Purinergic Signalling (2010) 6:429–445 439 Fig. 7 Distribution of P2X7- and P2X4-like immunoreactivity in the anterior telencephalon. a Immunoreactivity to a P2X7 antibody (green fluorescence) and cell nuclei staining with propidium iodide (red fluorescence) of a slice of the anterior telencephalon is shown in a . 1 Another slice treated with the anti-P2X7 antibody after preadsorption with the immunizing protein and counterstained with propidium iodide is shown in a . Higher magnification of the region indicated 2 by the white rectangle in a1 is shown in a3-1–a3-3. Note the clear somatic staining pattern of OB cells. b Immunoreactivity to a P2X4 antibody (b1) of another slice of the anterior telencephalon. Higher magnification of the P2X4-like immunoreactivity of the PVZ (b2). Note the stained tubular structures starting from the lateral ventricle and extending towards the OB (see arrows). Abbreviations: MOB main olfactory bulb, AOB accessory olfactory bulb, PVZ periven- tricular zone, lv lateral ventricle, mcl mitral cell layer, gcl granule cell layer Ultrastructural analysis of the PVZ and quantification of the proliferation rate of its neuronal stem cells Cells of the PVZ of the lateral ventricle have been shown to actively proliferate in larval X. laevis [4], and recent studies have identified these cells as true neuronal stem cells (reviewed in [3]). Nevertheless, the data describing the ultrastructure of the PVZ of the anterior telencephalon in larval X. laevis and amphibians in general is rather limited. In the present study, we set out to describe the architecture of this brain area and to get knowledge about the normal proliferation rate of its neuronal stem-/progenitor cells. The different morphology of the PVZ cells and the cells of the adjacent OB allowed to determine the borders between these two brain areas (see Fig. 1). This was very important for the further experiments of the present work. The PVZ of the anterior and medial part of the lateral ventricles consists of four to five cell layers with no apparent ependymal epithelium. This general appearance resembles that of the embryonic ventricular zone of higher vertebrates [45, 46]. In contrast, the PVZ situated laterally to the ventricle is thinner, with only one to two cell layers, resembling the typical ependymal ventricle lining present in adult higher vertebrates [45, 46]. The occurrence of mitotic cells and the presence of cells with chromatin in different states of aggregation showed that cells of the PVZ actively proli- ferate. Virtually all of the cells in close contact with the ventricular lumen are devoid of cilia or microvilli. In higher vertebrates, cells contacting the ventricular lumen have been shown to be ciliated [7, 8, 47]. Only very few cells in direct contact with the ventricles displayed a single cilium. Similar cells with a single primary cilium have been described in the adult ventricular zone of higher vertebrates. These cells have been reported to be related to radial glia, to actively proliferate and to produce new neurons and neuroblasts [7, 8]. The presence of large cytoplasmic expansions into the ventricular lumen and some multi- vesicular protrusions expanding into the ventricle suggest that cell material is releases into the lateral ventricles. Similar protrusions have been described in embryonic and adult cells lining the cerebral ventricles in several other species [48–50]. A more thorough description of the PVZ of larval X. laevis and a detailed classification of the different cell types populating this brain area was beyond the scope of this study. Future work will certainly bring more insight in the functional organization of the larval PVZ of this species. To further identify the border between the PVZ and the OB and to quantify the proliferative capacity of the PVZ cells, we adopted a BrdU incorporation assay. It is known