Lateral ventricular wall whole mount immunofluorescence from brains of Lrig1-T2A-iCreERT2/+; ROSA26-Ai14/+ mice at 7 days after very low dose induction

Summary of lateral ventricular wall whole mount immunofluorescence analyses from brains of Lrig1T2A-iCreERT2/+; ROSA26Ai14/+ 1c2 mice induced once with a very low dose of tamoxifen at 3 months of age, 7 days after induction.At very low doses of tamoxifen, only non-proliferative cells can be labeled in some mice. This could be determined post hoc.Note the distinctive morphologies of the Lrig1+ cells labeled.As one can see at https://doi.org/10.6084/m9.figshare.12488696.v4, when sparsely induced and chased for a very long time (1 year), neuroblasts and proliferating clusters were observed in the whole mounts. Neuroblasts are newly born cells that migrate out of the ventricular walls into the olfactory bulbs in 1-2 weeks of time. Once born, they do not persist for an extended length of time in the ventricular wall. Because of this reason, we can infer that some cells that were also in the whole mounts must have given birth to the neuroblasts in the old mice near the time of the perfusion. Providing a hint to the identity of these stem cells, the grape-like clusters of proliferating cells were composed of round cells (likely TA cells based on Ascl1 or Ki-67 immunoreactivity) and cells with branches and a basal process (see high resolution images at https://doi.org/10.6084/m9.figshare.12720998.v2). This suggested that the cells with the branches and a basal process are the stem cells that ultimately give rise to the neuroblasts. The ependymal cells could be ruled out because proliferating clusters composed of ependymal cells were not observed. Interestingly, other types of cells with different morphologies that could also be stem cells (i.e., B1 stem cells) were not observed (in young or old mice), suggesting Lrig1 reveals only one subset of the stem cells, i.e., the cells shown here with the branches and a basal process, and that these cells are neurogenic.