Retinoic acid (RA) is an important morphogen involved in
retinal development. Perturbations in its levels cause
retinal malformations such as
microphthalmia. However, the cellular changes in the retina that lead to this phenotype are little known. We have used the zebrafish to analyse the effects of systemic high RA levels on retinogenesis. For this purpose we exposed zebrafish embryos to 0.1μM or 1μM RA from 24 to 48h post-fertilisation (hpf), the period which corresponds to the time of
retinal neurogenesis and initial
retinal cell differentiation. We did not find severe alterations in 0.1μM RA treated animals, but the exposure to 1μM RA significantly reduced
retinal size upon treatment, and this
microphthalmia persisted through larval development. We monitored histology and cell death and quantified both the proliferation rate and cell differentiation from 48hpf onwards, focusing on the retina and optic nerve of normal and 1μM treated animals.
Retinal lamination and initial neurogenesis are not affected by RA exposure, but we found widespread apoptosis after RA treatment that could be the main cause of
microphthalmia. Proliferating cells increased their number at 3days post-fertilisation (dpf) but decreased significantly at 5dpf maintaining the microphthalmic phenotype.
Retinal cell differentiation was affected; some cell markers do not reach normal levels at larval stages and some cell types present an increased number compared to those of control animals. We also found the presence of young axons growing ectopically within the retina. Moreover although the optic axons leave the retina and form the optic chiasm they do not reach the optic tectum. The alterations observed in treated animals become more severe as larvae develop.