Radial Glial Cells Are Stem Cell-Like Neural Progenitors in the Adult

Radial glial cells are stem cell-like neural progenitors in the adult mammalian brain and can give rise to a number of differentiated neural cell types. Knowing that quiescent radial glial cells uniquely express the cytoskeletal protein nestin, a researcher performed clonal analysis in mice to study the characteristics of radial glial cells and their descendants. She created transgenic mice expressing Cre recombinase under the control of the nestin regulatory sequences. She also created transgenic mice that carry the gene encoding the green fluorescent protein (GFP) under the control of a ubiquitous promoter. The GFP-encoding gene, however, is inactive unless recombination by Cre removes a blocking sequence between the promoter and the gene. After mating these mice, she obtains adult mice carrying both transgenes and injects them with a low dose of tamoxifen to activate Cre sporadically. After a month, she uses microscopy to study the cells of the brain hippocampus. She discovers that most neural cell types in the tissue, including neurons, develop from the radial glial cells. Oligodendrocytes, however, appear to derive from another stem-cell population. Based on her conclusions, which cells do you think expressed GFP after a month: neurons (N) or oligodendrocytes (O)? If a single isolated GFP-expressing cell is observed in the tissue, would you expect it to be a differentiated cell (D) or a radial glial cell (R)? If a larger clone is observed to be entirely made of radial glial cells, would a symmetric (S) or asymmetric (A) model be favored for the strategy of cell divisions in these cells? Write down your answer as a three-letter string, e.g. NDA.