Sex-dependent structural asymmetry of the medial habenular nucleus of the chicken brain
ABSTRACT An investigation of structural asymmetry in the avian brain was conducted on the epithalamic medial habenular nucleus of the chicken. Twelve male and ten female two-day-old chickens were used for a morphometric evaluation of asymmetry. The medial habenular nucleus was measured from paraffin-wax-embedded, 8 micron-thick sections by use of a semiautomatic image analyser. The volumes of the right and left medial habenula of each animal were statistically analysed ('within animal experimental design'). The right medial habenula in males showed significant group asymmetry. In contrast, females failed to demonstrate group bias in favour of either hemisphere. However, individual females were lateralised, with either a larger right or left medial habenula. Although individuals of both sexes were lateralised, there was no significant sex difference in volume in either the right or left medial habenula. We propose that sex-linked structural asymmetry may be influenced by steroid hormonal effects in the central nervous system, and that such asymmetry could be more prevalent in the non-mammalian vertebrate brain than previously considered.
- SourceAvailable from: Adam R ReddonBioscience Hypotheses 01/2009; 2(6-6):383-387. DOI:10.1016/j.bihy.2009.06.007
- [Show abstract] [Hide abstract]
ABSTRACT: Domestic chicks were tested for eye use while feeding on the floor in the presence of a dummy mask which could either look at the location where the chicks were feeding or in the opposite direction. Animals completely naïve of visual experience of human eyes and gaze showed a more intense fear response when directly looked at (as shown by higher latency to approach the food) and a preferential use of the left eye (mainly feeding structures in their right hemisphere) to monitor the dummy mask. This response, seemingly predisposed, could be reversed in chicks with experience of human eyes and gaze directed toward them, which showed higher latency to approach the food and preferential left eye use when the dummy mask looked away from them. The results are discussed in relation to evidence for a right hemisphere involvement in fear responses and detection of predators in the vertebrate brain.Behavioural Brain Research 03/2007; 177(1):15-21. DOI:10.1016/j.bbr.2006.11.020 · 3.39 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: The epithalamus is a major subdivision of the diencephalon constituted by the habenular nuclei and pineal complex. Structural asymmetries in this region are widespread amongst vertebrates and involve differences in size. neuronal 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 habenula are often associated with asymmetries in neuronal organisation, although these two types of asymmetry follow different evolutionary courses. While the former is more conspicuous in fishes (with the exception of teleosts), asymmetries in neuronal organisation are more robust in amphibia and reptiles. Connectivity of the parapineal organ with the left habenula is not always coupled with asymmetries in habenular size and/or neuronal organisation suggesting that, at least in some species, assignment of parapineal and habenular asymmetries may be independent events. The evolutionary origins of epithalamic structures are uncertain but asymmetry in this region is likely to have existed at the origin of the vertebrate, perhaps even the chordate, lineage. In at least some extant vertebrate species, epithalamic asymmetries are established early in development, suggesting a genetic regulation of asymmetry. In some cases, epigenetic factors such as hormones also influence the development of sexually dimorphic habenular asymmetries. Although the genetic and developmental mechanisms by which neuroanatomical asymmetries are established remain obscure, some clues regarding the mechanisms underlying laterality decisions have recently come from 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 epithalamic laterality within the population. Between species, the laterality of asymmetry is variable and a clear evolutionary picture is missing. We propose that epithalamic structural asymmetries per se and not the laterality of these asymmetries are important for the behaviour of individuals within a species. A consistency of the laterality within a population may play a role in social behaviours between individuals of the species.Journal of Anatomy 199(Pt 1-2):63-84. · 2.23 Impact Factor