Neocortical projections of the suprageniculate and posterior thalamic nuclei in the marsupial brush-tailed possum, Trichosurus vulpecula (Phalangeridae), with a comparative commentary on the organization of the posterior thalamus in marsupial and placental mammals.

Axonal transport methods were used to determine the extent and organisation of neocortical projections from the suprageniculate (SG) and posterior (PO) thalamic nuclei in the brush-tailed possum. Our findings show that SG projects extensively to the auditory cortex, overlapping the cortical projection field of the medial geniculate nucleus, and to the immediately neighbouring association cortex. Though the input relationships of SG appear similar to those reported for other mammals, placental and marsupial, a strong SG projection to auditory cortex has not been reported previously. Neocortical relationships of PO are characterised by an orderly point-to-point projection to all but the most rostral parts of the motor-somaesthetic cortex. There is also a substantial projection to the entire posterior parietal association cortex. The PO-neocortex projection is reciprocally organised. The PO-neocortical projection in the possum is similar to that reported in the Virginia opossum, rat, and several other mammals. There is a major difference in organisation in comparison with certain monkeys where the PO projection is much more restricted and does not involve the motor and somaesthetic cortex. We conclude that PO is similarly organised in many, though not all, mammals, including the marsupials, rodents, insectivores, and prosimian primates. The possum SG, on the other hand, is clearly distinct from other mammals in its extensive projection to auditory cortex, though we cannot say at present whether this a general property of marsupial mammals or a peculiarity restricted to this species and possibly its close relatives.

A description of the dorsal thalamus of the marsupial native cat, Dasyurus viverrinus (Dasyuridae).

The nuclear architecture of the dorsal thalamus in the marsupial native cat, Dasyurus viverrinus, is described. The nine midline and intralaminar nuclei comprise a large and prominent part of the thalamus. The lateroposterior nucleus is clearly divided into medial and lateral divisions. A ventroanterior nucleus is distinguished from the ventrolateral nucleus, and the ventroposterior complex is divided into cytoarchitecturally distinctive medial and lateral divisions. There is a large posterior nucleus. The medial geniculate nucleus displays an external or principal division and an internal division. The dorsal lateral geniculate nucleus has three cell laminae. Though the dasyurus thalamus presents features that are intermediate between those displayed by the American opossum, Didelphis virginiana, and the Australian brush-tailed possum, Trichosurus vulpecula, overall the two Australian forms resemble each other more closely than either resembles the American from.

An analysis of some thalamic projections to parietofrontal neocortex in the marsupial native cat, Dasyurus viverrinus (Dasyuridae).

Thalamocortical projections to parietofrontal neocortex in the marsupial native cat, Dasyurus viverrinus, were examined using the retrograde transport of horseradish peroxidase. The results show that the organisation in Dasyurus is similar to that reported in another Australian marsupial, the brush-tailed possum, Trichosurus vulpecula, and is different to that found in the primitive American form, Didelphis virginiana. In Daysurus, as in Trichosurus, the areas of neocortex receiving projections from the trigeminothalamic system do not appreciably coincide with those areas receiving input from the cerebellothalamic system. In Didelphis there appears to be complete overlap of these areas of motor and somaesthetic neocortex. These results suggest that the motor--somaesthetic organisation in Australian marsupials is divergent from the presumed stem pattern seen in the American forms. If the Australian marsupials, as is now believed, were isolated from the stem Polyprotodonta in the upper Cretaceous, it would appear that the Australian marsupials may have begun to develop certain aspects of their characteristic CNS configuration very early, as the oldest known marsupial fossils are not much older than this.

Relationships of the visual cortex in the marsupial brush-tailed possum, Trichosurus vulpecula, a horseradish peroxidase and autoradiographic study.

The ascending and descending relationships of visual cortex in Trichosurus were determined using HRP and autoradiographic methods. The visual thalamus, LGNd and LP, was found to project to three cytoarchitecturally distinct areas of cortex with each of these regions displaying differing patterns of connectivity with other centres. The striate and peristriate cortices received homotypically organized projections from LGNd as well as projections from both divisions of LP. The posterior parietal cortex received projections from both LGNd and LP as well as from the latero-intermediate, posterior, ventroanterior and intralaminar thalamic nuclei. The cortical projection fields of LGNd and LP were co-extensive. Descending projections to thalamic centres were reciprocal with the exception of a descending only projection to the thalamic reticular nucleus. The striatogeniculate projection was homotypically organized as was the striatocollicular projection which extended only to the superficial layers of that nucleus. The posterior parietal projection to the superior colliculus involved the deeper layers of that nucleus. The entire visual cortex projected to the pretectum and to the pontine nuclei. The organization of visual cortex in Trichosurus differs from that reported in the American marsupial, Didelphis, in that the LGNd and LP complexes possess a very broad and co-extensive cortical projection, a finding substantially different to what has been reported in placental mammals as well. The organization of thalamic relationships with the posterior parietal cortex would also appear to be unusual with this region receiving convergent projections from at least seven separate thalamic nuclei.

