Self-renewal of stem cells and differentiation of nerve cells in the developing chick retina

Data on proliferation and self-renewal of stem cells in the developing chick retina were obtained on the basis of measurements of cells labeled by [3H]thymidine pulses in conjunction with the rate of increase in total cell number. Duration of S-phase was found to be about 4 h between stages 4 and 9 days. Self-renewal drops below the critical value of 50% (implying a transition from increase to depletion of absolute number of stem cells in the tissue) around day 7.6. The spatial order of cell proliferation was studied by measurements taken on subregions of retinas at various stages of development. Proliferating cells forming the ventricular layer increase in all regions of the retina up to day 7, though the proportion of proliferating cells is lowest in the center. From day 5 on it is higher in the nasal as compared to the temporal part of the tissue. After day 7 self-renewal of stem cells drops below 50%, stem cells become depleted and withdraw gradually from mitosis. The process is initiated in the center slightly temporal to the dorsal end of the optic fissure and then spreads rapidly towards the periphery, reaching the temporal margin first. The findings imply that while cells at the periphery are younger on the average, those cells which have become postmitotic at an early stage are not confined to a small central core of the fully developed retina, because the tissue continues to grow and produce postmitotic cells in all regions of the retina up to day 7.