PHOTORECEPTOR MORPHOGENESIS IN THE HUMAN RETINA - A SCANNING ELECTRON-MICROSCOPIC STUDY

There are a number of scanning electron microscopic (SEM) studies on r etinal photoreceptors of vertebrates. However, most of these are conce rned with the adult retina, and only a very few deal with developing p hotoreceptors. In man, SEM studies have not been carried out on photor eceptor morphogenesis during fetal or postnatal stages. Hence, the pre sent study was undertaken to examine the sequential morphological chan ges in developing photoreceptors during different gestational ages in the human retina. Retinas of human fetuses of gestational ages of 10-2 5 weeks and from autopsy of a 5-month-old infant were processed for SE M. The observations show some new information on the morphogenesis of photoreceptors. At 10-11 weeks, the outer and inner neuroblastic zones are well developed and separated from each other by the layer of Chie vitz. By 15-16 weeks, the photoreceptor precursors appear as spherical inner segments on the scleral surface of the outer neuroblastic zone. Cilia develop as: small protrusions from the apical ends of the inner segments. Photoreceptor inner segments become arranged in mosaic patt ern by 18-19 weeks. In the mosaic, large cone inner segments (putative blue cones) stand out prominently from the remaining small cone inner segments (prospective red/green cones). The rod inner segments are id entifiable and show cilia. Between 19-20 and 24-25 weeks, the cone inn er segments elongate and change in shape from spherical to oval. At 24 -25 weeks, the outer segments develop from the distal ends of rod cili a. At this period, the inner segments of rods and cones are interconne cted by protoplasmic projections. Although the precursors of both rods and cones appear to be in a similar state of development at 14-15 wee ks gestation, the rods undergo morphological maturation earlier than d o the cones. Photoreceptor development in the anterior retina lags beh ind that of the posterior retina by about 10 weeks. At 5 months after birth, the posterior retina possesses fully developed photoreceptors t hat are comparable to those of the adult. However, the photoreceptors in the ora serrata resemble those in the posterior retina of 24-25 wee ks gestation. (C) 1998 Wiley-Liss, Inc.