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.

Evolutionary Plasticity of Habenular Asymmetry with a Conserved Efferent Connectivity Pattern

“…Studies in amphibians and birds have shown that habenular asymmetry can be influenced by steroid hormones and thus be a subject of variability among sex. For example, left-sided asymmetry of the dorsal Hb in Rana esculenta is more pronounced in females than males, especially during the reproductive season [33] whereas in chicks the medial Hb shows right-sided volumetric asymmetry only in males [9]. We found that sex-specific differences in volumetric asymmetries of the Hb are infrequent in teleosts, and thus are not a primary source of inter-species variations.…”

“…For example, in the sea lamprey Petromyzon marinus the right Hb is considerably larger and more stratified than the left Hb [4] while in the frog Rana esculenta the left Hb has a unique lateral subnucleus that is not observed in the right Hb [5]. In amniotes, habenular asymmetry is prominent only in a few species of lizards such as Uta stansburiana [6] but in most cases it involves only subtle volumetric differences between left and right habenular nuclei [7], [8], [9]. In teleosts, scattered reports of qualitative nature have shown that the Hb of different species can be either symmetric or enlarged on one side (left or right) [3] and those discrepancies seem to extend to individuals within a single species i.e.…”

Directional asymmetry of the zebrafish epithalamus guides dorsoventral innervation of the midbrain target

“…The degree of habenular asymmetry can vary seasonally in frogs, presumably correlated with the mating period (Kemali et al, 1990). In chickens, L-R habenular differences are sexdependent and influenced by hormonal levels (Gurusinghe and Ehrlich, 1985;Gurusinghe et al, 1986).…”

Early- and Late-Light Embryonic Stimulation Modulates Similarly Chicks’ Ability to Filter out Distractors

“…A further complication may derive from the fact that the habenular nuclei are asymmetric in several species [44,45] and hence might be differentially stimulated by the action of light. By contrast, in birds the habenulae are assumed to be symmetric, despite one study on chicks showed that there can be individual asymmetries and males tend to present a larger right medial habenula [46]; unfortunately, there was no mention of the incubation condition in this study and hence whether it applies to our results or not is open to speculation. On the basis of atlases of different avian species, it appears that the pineal gland, projecting to the habenulae, is larger in absolute size in chicks than, for instance, in pigeons.…”