The formation of reparative dentine and Hohl cells in the dental pulp

The odontoblasts of dental pulp tissue form primary and secondary dentine. If the pulp tissue is exposed, the odontoblast layer will be destroyed. The aim of direct pulp capping is to induce new hard tissue formation in order to keep the pulp tissue vital. If direct pulp capping is successful, tertiary dentine is formed. This hard tissue is denoted as 'reparative dentine' and defined as a dentine matrix, which is formed by a new generation of odontoblast-like cells after an appropriate stimulus. Thus, reparative dentine is not formed by the original post-mitotic odontoblasts. Until recently it has been unclear which pulp cells differentiate into these odontoblast-like cells. Besides multipotent adult stem cells, fibroblasts and un-differentiated mesenchymal cells are mentioned in the literature as progenitor cells for odontoblast-like cells, because their cell-division rate increases significantly after direct pulp capping. The increase in the cell division rate of fibroblasts can be explained by the formation of granulation tissue, rather than the differentiation into secondary odontoblasts, as often described. A review of the relevant literature demonstrated that Hhl cells play a role in the formation of new odontoblast-like cells, and may well be activated after pulp tissue damage to form reparative dentine. Thus, for direct pulp capping, only wound dressings such as calcium hydroxide or mineral trioxide aggregate (MTA) should be used, as they are able to induce the proliferation of 'progenitor cells' (e.g. Hhl cells) in order to stimulate the formation of reparative dentine.