Expression of the Transcription Factor Pax6 in the Lobe of the Facial Nerve of the Carp Brain

Abstract

Using immunoperoxidase labeling, we examined the distribution of the transcription factor Pax6 in the unpaired lobe of the facial nerve (lobus impar nervi facialis, LINF) of the brain of adult individuals of the carp (Cyprinus carpio). A significant fraction of the cells with expression of the Pax6 protein was localized in the dorsal, lateral, and basal zones of the external LINF layer. Cells with intense Pax6-positivity were usually of a rounded or slightly elongated shape; in most cases they were not grouped. The average diameter of most labelled cells was 5.8 to 9.6 μm; in the basal zone, some cells were larger (mean diameter up to 11.4 μm). The density of localization of Pax6-positive cells within basal zones of the external LINF layer was noticeably greater than that in the dorsal and lateral portions of the above layer. Within the internal LINF layer, cells with a relatively high content of Pax6 were observed, but the optical density of the immunolabelled substance in cells of this layer was several times lower than that in the external layer. Labelled cells in the internal LINF layer formed significant spatial accumulations (niches) separated by regions with no immunonepositivity. Cross-sectional areas of such niches varied from about 460 to 2070 μm2. A considerable part of the cells localized in the internal layer should probably be considered units in the course of migration. Basal regions of the LINF probably represent the most significant region where post-embryonic (“adult”) neurogenesis is realized within this brain structure. Here, cells are formed de novo (mostly in the external layer of the matrix zone), migrate, and undergo differentiation.