Relationships between seasonal thermal variations and cell proliferation in heterothermic vertebrates, as revealed by PCNA expression in the brain of adult <em>Podarcis sicula</em>

Abstract

In adult terrestrial heterothermic vertebrates, spontaneous or experimentally induced plasticity of the brain is widely demonstrated. This phenomenon is more pronounced in Amphibians than in lacertilians, the most investigated among the Reptiles. In the lizard it has been observed that the summer photoperiod and temperature exert a positive influence on the proliferative activity of cerebral putative stem cells, which then differentiate into glial or neuronal cells. In the present investigation, the behaviour of proliferating neural cells has been investigated by immunocytochemistry in the brain of normal adult Podarcis sicula caught in their habitat in summer. The results, qualitatively evaluated, were compared with those on normal lizards of the same species, caught from the wild in late spring and previously analysed by the same author. The comparison showed that summer environment stimulates cell proliferation, although to a limited extent. This response seems to involve the quiescent cells which mainly populate the ependymal layer of the forebrain, the telencephalic hemispheres being the best provided with these undifferentiated cells, while no substantial differences from spring values were found in more caudal cerebral portions. The comparison between present findings and previously reported ones indicates that summer environment stimulates the proliferation of putative neuronal stem cells but only in the forebrain; this proliferation might not be sufficient to support the regeneration upon partial removal of this portion of the encephalon, and previous findings of a much higher regenerative capacity in the lizard brain upon cerebral injury might depend on factors linked to injury itself or to the studied species and their habitat.