Although progenitor cells in developing vertebrate retina are capable of producing all retinal cell types, they are competent to produce only certain cell types at a given time, and this competence changes as development progresses. We asked whether a change in progenitor cell competence is primarily responsible for ending production of a specific cell type, the retinal ganglion cell. Reducing Notch expression using an antisense oligonucleotide in vitro or in vivo increased ganglion cell genesis. The antisense treatment could reinitiate ganglion cell genesis after it had terminated in a region of the retina, but only for a brief period. The failure of the Notch antisense treatment to reinitiate ganglion cell production after this period was not due to the lack of receptor or ligand expression, as both Notch-1 and Delta-1 were still expressed. The failure of the Notch antisense treatment to reinitiate ganglion cell production is consistent with the suggestion that the intrinsic competence of progenitor cells changes as development progresses. Because reducing Notch signaling can reinitiate ganglion cell production for a brief period after ganglion cell production has normally ceased, it appears that ganglion cell production initially ends in a region of the retina because of cell-cell interactions and not because progenitor cells lose the competence to make ganglion cells. Notch signaling appears to temporarily prevent production of ganglion cells in a region, while some other signal must initiate a change in progenitor cell competence, thus permanently ending the possibility of further ganglion cell production.

Progenitor cells in the early developing nervous system can divide symmetrically, giving rise to two daughter cells that divide again, or asymmetrically, giving rise to one cell that differentiates and one that divides again. It has been suggested that the orientation of the cell cleavage plane during mitosis determines the type of division. A marker of early cell differentiation, the RA4 antigen, was used to identify regions of the developing chick retina with and without differentiating cells, and the orientation of the cleavage plane was characterized for mitotic figures in each region. No difference was found in the frequency of any orientation between the regions with or without differentiating cells. Furthermore, in the region of the retina with differentiating cells, the RA4 antigen was present in mitotic figures with every possible orientation. Thus, the orientation of the cleavage plane appears to be unrelated to whether or not a division produces a cell that differentiates. It has also been suggested that the intracellular protein Numb mediates neurogenesis via asymmetric localization during cell division. Numb localization was compared with expression of markers of early cell differentiation, the RA4 antigen and Delta. Differentiating and nondifferentiating cells were found both with and without Numb expression. Cells with a cleavage plane parallel to the retinal surface were polarized, such that Numb and/or the RA4 antigen, when present, were only in the daughter cell farthest from the ventricle. These findings indicate a need to reconsider current hypotheses regarding the key features underlying symmetric and asymmetric divisions in the developing nervous system.

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Progenitor cells in the developing retina initially divide so that each division produces two cells that divide again. Subsequently, progenitor cells change their mode of division so that one or both cells produced by a division can withdraw from the mitotic cycle and differentiate. We asked how these two progenitor cell stages differ molecularly and what controls the switch in the mode of division. We show that early preneurogenic progenitor cells express the transcription factor, Sox2, and the Notch ligand, Delta1. More mature neurogenic progenitor cells express Sox2 and the bHLH transcription factor, E2A, and not Delta1. Notch signaling maintains progenitor cells in the preneurogenic state. Sonic hedgehog expressed by newly differentiating cells initiates maturation of progenitor cells from preneurogenic to neurogenic at the neurogenic front, possibly by down-regulating Delta1 expression. Our results show that the preneurogenic-to-neurogenic transition is a highly organized unidirectional step made in unison by neighboring cells.