## 1 Introduction  
The clinical treatment of large-size bone defects requires an interdisciplinary approach of bone regeneration techniques to restore the anatomical, physical, and functional conditions after acquired trauma, infection, neoplasm, or surgical resection [1]. The procedure by autologous bone grafts represents the gold standard way for bone regeneration due to their self-capabilities of osteoconduction, osteoinduction, and osteogenesis [2, 3]. The osteeconduction process is a physiological mechanism based on the new bone formation on a biomaterials and scaffolds surface. This property is associated to the substrain of the bone healing process by a space-maintaining capacity in fractures or damaged hard tissue defects [4]. These components represent key factors able to enhance the integration of the graft, to promote the remodeling of the bone substitute and to induce the new bone formation [5]. The osseointegration of the bone graft is characterized by necessary components such as osteogenic progenitor cell differentiation, secretion of osteeconductive matrices, and release of osteoinductive growth factors conveyed by organized blood clot [5; 6]. On the contrary, the autologous bone graft approach is related to considerable increase in surgical procedures and a higher postoperative patient morbidity, in favor of new-generation biomaterials with heterologous and synthetic origin and biomaterials' bioactive coating [7]. Human dental pulp stem cells (hDPSCs) represent an easy-access reliable source of multipotent mesenchymal stem cells (MSCs). The hDPSCs are able to differentiate into osteoblasts and can be seeded on a bioscaffold and successfully applied for tissue engineering of bone defects [8; 9; 10]. The treatment of severe defects of the jaws requires bone regeneration while the augmentation procedures with bone substitutes and barrier membranes are indicated for localized ridge atrophy [11; 12]. The gold standard of graft materials for bone regeneration is autologous bone, due to osteogenic, osteoinductive, and osteeconductive properties. The application of natural substitutes such as allografts and xenografts seems to be an alternative [13; 14]. The using of xenografts is common and may have a \"similar\" structure of the component being replaced. Equine bone has been proposed as a bone substitute alone or in combination with other materials [15]. Bone blocks are proposed for large bone defect treatment due to their biocompatibility and osteoconductivity properties. Plenty of techniques have demonstrated that equine bone derived materials have osteogenic effects and promote bone regeneration acting as little resorbable scaffolds [16; 17; 18; 19]. Our previous results showed that equine bone blocks enhanced hDPSCs differentiation [20]. Melatonin (N-acetyl-5-methoxyryptamine) is a natural hormone able to improve bone health because it targets the overall remodeling process through its dual actions on osteoblasts and osteoclasts and is an important mediator in bone formation [21; 22; 23; 24]. Moreover, the association of melatonin combined with xenografts induced in dog jaws models the promotion of osteointegration with higher bone-to-implant contact compared to the scaffold alone [25]. There are various phases to the bone remodeling process that include the activation of osteoclast precursors to initiate bone resorption, followed by the activation of osteoblasts to form and mineralize new bone matrix [26]. In our previous study, we demonstrated that equine bone block enhanced hDPSC differentiation and the effect was further enhanced by the simultaneous presence of an equine bone block and 100 uM of melatonin that stimulated early stages of cell differentiation. This study led us to conclude that hDPSCs/equine bone blocks and melatonin seemed to be an effective biocompatible system useful for bone tissue engineering due to the osteogenic results obtained [20].