Hertwig’s epithelial root sheath cell behavior during initial acellular cementogenesis in rat molars

Abstract

This study was designed to examine developing acellular cementum in rat molars by immunohistochemistry, to elucidate (1) how Hertwig's epithelial root sheath disintegrates and (2) whether epithelial sheath cells transform into cementoblasts through epithelial-mesenchymal transition (EMT).Initial acellular cementogenesis was divided into three developmental stages, which can be seen in three different portions of the root: portion 1, where the epithelial sheath is intact; portion 2, where the epithelial sheath becomes fragmented; and portion 3, where acellular cementogenesis begins. Antibodies against three kinds of matrix proteinases, which degrade epithelial sheath-maintaining factors, including basement membrane and desmosomes, were used to investigate proteolytic activity of the epithelial sheath. Tissue non-specific alkaline phosphatase (TNALP) and keratin were used to investigate EMT. Epithelial sheath cells showed immunoreactivity for all three enzymes at fragmentation, which suggests that epithelial sheath disintegration is enzymatically mediated. Dental follicle cells and cementoblasts showed intense immunoreactivity for TNALP, and from portion 1 through to 3, the reaction extended from the alveolar bone-related zone to the root-related zone. Cells possessing keratin/TNALP double immunoreactivity were virtually absent. Keratin-positive epithelial sheath cells showed negligible immunoreactivity for TNALP, and epithelial cells did not appear to migrate to the dental follicle. Together, these findings suggest that a transition phenotype between epithelial cells and cementoblasts does not exist in the developing dental follicle, and hence that epithelial sheath cells do not undergo EMT during initial acellular cementogenesis.In brief, this study supports the notion that cementoblasts derive from the dental follicle. words

Hertwig’s Epithelial Root Sheath Fate during Initial Cellular Cementogenesis in Rat Molars

“…We agreed with the classical mesenchymal hypothesis in our previous reports, which focus on acellular cementogenesis [ 28 – 30 ]. The present study was designed to examine developing cellular cementum in rat molars by immunohistochemistry (IHC) and by TdT-mediated dUTP nick end labeling (TUNEL), to elucidate the fate of Hertwig’s epithelial root sheath, in particular, whether epithelial sheath cells undergo EMT and how many epithelial sheath cells die by apoptosis during initial cellular cementogenesis.…”

Histology of human cementum: Its structure, function, and development

“…Therefore, in the in vivo studies epithelial cell rests and embedded epithelial cells were probably misidentified as cementoblasts and cementocytes, respectively. Findings supportive of the classical mesenchymal hypothesis have also accumulated in in vivo studies using mouse and rat teeth [56], [58], [59], [69], [70], [71]. In these studies any epithelial sheath-derived cells did not express mesenchymal or cementoblast characteristics, e.g.…”

“…This hypothesis will be referred to as an alternative epithelial hypothesis [45]. The alternative epithelial hypothesis has now almost surpassed the classical mesenchymal hypothesis, because the alternative epithelial hypothesis can explain why epithelial sheath cells decrease in number during epithelial sheath disintegration [56], [57], [58], [59]. EMT is a normal phenomenon in embryos in several developing organs, e.g.…”

“…The epithelial cells decrease in number during epithelial sheath disintegration when observed in histological sections [56], [57], [58], [59]. If EMT does not occur, how can the cell number reduction be explained?…”

“…Apoptosis may be another possible cause for the cell number reduction, but it has been shown that apoptosis occurs in epithelial cell rests of Malassez and epithelial cells embedded in cementum, but does not in the disintegrating epithelial sheath [59], [72], [73]. Recent studies [56], [58], [59] have suggested that the cell number reduction may be related to the growth rate and shape of the epithelial sheath (Fig. 12).…”