Comparative expression patterns ofSox2andSox19genes in the forebrain of developing and adult turbot (Scophthalmus maximus)

sox19 regulates ovarian steroidogenesis in common carp

In teleost, ovarian steroidogenesis governed by the neuroendocrine system is also regulated by several transcription factors of gonadal origin. Investigating the synchronized interactions between the transcriptional and the hormonal factors is vital to comprehend the mechanisms that lead to gonadal differentiation. This study signifies the role of sry-related box (sox) 19 in ovarian steroidogenesis regulation of the common carp, Cyprinus carpio. Analysis of tissue distribution displayed higher sox19 expression in brain and ovary, and gonadal ontogeny showed higher expression of sox19 at 80 days post hatch (dph). Higher sox19 mRNA expression during spawning and increase of sox19 post human chorionic gonadotropin induction substantiate gonadotropin dependency. Estradiol-17β treatment but not 17α-methyl-di-hydroxy-testosterone to 50 dph common carp for inducing mono-sex, elevated sox19 expression substantially. Sox19 protein was observed in granulosa cells of the follicular layer in common carp ovary. Higher sox19 expression was detected in isolated granulosa and theca cells, in vitro. Transient gene silencing with sox19-siRNA caused downregulation of various ovary-related genes including those specific to activator protein-1 factors, fibroblast growth factors, wnt-signaling, steroidogenic genes along with certain transcription factors. Serum 17α, 20β-dihydroxy-4-pregnen-3-one and estradiol-17β reduced significantly post sox19 silencing, in vivo. Concomitantly, a decrease in aromatase activity was detected post sox19-siRNA treatment, in vivo. This study demonstrates the impact of sox19 in the regulation of common carp ovarian growth and steroidogenesis.

Adult neurogenesis in the central nervous system of teleost fish: from stem cells to function and evolution

Adult neurogenesis, the generation of functional neurons from adult neural stem cells in the central nervous system (CNS), is widespread, and perhaps universal, among vertebrates. This phenomenon is more pronounced in teleost fish than in any other vertebrate taxon. There are up to 100 neurogenic sites in the adult teleost brain. New cells, including neurons and glia, arise from neural stem cells harbored both in neurogenic niches and outside these niches (such as the ependymal layer and parenchyma in the spinal cord, respectively). At least some, but not all, of the stem cells are of astrocytic identity. Aging appears to lead to stem cell attrition in fish that exhibit determinate body growth but not in those with indeterminate growth. At least in some areas of the CNS, the activity of the neural stem cells results in additive neurogenesis or gliogenesis - tissue growth by net addition of cells. Mathematical and computational modeling has identified three factors to be crucial for sustained tissue growth and correct formation of CNS structures: symmetric stem cell division, cell death and cell drift due to population pressure. It is hypothesized that neurogenesis in the CNS is driven by continued growth of corresponding muscle fibers and sensory receptor cells in the periphery to ensure a constant ratio of peripheral versus central elements. This 'numerical matching hypothesis' can explain why neurogenesis has ceased in most parts of the adult CNS during the evolution of mammals, which show determinate growth.

Organization of radial glia reveals growth pattern in the telencephalon of a percomorph fish Astatotilapia burtoni

In the brain of teleost fish, radial glial cells are the main astroglial cell type. To understand how radial glia structures are adapting to continuous growth of the brain, we studied the astroglial cells in the telencephalon of the cichlid fish Astatotilapia burtoni in small fry to large specimens. These animals grow to a standard length of 10-12 cm in this fish species, corresponding to a more than 100-fold increase in brain volume. Focusing on the telencephalon where glial cells are arranged radially in the everted (dorsal) pallium, immunocytochemistry for glial markers revealed an aberrant pattern of radial glial fibers in the central division of the dorsal pallium (DC, i.e., DC4 and DC5). The main glial processes curved around these nuclei, especially in the posterior part of the telencephalon. This was verified in tissue-cleared brains stained for glial markers. We further analyzed the growth of radial glia by immunocytochemically applied stem cell (proliferating cell nuclear antigen [PCNA], Sox2) and differentiation marker (doublecortin) and found that these markers were expressed at the ventricular surface consistent with a stacking growth pattern. In addition, we detected doublecortin and Sox2 positive cells in deeper nuclei of DC areas. Our data suggest that radial glial cells give rise to migrating cells providing new neurons and glia to deeper pallial regions. This results in expansion of the central pallial areas and displacement of existing radial glial. In summary, we show that radial glial cells can adapt to morphological growth processes in the adult fish brain and contribute to this growth.