Background: Low level gestational lead exposure (GLE) produces increased number of rod photoreceptors and supernormal eletroretinograms (ERGs) in children and rodents. This contrasts with known apoptotic loss of rod photoreceptors and subnormal ERG of high level adult Lead exposure. Objectives: The goal of this study was to decipher the underlying cellular and molecular mechanisms of this novel GLE phenotype in our murine GLE model. We hypothesized that cell cycle reentry of retinal progenitor cells (RPCs) and cell cycle exit, and rod photoreceptor cell fate specification of RPCs would be higher in GLE relative to control retinas and would occur with concurrent molecular changes. Methods: Female C57BL/6 mice were exposed to lead through drinking water 2 weeks before mating, throughout gestation till postnatal day 10 (PN10). Blood lead concentrations ([BPb]) in controls and GLE pups were ≤ 1 and 25 µg/dL, respectively, at PN10. Retinas from pups aged PN2 to PN10 were used to perform retinal experiments. Results: Cumulative BrdU labeling revealed that GLE shortens cell cycle length at G1-phase and accelerates cell cycle. Affymetrix gene expression array, qPCR, Western blot and confocal microscopy showed that GLE increases alters NOTCH1-HES1 pathway, cell cycle regulators (Cyclins, INKs, p27KIP, RB phophorylation) and bHLH (ASCL1, OTX2, HES6) rod photoreceptor differentiation regulators, without change in spatiotemporal expression pattern. In both GLE and control retinas, we found a spatial coupling of regulators of cell cycle exit (p27KIP), cell cycle inhibitors (p16, p19 INKs), cell cycle reentry (HES1), cell fate specification (ASCL1, OTX2, HES1) in the SVZ, while cell cycle progression regulators (Cyclins) were spatially uncoupled and located in the inner retina. ChIP-qPCR showed increased activity of Cyclin D1 and ASCL1 promoters in GLE retinas. Conclusions: Our findings show that GLE shortens cell cycle to increase proliferation and rod photoreceptor differentiation of RPCs, and this is mediated through differential expression of the NOTCH1-HES1 signaling, cell cycle regulators (Cyclins, INKs, CIP/KIP) and differentiation factors (ASCL1, OTX2). Our results suggest that there is greater cross-talk between the spatiotemporally coupled processes of cell cycle inhibition (INKS, KIP), cell fate specification factors (ASCL1, OTX2, HES1) and cell cycle reentry mediated by HES1 than with spatiotemporally uncoupled cell cycle progression regulation by Cyclins, and this spatiotemporal segregation may provide an insurance to proper transition of RPCs through these developmental stages. These results provide new insights into the molecular mechanism of retinal development and GLE effects. Abbreviations: AC: Amacrine cell, BC: Bipolar cell, BrdU: 5-bromo-2-deoxyuridine, E: embryonic day, GCL: ganglion cell layer, GLE: gestational lead exposure, INL: inner nuclear layer, IR: immunoreactive, LSCM: laser scanning confocal microscopy, NBL: neuroblastic layer, ONL: outer nuclear layer, PH3: phosphohistone H3, PN: postnatal day, RPC: Retinal progenitor cell Also available at: https://gitlab.com/smukher2/pbexposure_molecularbiology_aug2011 Source: This electronic version of my PhD thesis is a copy of the same available in University of Houston, Houston, TX, U.S. Library.