Critical Periods of Eye Development in Vertebrates with Special Reference to Humans

Abstract

This review highlights similarities in human, rodent and avian eye development, focusing on morphogenesis, proliferative activity, apoptosis and mechanistic aspects during the development of the retina, the iris and the cornea. Retinal morphogenesis encompasses four stages: (1) appearance of the ganglion cell layer, (2) formation of the outer plexiform layer separating the inner and outer nuclear layers, (3) formation of immature photoreceptor segments and (4) development of outer photoreceptor segments. The first two stages occur very early during development, i.e. within the first 10 days of incubation in chickens, during the first postnatal week in rodents and until 28 weeks of gestation in humans. In contrast, the formation of receptor inner and outer segments takes considerably less time, i.e. about another 4 days in chickens, 7 days in rodents and 12 weeks in humans. Lesion experiments in chickens and rodents indicate that catecholaminergic input is a prerequisite for proper photoreceptor development, whereas synaptic contact of retinal ganglion cells with higher CNS centers is not necessary. Although already observed very early during the development of the human retina, apoptotic cell death only becomes significant after 23 weeks of gestation and further increases during late gestation. The majority of dying cells in the human and avian retina represent bipolar cells, and the number of apoptotic cells is highest during the formation of inner and outer photoreceptor segments. The similarities outlined in the present review indicate that the basic developmental mechanisms are the same in the primate, rodent and avian retina.