Sex differences in adult cell proliferation within the zebrafish (Danio rerio) cerebellum

It has been reported that neurons generated in the adult brain show sex-specific differences in several brain regions of lower vertebrates and mammals. The present study questioned whether cell proliferation and survival in the adult zebrafish (Danio rerio) cerebellum, the most mitotically active area of adult teleost brain, is sexually differentiated. Adult zebrafish were treated with the thymidine analogue 5'-bromo-2'-deoxyuridine (BrdU) and allowed to survive for 24 h (short-term) and for 21 days (long-term). BrdU immunohistochemistry allowed visualization of cells incorporating BrdU at the S phase of mitosis. At short-term survival, male zebrafish had a higher number of labelled cells at proliferation sites of the molecular layer of corpus cerebelli (CCe) and the granular layer of the caudal lobe of the cerebellum (LCa) than did females. In long-term survival, BrdU-positive cells were found at their final destination, but only the granular layer of the medial division of the valvula cerebelli showed sex-specific differences in the number of labelled cells. This higher mitotic activity in male cerebellum might be related to sex-specific motor behaviour observed in male zebrafish. To investigate the role of programmed cell death, the terminal deoxynucleotidyl-mediated dUTP nick-end-labelling (TUNEL) method was applied. The vast majority of apoptotic figures occurred in the granular cell layer of valvula and CCe, only in a few cases within the BrdU-retaining cells. Apoptosis was found specifically at the sites of the final destination of proliferating cells, indicating that the close relation of cell birth and death might represent a possible plasticity mechanism in the adult zebrafish cerebellum.