Cell Dynamics in Tooth Root Formation: Hertwig's Epithelial Root Sheath and Cellular Cementogenesis

Abstract

Tooth root formation is initiated by the elongation of Hertwig's epithelial root sheath (HERS). HERS cells are widely accepted to control root formation, but the mechanisms regulating HERS growth have not been elucidated. To solve this problem, we have established a new culture method for postnatal mouse mandibular molars and, using it, examined the role of insulin-like growth factor-I (IGF-I) during tooth root formation. Our immunohistochemical study revealed the specific localization of the IGF-I receptor in HERS in the tooth root. In control cultures of molars prepared from postnatal 5- and 20-day-old mice, normal development of both HERS and periodontium proceeded as seen in vivo. However, the presence of 100 ng/ml IGF-I in the culture medium resulted in further elongation of HERS and increased cell proliferation in its outer layer of 5-day-old mice molars, and in promotion of cellular cementogenesis at the root surface of 20-day-old mouse molars. Thus, our results show that IGF-I played different roles depending on the stage of root formation , and 100 ng/ml of recombinant human IGF-I (Chemicon International, Temecula, CA, USA) was added to the culture medium for experiments. The specimens for light or electron microscopy were fixed and decalcified by conventional methods prior to embedment in paraffin or Epon mixture for preparing sections. To determine the length of HERS and formation