PROLIFERATION ZONES IN THE BRAIN OF ADULT GYMNOTIFORM FISH - A QUANTITATIVE MAPPING STUDY

Whereas in mammals postnatal neurogenesis, gliogenesis, and angiogenes is appear to be kept at low rates, in fish the capability for the prod uction of new brain cells during adulthood is very pronounced. Many of the newly generated cells originate from germinal layers that maintai n their proliferative activity during adulthood. By employing incorpor ation of the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) into mi totic active cells, we have quantitatively mapped such proliferation z ones in the brain of adult Apteronotus leptorhynchus (Gymnotiformes, T eleostei). In the telencephalon, diencephalon, mesencephalon, and rhom bencephalon, the total number of BrdU-labelled cells was low, making u p approximately 25% of all mitotic active cells in the brain. Many of these cells were scattered over wide areas. Otherwise, zones of high p roliferative activity were typically located at or near the surface of ventricular, paraventricular, and cisternal systems. Approximately 75 % of all BrdU-labelled cells found in the brain of adult Apteronotus l eptorhynchus were situated in the cerebellum. Zones displaying prolife rative activity were restricted to small areas, such as narrow stripes around the midline of corpus cerebelli and valvula cerebelli, the bou ndary between corpus and valvula, and a large portion of the area cove red by the eminentia granularis medialis. Counts indicate that, on ave rage, 100,000 cells, corresponding to approximately 0.2% of the total population of cells in the brain of adult Apteronotus leptorhynchus, a re in S-phase within a period of 2 hours. At least part of these newly generated cells is added to the population of already existing cells. This leads to a permanent growth of the brain with increasing size of the fish, a process that appears to slow down only in individuals of relatively advanced age. (C) 1995 Wiley-Liss, Inc.