Quantitative analysis reveals dominance of gliogenesis over neurogenesis in an adult brainstem oscillator.

Neural stem/progenitor cells in the neurogenic niches of the adult brain are widely assumed to give rise predominantly to neurons, rather than glia. Here, we performed a quantitative analysis of the resident neural progenitors and their progeny in the adult pacemaker nucleus (Pn) of the weakly electric fish Apteronotus leptorhynchus. Approximately 15% of all cells in this brainstem nucleus are radial glia-like neural stem/progenitor cells. They are distributed uniformly within the tissue and are characterized by the expression of Sox2 and Meis 1/2/3. Approximately 2-3% of them are mitotically active, as indicated by expression of proliferating cell nuclear antigen. Labeling of proliferating cells with a single pulse of BrdU, followed by chases of up to 100 days, revealed that new cells are generated uniformly throughout the nucleus and do not undergo substantial migration. New cells differentiate into S100+ astrocytes and Hu C/D+ small interneurons at a ratio of 4:1, reflecting the proportions of the total glia and neurons in this brain region. The continuous addition of new cells leads to a diffuse growth of the Pn, which doubles in volume and total cell number over the first 2 years following sexual maturation of the fish. However, the number of pacemaker and relay cells, which constitute the oscillatory neural network, remains constant throughout adult life. We hypothesize that the dominance of gliogenesis is an adaptation to the high-frequency firing of the oscillatory neurons in this nucleus.