Characterisation and computational modelling of retinal stem cells in medaka (Oryzias latipes)

The central functional unit of the vertebrate eye is the retina, composed of neural retina (NR), retinal pigmented epithelium (RPE), and non-visual retina (NVR). In amphibians and fish, the retina grows throughout life via different pools of stem cells (SCs). In this work, I combined experimental and computational approaches to elucidate SC dynamics in the three retinal tissues of the teleost fish medaka (Oryzias latipes). I developed a cell centred agent based model to recapitulate post-embryonic growth of the NR and RPE. By accounting for 3D tissue geometry and continuous growth, the model reconciled conflicting hypotheses, demonstrating that competition between SCs is not mutually exclusive with lifelong coexistence of multiple SC lineages. To understand how NR and RPE regulate their proliferative output to coordinate growth rates, I developed quantitative methods to compare experiment and simulation. I tested the experimental data against simulations implementing two modes of feedback between cell proliferation and organ growth. Thus, I identified that the NR acts upstream to set the growth pace by sending an inductive growth signal, while the RPE responds downstream to this signal. Leveraging the model, I showed that NR SCs compete for niche space, but tissue geometry biases cells at certain positions to win this competition. Further, NR SCs modulate division axes and proliferation rate to change organ shape and retinal topology. Motivated by model predictions, I experimentally characterised the large SC population of the RPE, which consisted of both cycling and non-cycling quiescent cells. Putative sister cells exhibited similar temporal dynamics in local clusters, indicating that quiescence was the major mechanism for regulating proliferative output in the RPE. Finally, I experimentally showed that the NVR grows post-embryonically from a primordium, and shared all known markers for NR SCs in the same spatial distribution. Unlike NR and RPE, the NVR lacked a dedicated niche, instead proliferative cells were distributed throughout the tissue. Lineage tracing revealed a continuous relationship between RPE, NVR, and NR. Thus, the SCs of NR and RPE, and all cells of the NVR displayed plastic multipotency capable of generating all retinal tissues. By taking advantage of the positive feedback loop between experiment and simulation, this work shines a new light into a fundamental problem – growth coordination of different SC populations in a complex vertebrate organ

Left/right asymmetric collective migration of parapineal cells is mediated by focal FGF signaling activity in leading cells

International audienceThe ability of cells to collectively interpret surrounding environmental signals underpins their capacity to coordinate their migrationin various contexts, including embryonic development and cancer metastasis. One tractable model for studying collectivemigration is the parapineal, a left-sided group of neurons that arises from bilaterally positioned precursors that undergo acollective migration to the left side of the brain. In zebrafish, the migration of these cells requires Fgf8 and, in this study, we resolvehow FGF signaling correlates with—and impacts the migratory dynamics of—the parapineal cell collective. The temporal and spatialdynamics of an FGF reporter transgene reveal that FGF signaling is activated in only few parapineal cells usually located at theleading edge of the parapineal during its migration. Overexpressing a constitutively active Fgf receptor compromises parapinealmigration in wild-type embryos, while it partially restores both parapineal migration and mosaic expression of the FGF reportertransgene in fgf8−/− mutant embryos. Focal activation of FGF signaling in few parapineal cells is sufficient to promote the migrationof the whole parapineal collective. Finally, we show that asymmetric Nodal signaling contributes to the restriction and leftwards biasof FGF pathway activation. Our data indicate that the first overt morphological asymmetry in the zebrafish brain is promoted byFGF pathway activation in cells that lead the collective migration of the parapineal to the left. This study shows that cell-state differencesin FGF signaling in front versus rear cells is required to promote migration in a model of FGF-dependent collective migration