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Abstract

Many vertebrate species show life-long growth facilitated by distinct and dedicated stem cell systems. Understanding the initial establishment and life-long maintenance of stem cells and their environment is, therefore, a key to understanding the transition between these species and mammals that have lost life-long growth capacity. In teleost fish, the retinal stem cell niche, called the ciliary marginal zone (CMZ), facilitates retinal growth and the proliferative behavior of stem cells and their progenitors, ensuring organ growth and functionality throughout life. The mature CMZ is found in the distal periphery of the retina, and it is composed of concentric rings containing stem (distal) and progenitor cells (more proximal), respectively. While stem and progenitor cells within the CMZ and their relative contribution to shape and function are well established, the origin of retinal stem cells (RSCs) remains obscure. Previous studies suggested the existing link between eye morphogenesis and RSC establishment and for a predefined RSC population at the early stages of eye development. Therefore, the objective of this thesis is centered on understanding the processes and mechanisms behind the development of adult RSCs in teleost fish. To describe RSCs in more depth, I conducted single-cell RNA sequencing of the medaka (Oryzias latipes) retina and unveiled novel markers for RSCs and retinal progenitor cells (RPCs). To address whether the establishment of RSCs is coupled to the optic cup morphogenesis, I inspected the pattern of RSC marker expression in the conditions of interrupted eye morphogenesis through either drug treatment or genetic mutations. I could show that stem cell marker expression is still initiated in the CMZ upon defected eye morphogenesis, indicating that all retinal progenitors retain the potency of acquiring retinal stem cell identity. Moreover, I analyzed the changes in the proliferative behaviour of retinal stem cell marker-expressing cells within the peripheral retina utilizing BrdU incorporation and tracing their progeny after Cre/loxP-mediated recombination. I observed that the fast-proliferating retinal progenitors in the retinal periphery switch to a slower proliferative behavior during the embryonic development of medaka. Via long-term clonal analysis, I showed when the retinal peripheral cells become clonal, i.e., bona fide stem cells. Taken together, my findings challenge the notion of a pre-existing early RSC population in eye development and argue for presence of mechanisms by which embryonic RPCs at the retinal periphery mature into adult stem cells in the medaka CMZ.