Ultrastructural diurnal changes of the retinal photoreceptors in the embryo of a viviparous teleost ( Poecilia reticulata P.)

All diurnal changes studied — ellipsosome excepted — start at midgestation, following differentiation of photoreceptors and pigment-epithelium cells. These are: (1) shedding of the tips of the light-sensitive photoreceptor outer segments and subsequent phagocytosis by the pigment-epithelium; (2) retinomotor movements of pigment-epithelium processes, rods and cones; (3) changes of cone square-mosaics into row-mosaics at night. Newly-differentiated photoreceptors in the embryo are, therefore, already vulnerable to disruption of cyclical systems. Several inherited human retinal diseases, such as Retinitis pigmentosa, are thought not to affect differentiation of photoreceptors but their cyclical renewal pathways. The retina of the guppy-embryo is, therefore, a valuable model for such studies.