Continued growth and circuit building in the anamniote visual system

Abstract

Fish and amphibia are capable of lifelong growth and regeneration. The two core components of their visual system, the retina and tectum both maintain small populations of stem cells that contribute new neurons and glia to these tissues as they grow. As the animals age, the initial retinal projections onto the tectum are continuously remodeled to maintain retinotopy. These properties raise several biological challenges related to the control of proliferation and differentiation of retinal and tectal stem cells. For instance, how do stem and progenitor cells integrate intrinsic and extrinsic cues to produce the appropriate type and number of cells needed by the growing tissue. Does retinal growth or neuronal activity influence tectal growth? What are the cellular and molecular mechanisms that enable retinal axons to shift their tectal connections as these two tissues grow in incongruent patterns? While we cannot yet provide answers to these questions, this review attempts to supply background and context, laying the ground work for new investigations.

Zebrafish midbrain slow-amplifying progenitors exhibit high levels of transcripts for nucleotide and ribosome biogenesis

“…Our results confirm on live specimens that the OT is a typical cellular conveyor belt (Devès and Bourrat, 2012); it has zones of unmixed FAPs and SAPs at its periphery, a zone of cells exiting the cycle and a central zone of differentiating cells (Cerveny et al, 2012) (Fig. 7).…”

“…Cytological and molecular signatures may help to define cell type homologies from an 'evo-devo' perspective (Arendt, 2005). Synexpression of genes in retinal CMZ cells and midbrain progenitors has been noted (Cerveny et al, 2012;Ramialison et al, 2012) and the phenotypes of mutants for at least 18 PML-specific genes are illustrated on the ZIRC website (supplementary material Table S3). These mutants share strikingly similar neuroectodermal and ocular defects.…”

“…Each cell population, at a particular level of differentiation, is marked by concentric gene expression patterns. Similarly, the anamniote retina may be considered a cellular conveyor belt and, as discussed in a recent review (Cerveny et al, 2012), tectal cells and retina cells from the ciliary marginal zone (CMZ) share common molecular signatures and express many canonical proliferation markers.…”

“…Previously described as the 'caudal wall' (Palmgren, 1921) and recently as the 'posterior midbrain lamina' (Grandel et al, 2006), this structure wraps the embryonic OT both posteriorly and laterally; we therefore find it more appropriate to refer to this structure as the 'peripheral midbrain layer' (PML). Three recently published reviews (Cerveny et al, 2012;Grandel and Brand, 2013;Schmidt et al, 2013) pointed out the need to know more about the formation and function of this cell layer, which gives rise to different types of tectal cells. It is in close proximity with the her5-positive stem cells (SCs) at the midbrain-hindbrain boundary (MHB) and probably derives from this SC population.…”

A self-renewing division of zebrafish Müller glial cells generates neuronal progenitors that require N-cadherin to regenerate retinal neurons

“…In adult teleost fish, growth-related neurogenesis continues throughout the central nervous system (Kizil et al, 2012;Grandel and Brand, 2013), including in the retina, where neuroepithelial retinal progenitors in the ciliary marginal zone (CMZ; a stem cell niche) generate retinal neurons associated with ocular growth (Hitchcock et al, 2004;Raymond et al, 2006;Cerveny et al, 2012). In the teleost retina, neurogenesis continues not only in the CMZ, but also in central retina.…”