Multi-nucleated cells remove the main hyalinized tissue and start resorption of adjacent root surfaces.

Recent studies revealed that the initial root resorption occurred in the peripheries of the necrotic periodontal ligament (PDL) and was performed by mono-nucleated non-clast macrophage- and fibroblast-like cells (Brudvik and Rygh, 1993a, b). The aim of the present transmission electron microscopic (TEM) investigation was to study in more detail the cells involved in removal of the main hyalinized tissue and those involved in root resorption, occurring on the root surface situated beneath the main hyalinized tissue. Twelve male Wistar rats were used. The maxillary first molar was moved mesially by a fixed orthodontic appliance for 7 and 10 days. The results indicate that multi-nucleated giant cells (MNGC) without a ruffled border surface, as well as mono-nucleated macrophage-like cells were responsible for removal of the necrotic tissue and also for resorption of the surface parts of the root cementum. Although the present MNGC showed many morphological traits similar to the observed odontoclasts and osteoclasts, except for their lack of ruffled borders, it is assumed that they are derived from the mono-nucleated phagocytic system. Multi-nucleated clast-like cells with ruffled border were never observed near the remnants of the necrotic tissue. Such cells were found only in the resorption lacunae on root and bone surfaces.