Characterisation of maturation of photoreceptor cell subtypes during zebrafish retinal development.

Photoreceptor cells (PRCs) mature from simple epithelial cells, a process characterised by growth and compartmentalisation of the apical membrane into an inner and an outer segment. So far, a PRC subtype-specific description of morphological and cellular changes in the developing zebrafish retina is missing. Here, we performed an in-depth characterisation of four of the five PRC subtypes of the zebrafish retina between 51 and 120 h post fertilisation, including quantification of the size of different compartments, localisation of polarity proteins and positioning of organelles. One of the major findings was the anisotropic and subtype-specific growth of the different PRC compartments. In addition, a transient accumulation of endoplasmic reticulum in rod PRCs, changes in chromatin organisation in UV sensitive cones and differential expression of polarity proteins during the initial stages of PRC maturation were observed. The results obtained provide a developmental timeline that can be used as a platform for future studies on PRC maturation and function. This platform was applied to document that increased exposure to light leads to smaller apical domains of PRCs.