Maturation of Photoreceptor Cells During Zebrafish Retinal Development

Zebrafish have been used as a model to study the vertebrate retina due to its functional and structural similarities to the human retina. Photoreceptor cells (PRCs) are highly specialised type of neurons present in the retina. In zebrafish, PRCs can be divided into 5 different subtypes, rods and green, red, blue and UV sensitive cones. Mature PRCs are composed of different morphological compartments (basal domain, inner segment and outer segment), which are essential for their phototransduction ability. During development, these cells are known to arise from columnar neuroepithelium precursor cells and undergo a maturation process to become compartmentalised10. However, a detailed characterisation of this process is lacking in zebrafish. In this project, I aimed to establish and characterise in detail the stages of PRC maturation in zebrafish. Next, I aimed to investigate the role of candidate genes in this PRC maturation process. To label the plasma membrane of all cells, a zebrafish transgenic line was utilised. Furthermore, a novel zebrafish transgenic line that labels the outer segments of red sensitive PRCs was generated. This transgenic line enabled visualisation and volume quantification of outer segments of red sensitive cones. The use of both transgenic lines in combination with antibody stainings indicated that, from 72 hours post fertilisation (hpf) onwards, subtypes of PRCs exhibit differences in growth rate and morphology of their cell compartments. Additionally, differences in mitochondrial clusters and nuclei positioning were observed during the maturation process. From 72 hpf to 120 hpf, rough endoplasmic reticulum accumulation emerged specifically in rod like PRCs. Changes in chromatin organisation were observed in UV sensitive cones like PRCs from 120 hpf onwards. This showed that a high degree of complexity was observed even within the cone PRC subtypes. Lastly, the role of a candidate gene, crb2b, was examined by comparing PRC maturation process in WT and crb2b mutants. My results indicate that loss of Crb2b does not show obvious defects in PRC maturation. Results obtained in this dissertation provided a comprehensive characterisation of six independent PRC maturation stages using the criteria of cell compartmentalisation and growth, organellar distribution and localisation of cell polarity related protein complexes. This defined developmental timeline provides a platform to further study PRC maturation and function