As reported herein, the testes from rats in both the low-dose and high-dose adjudin-treated groups displayed a phenotype in which virtually all the seminiferous tubules were devoid of germ cells, yet the number of Utf1-positive cell population, representing As, Apr and short chain of Aal spermatogonia (van Bragt et al. 2008), was maintained in both treatment groups at a level similar to that of normal adult rats, and was not reduced. However, the number of Plzf-positive cells in rats from the high-dose group was significantly lower than that of normal rats (reducing from ∼2.8 Plzf-positive cells/cross-section of tubule in normal rats to ∼1.2 Plzf-positive cells/cross-section of tubule in the high-dose group) by the end of the experimental period at 24/30-week post-adjudin treatment. Therefore, we speculate that the type of spermatogonia which had their number reduced was more advanced Aal spermatogonia. This speculation is reached since Plzf is expressed from As to long chain of Aal spermatogonia (Suzuki et al., 2009) which include more advanced spermatogonia, whereas the Utf1-positive cells which represent less differentiated spermatogonia (van Bragt et al. 2008) was not reduced in both treatment groups versus controls. Thus, the reduced population of Plzf-positive cells in the tubules from high-dose-treated group vs. control and the low-dose groups could be the result of spermatogonia (note: spermatogonia were not depleted in rats from the high-dose group as the population of Utf1-positive cells in the tubules was maintained) that failed to proceed beyond short chain of Aal. This thus hinders re-initiation of spermatogenesis, and we speculate that this is due to the unfavourable microenvironment in the basal compartment, perhaps at the stem-cell niche, for spermatogenesis because of the disruption of the BTB. For instance, ‘unwanted’ but yet-to-be identified signals originated in the apical compartment could no longer be ‘prevented’ from reaching the stem cell niche because the BTB had been disrupted. This argument was supported by the observations that the number of Plzf-positive cells in low-dose-treated group was also significantly reduced by 6/8-week and 12-weeks post-adjudin treatment, but their number rebounded when BTB was ‘resealed’ by 20 weeks and thereafter. Besides, this postulate is supported by earlier studies reporting that after exposure of rodents to various toxicants or irradiation to induce infertility, type A spermatogonia were still present in tubules of animals with azoospermia. These include rats treated with 2, 5-hexanedione (Boekelheide & Hall, 1991), irradiation (Kangasniemi et al., 1996), dibromochloropropane (Meistrich et al., 2003) and procarbazine (Meistrich et al., 1999). In the above studies, it was shown that although the number of spermatogonia was reduced, some of them survived the toxicant exposure and maintained a constant number as manifested by active proliferation of spermatogonia based on mitotic index and proliferation assay (Allrad & Boekelheide, 1996; Shuttlesworth et al., 2000). However, the proliferating spermatogonia rapidly underwent apoptosis and thus failed to differentiate beyond type A spermatogonia (Allrad & Boekelheide, 1996; Shuttlesworth et al., 2000). Unfortunately, the integrity of the BTB was not investigated in any of these earlier studies. Collectively, the findings reported herein and those reported earlier thus support the notion that the SSCs/spermatogonia in the testes following toxicant treatment [note: adjudin is a ‘toxicant’ in the sense that it affects germ-cell adhesion and spermatogenesis even though it did not cause any mortalities at up to 40 times of the effective dose (50 mg/kg b.w., by gavage) to induce infertility (Mruk et al., 2006)] can proliferate and differentiate to re-initiate spermatogenesis in an optimal microenvironment in the epithelium with an intact BTB.