The cerebral cortex of the koala is relatively smooth (lissencephalic) for its size. The lateral ventricles are large and the cerebral cortex is relatively thin for the endocranial volume.
The cortex is laminated like all other mammals, but the cytoarchitectural differentiation between the six isocortical layers is less obvious than in other marsupials and eutherians with a similarly sized brain. The motor cortex has a prominent layer 5 with large pyramidal neurons (see Fig. 1a in Plate 1) and the granular layer 4 is also prominent in the primary visual (Fig. 2a in Plate 2) and primary somatosensory cortices (Fig. 1b in Plate 1), but the isocortical layers are thin and less distinct in other cortical regions and the primary auditory cortex has only an indistinct layer 4 (Fig. 2a).
The primary somatosensory cortex lacks the barrel structures seen in the brains of macropod and phalangerid diprotodontids (Weller, 1972; Weller and Haight, 1973; Weller, 1993; Waite et al., 1991). Putative body representation areas have been labelled on Fig 1b, but these are based on extrapolation from other diprotodontids and have not been confirmed by electrophysiology.
The primary auditory cortex is also difficult to identify definitively without electrophysiology and has been labelled here based on extrapolation from the cortical sensory maps of other diprotodontid species (e.g. tammar wallaby and brush-tailed possum).
The entorhinal cortex is easily identified by the presence of a lamina dissecans (Fig. 2b).
Acknowledgments
I would like to thank Dr Leo Joseph of the Australian National Wildlife Collection, CSIRO, for access to the sectioned and stained koala brain from the Nelson Brain Collection in Canberra.
References
Waite PM, Marotte LR, Mark RF (1991) Development of whisker representation in the cortex of the tammar wallaby Macropus eugenii. Developmental Brain Research, 58, 35-41.
Weller WL (1972) Barrels in somatic sensory neocortex of the marsupial Trichosurus vulpecula (brush-tailed possum). Brain Research, 43, 11-24.
Weller WL, Haight JR (1973) Barrels and somatotopy in S1 neocortex of the brush-tailed possum. Journal of Anatomy, 116, 474.
Weller WL (1993) SmI cortical barrels in an Australian marsupial, Trichosurus vulpecula (brush-tailed possum): structural organisation, patterned distribution and somatotopic relationships. Journal of Comparative Neurology, 337, 471-492.
Figures
Figures 1a, b (Plate 1). Sections 330 and 570 through the cerebral hemisphere of an adult koala showing cortical regions. These sections are respectively 9.9 and 17.1 mm from the rostral pole of the brain.
Figures 2a, b (Plate 2). Sections 750 and 910 through the cerebral hemisphere of an adult koala showing cortical regions. These sections are respectively 22.5 and 27.3 mm from the rostral pole of the brain.
