Ontogeny of retinal oil droplets in the chick embryo

Abstract

The origin of the colored oil droplets in the chick cones was investigated by subjecting retinas from staged embryos to correlative biochemical (visible spectrophotometric assays of chromatographically separated extracts) and microscopic analyses. The carotenoid pigments responsible for the coloration of the oil droplets are detected spectrophotometrically before they can be visualized microscopically in the form of colored droplets. The spectrophotometric data reveal that each pigment arises independently in the following sequence: golden yellow, yellowish green, and red. The golden yellow hydrocarbon is first detected at stage 27 at which time the retina consists of a thick, unstratified, neuroepithelial layer with an active proliferative zone, but lacks visual cells. Galloxanthin, the yellowish green pigment, arises at stage 34. The retina is still devoid of visual elements at this stage but is organized into several nuclear layers. The concentrations of the golden yellow and yellowish green carotenoids begin to increase rapidly at stage 37 when inner segments are differentiating, but oil droplet vacuoles have not yet appeared within them. Colorless droplets first evident in fresh preparations of this stage may not be colorless at all, but may contain the yellow pigments in sach low concentrations that their colors cannot be discriminated microscopically. The red pigment, astaxanthin, is first evident at stage 41 and immediately exhibits a steady increase in concentration. The retina possesses adult stratification at this time; all ten layers are present and oil droplet vacuoles are observed for the first time. On the basis of the spectrophotometric findings it is contended that the double cones are the first cones to differentiate in the chick retina and that the chief component (bearer of the golden yellow droplet) arises before the accessory element (carrier of the yellowish green droplet).