Ultrastructural features of odontoclasts that resorb enamel in human deciduous teeth prior to shedding

Abstract

Three dental hard tissues, i.e., cementum, dentin, and enamel, are resorbed by multinucleated cells referred to as "odontoclasts." These cells have morphological and functional characteristics similar to those of bone-resorbing osteoclasts. However, concerning enamel resorption, which is a process that may occur during tooth eruption, satisfactory ultrastructural data on odontoclastic resorption are still lacking. Ultrastructural and histochemical characteristics of odontoclasts resorbing enamel of human deciduous teeth prior to shedding were examined by means of light microscopy and transmission and scanning electron microscopy. Odontoclasts that that resorbed enamel were tartrate-resistant acid phosphatase (TRAP)-positive multinucleated giant cells that were essentially the same as those that resorbed dentin and cementum. Ultrastructurally, they had numerous mitochondria, lysosomes, and free polysomes in their cytoplasm. In addition, they were characteristically rich in large cytoplasmic vacuoles containing enamel crystals in the cytoplasm opposite the ruffled border. Although they extended a well-developed, ruffled border against enamel surface, a clear zone--an area typically devoid of organelles--was rarely seen in these cells. In many cases, the cells were in very close contact with the enamel surface by the peripheral part of their cytoplasm. The enamel prisms at the resorption surface contained more loosely packed and electron-lucent enamel crystals compared with those of unresorbed, intact enamel. Furthermore, numerous thin needle- or plate-like enamel crystals that were liberated from the enamel matrix were found in the extracellular channels of the ruffled border and in various-sized cytoplasmic vacuoles in their cytoplasm. The superficial layer of the enamel matrix undergoing odontoclastic resorption stained positively with toluidine blue and for TRAP activity. The results of the present study suggest that odontoclasts resorbing enamel secrete acids as well as organic components, including hydrolytic enzymes, into the resorption zone underlying their ruffled border and that they phagocytose crystals that have been liberated from the partially demineralized enamel matrix by acids, subsequently dissolving them intracellularly.

Case report of an internal granuloma investigated by light and scanning electron microscopy

“…Concerning ultrastructural features of odontoclasts resorbing enamel, it could be demonstrated that these tartrate-resistant acid phosphatase-positive multinucleated cells were quite similar to the cells resorbing dentine or cementum [13]. Rich in mitochondria, lysosomes and free polysomes, these cells secrete acids, organic components and hydrolytic enzymes performing phagocytosis of enamel crystals then to be found in large cytoplasmic vacuoles to be dissolved intra-cellularly.…”

Internal inflammatory root resorption: the unknown resorption of the tooth

“…Domon et al (22) showed that in deciduous teeth undergoing resorption, the mean number of nuclei per odontoclast was 5.3, and only 2.9% of the resorbing cells were mononuclear. Comparative studies on cell ultrastructure have shown that odontoclasts resorbing dentin or cementum are similar to those resorbing enamel (23). Close similarity to bone osteoclasts was also documented.…”

“…Sahara et al (23) studied the resorption by odontoclasts of a superficial non-mineralized layer of predentin prior to the shedding of human deciduous teeth by light and electron microscopy. They found multinucleate cells on the predentin surface of the coronal dentine between the degenerated odontoblasts and resorption lacunae in the non-mineralized predentin.…”

Storage media enhance osteoclastogenic potential of human periodontal ligament cells via RANKL‐independent signaling

“…After replantation of avulsed tooth, the most common complication is root resorption (1), as the result of the action of odontoclasts (9, 10). The morphological and functional characteristics of odontoclasts are similar to those of bone‐resorbing osteoclasts, with both odontoclasts and osteoclasts being differentiated from monocyte and macrophage lineage cells (11, 12). Increasing amount of evidence suggests that PDL cells closely resemble osteoblasts that can induce osteoclast‐like cell formation if co‐cultured with osteoclastic precursors, such as peripheral blood mononuclear cells (PBMC) (13, 14), mouse bone marrow cells (15, 16), and monocyte/macrophage lineage (RAW 264.7 cells) (17).…”