Autonomous proliferation of neural precursors in the tadpole retina revealed after partial removal of the embryonic eyebud.
The cellular mechanisms that regulate cell division in the developing vertebrate nervous system are essentially unknown. Using the frog retina as a model system, we tested the hypothesis that proliferation is regulated by feedback signals that encode the total number of cells in the population. To alter the number of cells in the retina, we surgically removed approximately half of the embryonic eyebud. In a previous study, cell counts made immediately after partial ablation showed that the number of proliferating, undifferentiated cells was reduced to 70% of the number in the contralateral control eye, while the number of postmitotic, differentiated cells was reduced to 47%. In this study, we sought to determine whether these reduced cell number of affected proliferation, which was assessed by counting the total number of cells present at various times after surgery. The partially ablated retinas consistently had fewer cells than the contralateral retinas. Analyses of the cell numbers suggested that there was little or no change in mitotic rate in the partially ablated retinas and that the decreased production of both differentiated and undifferentiated cells was due almost entirely to the reduction in the number of dividing cells. There is thus no evidence that retinal stem cells up-regulate their production of new cells in response to a reduction of either the stem cells themselves or of their descendants. This lack of response to population size indicates that, in contrast to many non-neural stem cell systems, proliferation of neural stem cells is not regulated by a feedback mechanism but instead is largely autonomous.
Wetts
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Division of Neurosciences, Beckman Research Institute of the City of Hope, Duarte, CA 91010, USA.
Quon
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RF
D000818:Animals; D002452:Cell Count; D002454:Cell Differentiation; D002455:Cell Division; D005246:Feedback; D007814:Larva; D009799:Ocular Physiological Phenomena; D012016:Reference Values; D012160:Retina / Q000166:cytology*; D013234:Stem Cells / Q000166:cytology*; D014982:Xenopus laevis / Q000254:growth & development*