Connections of the Medial Telencephalic Wall in the Spotted African Lungfish

The extent and boundaries of the roof, or pallium, of the telencephalon in lungfishes have been debated for over 30 years, and two hypotheses exist. Proponents of a restricted pallium claim that the medial border of the pallium occurs in a dorsal position and that the entire medial hemispheric wall is formed by the septal nuclei. Proponents of an extended pallium claim that the medial border of the pallium occurs in a more ventral position and that the medial hemispheric wall is divided into a dorsal medial pallium and ventral septal nuclei, as in amphibians. Immunohistochemical data have generally been interpreted to support the hypothesis of an extended pallium, but disagreement still exists. To clarify the extent of the pallium in lungfishes, the connections of the dorsal and ventral divisions of the medial hemispheric wall in the Spotted African Lungfish were examined using a number of neuronal tracers. In amphibians and other tetrapods, the afferent projections to the medial pallium and the septal nuclei differ extensively, as do the commissural routes taken by decussating interhemispheric connections. Although the descending projections of the medial pallium and septal nuclei are very similar to one another in amphibians and other tetrapods, they do differ in that the septal nuclei and the ventral thalamus are extensively interlinked, whereas the medial pallium lacks such connections. These differences also characterize the connections of the dorsal and ventral divisions of the medial hemispheric wall in the Spotted African Lungfish, which supports the hypothesis of an extended pallium. The telencephalic organization in lungfishes thus appears remarkably similar to that in amphibians and reflects a pattern that almost certainly existed in the last common ancestor of lungfishes and tetrapods.