Retinal progenitor cells continue to proliferate throughout development in the absence of Rb and p107. (A-T) Immunostaining of P30 Chx10-Cre;Rb
-/- retinas by using antibodies against (A-D) BrdU to identify ectopically dividing cells; (E-H) calbindin, horizontal cells and Chx10, bipolar/progenitor cells; (M-P) GFAP, reactive Müller glia; (Q, R) cone arrestin, cone photoreceptors and (S, T) Pax6, amacrine/progenitor cells. (U, V) Electron micrographs showing that most retinal structures are disrupted in the absence of Rb and p107 and that continued proliferation eventually leads to retinoblastoma. (W-Z) Newborn Rb
-/- mice were injected with a retrovirus expressing Cre (LIA-Cre), and 3 weeks later, the retinas were sectioned and stained for alkaline phosphatase expression to identify the neurons that had lost Rb and p107. Amacrine cells (W) and bipolar cells (X) formed normally in the absence of Rb and p107. Rod photoreceptors (Y) failed to form normally. There were also several clones that spanned the INL or INL and ONL, which was indicative of hyperproliferation (Z). Similar experiments were carried out in E17.5 retinal explants by using a Cre-expressing retrovirus that also expresses nuclear LacZ (AA). These clones can readily be scored for cell number to determine if loss of Rb and p107 leads to deregulated proliferation. Abbreviations: GCL, ganglion cell layer; INL, inner nuclear layer; olm, outer limiting membrane; ONL, outer nuclear layer. Scale bars: B, D, F, H, J, L, N, P, R, T and W-AA, 10 μm.