The organization of neocortical projections from the ventrolateral thalamic nucleus in the brush-tailed possum, Trichosurus vulpecula, and the problem of motor and somatic sensory convergence within the mammalian brain.

The retrograde transport of HRP was used to determine the extent and organizational details of the VL neocortical projection in Trichosurus. The major, or external, part of VL projects homotypically to the anteromedial part of the parietofrontal cortex, overlapping the VP cortical projection field extensively, but not completely. Overlap between the VL field and SmI is most extensive in the area of rear limb representation, and decreases considerably in the area of the head representation. A small internal subdivision of VL projects discontinuously to the SmI head region, and appears to be limited to portions of the cortical barrel field. The VL cortical projection is much less extensive than that of VP, but does include a small area of cortex which does not receive a VP projection. Thus VL and VP do not display congruent cortical projections. Details of the thalamocortical relationships, as determined from both anatomical and physiological studies, are compared with other mammals.

The organization of neocortical projections from the ventroposterior thalamic complex in the marsupial brush-tailed possum, Trichosurus vulpecula: a horseradish peroxidase study.

Retrograde transport of horseradish peroxidase (HRP) was used to determine the extent and some of the organizational details of the cortical projection of the ventroposterior thalamic complex (VP) in the marsupial brush-tailed possum, Trichosurus vulpecula. The cortical projection field of VP is coincident with SI as determined by electrophysiological methods, and would appear not to overlap fully the primary motor cortex. Thus, in Trichosurus it appears that the motor and somatic sensory cortical regions are not fully congruent, unlike those of the American opossum, Didelphis, which are. Each division of VP projects discretely, in a non-overlapping manner, to various regions within SI. The ventrolateral subdivision or VPL projects medially and in a strict homotypic manner, though the proportion of VPL cells projecting to cortex is subject to a large amount of variation. The dorsomedial division of VP or VPM projects uniformly to cortex from all areas of that subnucleus, but the strict homotypy characteristic of VPL's projection was not as apparent. VPM also projects to two distinct regions within its cortical field. The posteromedial division of VP or VPP projects to an area of cortex that receives no other VP input but, on the basis of cortical mapping studies, appears to belong to SI. Projections from VPL (and presumably from VPM) to a small area of cortex in the extreme posterolateral part of the VP field correspond to the position expected for, and electrophysiologically confirmed to be, SII.

An atlas of the dorsal thalamus of the marsupial brush-tailed possum, Trichosurus vulpecula.

In several respects the dorsal thalamus of Trichosurus presents a level of organizational complexity considerably in advance of that seen in Didelphis. In particular, such features as the lamination of the dorsal lateral geniculate, the distinct subdivisions of the ventroposterior and lateroposterior complexes, and the more pronounced separation of the ventroanterior from the ventrolateral nucleus, are similar to conditions found in many eutherians. On the other hand, some features which Trichosurus shares with Didelphis, such as the lack of a median central nucleus, and the well developed midline nuclei, are usually said to be indicative of a more 'primitive' level of neural organization. In most instances the thalamic nuclei in Trichosurus are easily homologized with those of Didelphis and other mammals. The difficult areas are the lateral and posterior groups of nuclei, regions which have generally been troublesome in most mammals thus far examined. What does become apparent after this examination of the Trichosurus thalamus is that this representative of the diprotodontid radiation has produced some neurological features which strikingly parallel those of certain advanced eutherians while retaining other features generally considered to be generalized and primitive. Within the metatheria Trichosurus represents a considerable advance over Didelphis. However, as will be seen in later papers in this series, there are other features, particularly those of thalamocortical fibre distribution, which serve to set Trichosurus quite markedly apart from both Didelphis and at least certain eutherian mammals (Haight & Neylon, 1977b, c, 1978).

Morphological variation in the brain of the marsupial brush-tailed possum, Trichosurus vulpecula.

The brush-tailed possum, Trichosurus vulpecula, displays an unusual amount of phenotypic variation within its central nervous system. These variations are expressed in terms of gross brain morphology, as evidenced by the occurrence of both highly lissencephalic and gyrencephalic neocortices in apparently normal adult animals; by displacement and asymmetry of certain internal structures, notably within the ventral thalamic nuclei; and by the inconstant occurrence of at least one cell group within the thalamus. These variations are described and discussed with reference to possible functional, ontogenetic and phylogenetic significance.