Asymmetry in the epithalamus of vertebrates

The epithalamus is a major subdivision of the diencephalon constituted by t he habenular nuclei and pineal complex. Structural asymmetries in this regi on are widespread amongst vertebrates and involve differences in size, neur onal organisation, neurochemistry and connectivity. In species that possess a photoreceptive parapineal organ, this structure projects asymmetrically to the left habenula, and in teleosts it is also situated on the left side of the brain. Asymmetries in size between the left and right sides of the h abenula are often associated with asymmetries in neuronal organisation, alt hough these two types of asymmetry follow different evolutionary courses. W hile the former is more conspicuous in fishes (with the exception of teleos ts), asymmetries in neuronal organisation are more robust in amphibia and r eptiles. Connectivity of the parapineal organ with the left habenula is not always coupled with asymmetries in habenular size and/or neuronal organisa tion suggesting that, at least in some species, assignment of parapineal an d habenular asymmetries may be independent events. The evolutionary origins of epithalamic structures are uncertain but asymme try in this region is likely to have existed at the origin of the vertebrat e, perhaps even the chordate, lineage. In at least some extant vertebrate s pecies, epithalamic asymmetries are established early in development, sugge sting a genetic regulation of asymmetry. In some cases, epigenetic factors such as hormones also influence the development of sexually dimorphic haben ular asymmetries. Although the genetic and developmental mechanisms by whic h neuroanatomical asymmetries are established remain obscure, some clues re garding the mechanisms underlying laterality decisions have recently come f rom studies in zebrafish. The Nodal signalling pathway regulates laterality by biasing an otherwise stochastic laterality decision to the left side of the epithalamus. This genetic mechanism ensures a consistency of epithalam ic laterality within the population. Between species, the laterality of asy mmetry is variable and a clear evolutionary picture is missing. We propose that epithalamic structural asymmetries per se and not the laterality of th ese asymmetries are important for the behaviour of individuals within a spe cies. A consistency of the laterality within a population may play a role i n social behaviours between individuals of the species.