IN-VIVO CLONAL ANALYSES REVEAL THE PROPERTIES OF ENDOGENOUS NEURAL STEM-CELL PROLIFERATION IN THE ADULT MAMMALIAN FOREBRAIN

The adult mammalian forebrain contains a population of multipotential neural stem cells in the subependyma of the lateral ventricles whose p rogeny are the constitutively proliferating cells, which divide active ly throughout life. The adult mammalian brain is ideal for examining t he kinetics of the stem cells due to their strict spatial localization and the limited and discrete type of progeny generated (constitutivel y proliferating cells). Clonal lineage analyses 6 days after retroviru s infection revealed that under baseline conditions 60% of the constit utively proliferating cells undergo cell death, 25% migrate to the olf actory bulb and 15% remain confined to the lateral ventricle subependy ma (where they reside for approximately 15 days). Analysis of single c ell clones 31 days after retroviral infection revealed that the stem c ell divides asymmetrically to self-renew and give rise to constitutive ly proliferating cells. Following repopulation of the depleted subepen dyma the average clone size is 2.8 times larger than control, yet the absolute number of cells migrating to the olfactory bulb is maintained and the stem cell retains its asymmetric mode of division. The number of neural stem cells in the adult forebrain 33 days after repopulatio n of the subependyma was estimated using bromodeoxyuridine labeling of subepenydmal cells. There were calculated to be 1200-1300 cells betwe en the rostral corpus callosum and rostral anterior commissure; these data support a lineage model similar to those based on stem cell behav ior in other tissue types.