Non-clast cells start orthodontic root resorption in the periphery of hyalinized zones.

Previous research has indicated that orthodontically-induced root resorption is regularly associated with local trauma, most often with over-compression of the periodontal ligament (PDL). Recent research has revealed that the initial root resorption occurs in the periphery of the necrotic PDL, where mononucleated cells not stained by tartrate resistant acid phosphatase (TRAP), are the first cells to penetrate the root surface. The aim of the present transmission electron microscopic (TEM) investigation was to study in more detail the root surface during the initial penetration of cells into precementum and mineralized cementum. In 21 Wistar rats (age 40-45 days) the maxillary first molar was moved mesially by a closed coil spring for 6 and 12 hours, 1, 2, 3, 4, and 5 days. The results clearly indicate a role for mononucleated non-clast cells in the initial local removal of precementum and mineralized acellular cementum in the periphery not only near, but also at some distance from the compressed PDL. 1. Macrophage-like cells phagocytosed necrotic tissue in the middle of the PDL after 6 hours and near the root surface close to the hyalinized zone after 24 hours. 2. Fibroblast-like cells seemed to break down by phagocytic and collagenolytic activity the precementum near the hyalinized zone after 24 hours. 3. The surface layers of mineralized cementum were removed by mononucleated cells which occurred on the mineralized cementum after 3 days. 4. Multinucleated cells without ruffled border occurred in the PDL at some distance from the root surface after 24 hours. Multinucleated cells with a ruffled border towards the mineralized root surface were rare during the 5 days' period.