## 4 Discussion
This finding is in agreement with previous studies that showed the formation of mineralized tissue after the use of MTA [18; 20] or iloprost [7; 34] to cap mechanically exposed dental pulp. Interestingly, in the iloprost group, the reparative dentin was mostly found on the lateral walls of the exposure site at week one. This could be explained by the liquid nature of iloprost, making it almost impossible to control its deposition site. Thus, MTA was mixed with iloprost to control its delivery. This study revealed that pulp capping with either MTA or MTA-iloprost showed no significant difference in inflammatory pulpal responses and tertiary dentin formation. This could be due to the similarities in their composition, where the only difference is the presence of iloprost in the MTA-iloprost group. However, the formation of a continuous bridge was observed earlier in the MTA-iloprost group. Mineralization and angiogenesis are closely related. Iloprost is able to upregulate angiogenesis and increase VEGF expression and pulpal blood flow [4], which enhances the recruitment and differentiation of osteo/odontoprogenitor cells [4]. Additionally, VEGF was reported to induce the mineralization of bone marrow-derived mesenchymal stem cells and human dental pulp stem cells [35]. The results of the current study demonstrated that MTA mixed with iloprost had a higher angiogenic potential than MTA, as evidenced by dilated and engorged blood vessels in the histologic sections. Further studies are required to evaluate the angiogenic properties of MTA when mixed with iloprost. Another factor that was evaluated in this study was the quality of the newly formed calcified bridge. The majority of specimens in the MTA-iloprost group had a regular pattern of tubules, while the MTA group showed an irregular pattern of tubules. A possible explanation for this observation is that tertiary dentin formation often starts as irregular and/or atubular dentin, and then the formation of a tubular dentin-like matrix takes place later by the more palisaded odontoblast-like cells [36]. This confirms that the formation of dentin bridges in the MTA-iloprost group starts earlier than in the MTA group, and it achieves more regular tubularization. In our previous in vitro experiment, it was found that MTA mixed with iloprost upregulated the expression of differentiation markers, including bone sialoprotein (BSP) and osteepontin (OSP), compared with MTA ([PERSON] et al., 2021, unpublished data). The ability of BSP to induce the formation of homogenous reparative mineralized tissue when implanted directly on mechanically exposed pulp has been reported in a previous study [37], thus suggesting its role during reparative dentinogenesis. OSP is another hard tissue-related protein that is believed to have some role in the initiation of the reparative process by its deposition in the superficial layer in the exposed pulpal matrix, followed by the attraction and differentiation of odontblast-like cells [38]. Further research is required to study the mechanism of the signaling processes when MTA is mixed--with iloprost. Nonetheless, pulp capping therapy is performed when pulp exposure occurs due to caries, trauma, or atrogenic insult. The main limitation of the present study was that capping was performed immediately after exposure of normal pulp tissue; thus, it does not represent all clinical scenarios. Further in vivo studies on inflamed pulps are necessary to investigate the potential application of the combination of MTA and iloprost as pulp capping material.