The pineal complex and the habenular nuclei in the debate on the origin of cerebral asymmetry
Luigia CRISTINO and Vittorio GUGLIELMOTTI
Neural asymmetry is a peculiarity of vertebrates from cyclostomes to humans. The discovery of an asymmetric organization of the human brain, which dates back to Brocaís observations in 1865 on an area specialized for language, and to the pioneering experiments by Sperry on split brains, has paved the way for the concept of "hemispheric dominance". The frog habenula provides a paradigmatic example of the evolution of brain asymmetry. Habenular asymmetry was first reported by Goronowitsch in 1883 in a bone fish. Pioneering neuroanatomical studies in the early 20th century hypothesized that the development of habenular asymmetry could be viewed in the context of phylogenetic modifications of the pineal complex related to the animalís survival to environmental changes. Data in support of this hypothesis are far from being complete, but knowledge accumulated since then on diencephalic asymmetries in vertebrates. Ontogenetic and phylogenetic studies on the vertebrate brain indicates that morphological asymmetries evolved very early. Many of these data were reported in investigations carried out in our laboratory (at the Institute of Cybernetics of the National Research Council) on Rana esculenta in the last decades. In the frog, a striking habenular asymmetry was reported for the first time by Kemali and Braitenberg in 1969. In the subsequent decades, studies on the frog brain (e.g. with silver impregnation, electron microscopy, tract tracing, immunohistochemistry), and many other contributions on this theme have been obtained. Findings in the frog compared to recent genetic studies in fish suggest a strict interplay in the epithalamus between the pineal and habenular structures. The comparative analysis of data, from cyclostomes to mammals, suggests that transformation of epithalamic structures may play an important role in brain evolution. Asymmetry of the epithalamus in lower vertebrates occupies, therefore, a special place in the history of cerebral asymmetry.
Pavia, Italy, 2006