| Summary: | The habenular nuclei are diencephalic structures present in Vertebrates and they form, with the
associated fiber systems, a part of the system that connects the telencephalon to the ventral
mesencephalon (Concha M. L. and Wilson S. W., 2001). In representative species of almost all classes
of Vertebrates the habenular nuclei are asymmetric, both in terms of size and of neuronal and
neurochemical organization, although different types of asymmetry follow different evolutionary
courses. Previous studies have analyzed the spread and diversity of the asymmetry in species for which
data are not clear (Kemali M. et al., 1980). Notwithstanding that, it’s still not totally understood the
evolution of the phenomenon, and the ontogenetic mechanisms that have led to the habenular
asymmetry development are not clear (Smeets W.J. et al., 1983).
For the present study 14 species of Elasmobranchs and 15 species of Teleostean have been used.
Brains removed from the animals have been fixed using 4% paraformaldehyde in phosphate buffer;
brains have been analyzed with different tecniques, and I used histological, immunohistochemical and
ultrastructural analysis to describe this asymmetry.
My results confirm data previously obtained studying other Elasmobranchs species, in which the left
habenula is larger than the right one; the Teleostean show some slightly differences regarding the size
of the habenular ganglia, in some species, in which the left habenular nucleus is larger than the right.
In the course of studies, a correlation between the habits of life and the diencephalic asymmetry seems
to emerge: among the Teleostean analyzed, the species with benthic life (like Lepidorhombus boscii,
Platichthys flesus, Solea vulgaris) seem to possess a slight asymmetry, analogous to the one of the
Elasmobranchs, while in the other species (like Liza aurata, Anguilla anguilla, Trisopterus minutus)
the habenulae are symmetrical. However, various aspects of the neuroanatomical asymmetries of the
epithalamus have not been deepened in order to obtain a complete picture of the evolution of this
phenomenon, and new searches are needed to examine the species without clear asymmetry, in order to
understand the spread and the diversity of the asymmetry among the habenulae between the
Vertebrates.
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