A cytoarchitectonic atlas of the medial geniculate body of the opossum, Didelphys virginiana, with a comment on the posterior intralaminar nuclei of the thalamus.

The organization of the medial geniculate body and adjacent posterior thalamus of the Virginia opossum was studied in Nissl-, Golgi-, reduced silver, and myelin-stained preparations. Our chief goals were to define the cytoarchitectonic subdivisions and boundaries in Nissl preparations and to reconcile these with those observed with the Golgi method and in experimental material, to present these results in an atlas of Nissl-stained sections, and to compare the chief nuclear groups in the opossum and the cat medial geniculate body. In the opossum, the ventral division consists chiefly of the ventral nucleus. The ventral nucleus is divided into two main parts: the pars lateralis and the pars ovoidea, the former being relatively smaller in the oposum. The ventral nucleus of both species contains large principal neurons with bushy, tufted dendrites and smaller Golgi type II cells. However, the opossum has far fewer Golgi type II cells, and the texture of the neuropil is correspondingly different, although the primary ascending input from the midbrain arises from the central nucleus of the inferior colliculus in both species. The dorsal division consists of the dorsal nuclei, including the suprageniculate nucleus and the caudal part of the lateral posterior nucleus, the marginal zone, and the posterior limitans nucleus. These nuclei are identified in both species, although they are much smaller in the opossum. The neurons consist of medium-size and small somata with a predominantly radiate mode of dendritic branching and a lower cell concentration than in the ventral division. In both species the afferent brain stem input comes from the inferior colliculus, the lateral tegmental area, the intercollicular tegmentum, and the superior colliculus. The medial division contains several types of cells, which are heterogeneous in form and size, most having radiating dendrites and a low cellular concentration. This division is especially smaller in the opossum, although comparable inputs arise from various auditory and non-auditory sources in the midbrain and spinal cord in both species. A large intralaminar complex of nuclei occurs in the opossum, which have a more extensive distribution than previously appreciated. They not only occupy the intramedullary laminae but form a shell around the medial geniculate nuclei and adjoining main sensory nuclei. The intralaminar complex includes the posterior limitans, posterior intralaminar, posterior, parafascicular, posterior parafascicular, central intralaminar, limitans, and central medial nuclei, and the marginal zone of the medial geniculate body. These predominantly radiate, poorly branched cells are similar in both species. However, the opossum intralaminar nuclei are much more conspicuous than those of the cat. While it is possible to subdivide this complex, their basic structural similarity and topograpgical continuity suggest that the intralaminar neurons constitute a fundamental architecctonic category. These neurons have differentiated during ontogeny and associated thalamic nuclei, which differ broadly in their connections and physiological properties. These findings imply: (1) tha the patterns of structural organization in the main thalamic sensory nuclei and in the associated nuclei are comparable in both species, (2) that the cortical and brain stem connections with thse nuclei are to be compared in both species, and (3) that despite these similarities, there may be interspecific differences in the cytological and perhaps the synaptic arrangement of particular nuclei.