Morphogenesis of photoreceptor and retinal ultrastructure in a sub-human primate

Since the foveal photoreceptors in the duplex retina of adult human and other diurnal primates are closely associated with high visual acuity and spectral sensitivity, observations were made on the pre- and postnatal morphologic development of the macula and fovea in the retina of the diurnal rhesus monkey. Histologic findings on the level of development of the retina at birth revealed a macula with the characteristic foveal depression. Observations on the ultrastructural development of the foveal photoreceptors revealed that their outer and inner segments were relatively well differentiated even at birth. An examination of the prenatal morphogenesis of the photoreceptors in the eye of the fetus indicated that the initial differentiation of the receptors may take place as early as 100 to 125 days after conception. However, the receptors in the specialized fovea continued to develop for 1 to 2 months after birth, whereas the photoreceptors outside the central region of the retina had completed their fine structural development at birth. By the end of the second month after birth, the macular and foveal areas essential for high visual acuity and spectral sensitivity appeared structurally well completed. In general, major maturational changes observable after birth were apparent predominantly in the pigment epithelium of the retina.