Fetal topography of bovine rhodopsin mRNA suggests retinotopographically determined gene expression.

PURPOSE

To understand the developmental processes in the differentiating bovine retina, topographic accumulation of rhodopsin mRNA in staged fetal and adult retinas was analyzed.

METHODS

Isolated retinas were spread on a nylon membrane with the photoreceptor cells facing the membrane and dissected into 25-mm square tissue segments, sometimes with as many as 150 segments/eye. Subsequent to disruption of the tissue in each segment, rhodopsin and beta-actin mRNA levels were quantitated with a solution hybridization assay. Slight variations in RNA extraction efficiency and retinal segment size were corrected using beta-actin mRNA as an internal standard.

RESULTS

Analysis of multiple fetal and adult bovine retinas revealed a relatively static central-to-peripheral gradient of rhodopsin mRNA level that appears at the time of transcriptional induction (6 to 6.5 months of gestation) and persists into adulthood. After induction of rhodopsin mRNA expression, increase of rhodopsin mRNA levels was detected simultaneously in all retinal segments. Furthermore, the rate of increase in rhodopsin mRNA levels in peripheral and central regions was identical.

CONCLUSIONS

Fetal induction of rhodopsin mRNA expression occurs simultaneously in all photoreceptor cells across the retina, but the levels are set according to a topographically predetermined pattern. This suggests that regulation of accumulation of rhodopsin mRNA during development is determined according to spatial coordinates before gene induction, most likely in a nonphotoreceptor retinal cell type.