Objective: To evaluate histologically the newly formed tissues following regenerative endodontic procedures (REPs) in dogs using either blood clot or two different formulations of a chitosan hydrogel as scaffolds Methods: Apical periodontitis were induced by inoculating immature teeth (n=96) with oral plaque in Beagle dogs (n=4). Teeth were divided into four test groups according to the treatment protocol: I- MTA apical plug or regenerative procedures using different scaffolds; II- blood clot; III- sodium hialuronate: chitosan scaffold (HA:CS); and IV-pectin: chitosan scaffold (P:CS). All root canals were disinfected with a triple antibiotic paste protocol prior to revascularization. Eleven weeks' post-treatment, the animals were sacrificed and the jaw blocks harvested for histologic processing and morphological and histomorphometric analysis and statistical analysis (Kruskal-Wallis and Mann-Whitney U tests) were performed with a level of significance set at P 0,05. Results: The lumen of the root canal completely filled with MTA with creation of an apical bridge between root canal walls in 83% of the samples in group I. A vital vascularized tissue was found in Groups II, III and IV. For these groups, apical closure happened in 66.7% of the treatments and root growth was detected more often as an increase in thickness (85.6%) than in length (45.6%). The greatest amount of mineralized tissue inside the canal were observed in group II (blood clot), with significant differences compared to group III (P = 0.006), and group IV (P = 0.025). Radiographic evaluation post-treatment revealed that the apical region was remineralized with reestablishment of a periodontal ligament space. Histological evaluation revealed that apical mineralization was composed of of cellular cementum and dentine, surrounded by periodontal ligament, containing epithelial rests of Malassez, exhibiting a similar composition to the walls of any ordinary root apex, including the presence of foramina. Conclusions: The addition of the scaffolds to the blood in regenerative procedures did not improve the formation of new mineralized tissues along the root canal walls and the histologic evidence of the regeneration of a pulp-dentin complex. Despite of the scaffolds used, REPs allowed the continued development of root walls with presence of bone-like tissue, cementum and periodontal ligament often observed into the canal space. In spite of the theoretical possibility of a more effective regeneration with different scaffolds (group 3, 4), our results sustain that better results were found with the induction, formation and stabilization of a blood clot (group 2).REP's should be considered as a potential option for the treatment of immature permanent teeth with irreversible pulpitis or pulp necrosis. Schlagwörter: Immature permanent tooth, tissue regeneration, root canal, tissue engineering, chitosan scaffolds, apical papilla