Schematic flow-diagram of cytogenesis in the mammalian retina. After a phase of identical replication of retinal stem cells, a stepwise transition occurs via to early to late progenitor cells and, finally, radial glial cells which is accompanied by distinct restrictions/changes of potency (thick black arrows). The duration of each proliferation phase (i.e., the number of cell divisions) until transition to the next step is tightly controlled, in a species-specific manner, by a variety of factors (some of the hitherto-known factors are given at the left side). This allows a subsequent determination of (i) the size of the future retina and eye (stem cells; very variable), (ii) the contribution of primary photopic cells (early progenitors; not very variable), and (iii) the addition of complementary bipolar and amacrine cells (not very variable) and rod photoreceptor cells (very variable) by the late progenitors. There appear to be some key regulators that modulate the relation between early and late neuron production (e.g., DOPA and sonic hedgehog). One of the daughter cells of the late progenitor cells becomes a radial glial cell which normally differentiates as a Müller cell without further proliferation. Under pathological conditions (thick red arrows), however, Müller cell may de-differentiate into radial glial cells, and undergo proliferation. Under experimental conditions, even a re-transition to late progenitors has been induced whereas a re-transition into early progenitors was not yet achieved in mammals. Modified after Reichenbach et al. (1998); for further details, see Sections and 3.1.4. Although not indicated in this figure, the cell type specification/ratio of the neuronal progenies of both early and late progenitors is also controled by signal molecules; the interested reader is referred to Reichenbach et al. (1998), Dyer and Cepko (2001b), Lamb et al. (2007), and others
2.18 and 2.19).
2.18 and 2.19); the generation of some horizontal cells by these
late progenitors in marsupials is an exception among mammals.
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