Characteristics of cellular proliferation in the developing human retina.

The expression of proliferation-associated proteins Ki67 and PCNA was studied in the retinal rudiments of human embryos at 5-8 weeks of development; studies also addressed the numbers of nucleoli in the nuclei of neuroepithelial cells (with consideration of their distances to the apical surface) and DNA-synthesizing cells after transient (20 min) in vitro incubation in serum-free medium containing BrdU. The retinal rudiment of embryos at five weeks of development had neuroepithelium of the typical structure. BrdU-positive nuclei and nuclei with small numbers of nucleoli were located in the basal part of the ventricular zone. However, this organization was disrupted during the initial period of formation of the inner nuclear layer (six weeks). At this time, DNA-synthesizing cells were found even at the apical surface. Retinal rudiments of embryos at 6-7 weeks of development contained an additional area of cell proliferation in the Chievitz layer and the inner nuclear layer. In eight-week embryos, dividing cells were located in the outer nuclear layer, which again acquired the organization typical of neuroepithelium.