Epithelial cytodifferentiation and extracellular matrix formation in enamel‐free areas of the occlusal cusp during development of mouse molars: Light and electron microscopic studies

Abstract

Mandibular first molars in mice ranging in age from 18 days prenatal to 5 days postnatal were used for light and electron microscopic examinations of the enamel-free area (EFA) during development of the occlusal cusp (mesiobuccal cusp). Notable morphological changes in the inner enamel epithelium and the cells of the stratum intermedium were observed. At prenatal age of 18 days, the inner enamel epithelium of the EFA (EFA epithelium) was composed of a layer of columnar cells and covered by the cells of the stratum intermedium. Two days after birth, the EFA epithelium was made up largely of preameloblasts, with mitochondria located in the proximal side of the cells toward the stratum intermedium. The cells of the stratum intermedium were irregularly shaped, with wide intercellular spaces between them. At a postnatal age of 3 days, most of the EFA epithelial cells resembled maturation-stage ameloblasts, being short and columnar in shape and having nuclei located in their proximal side. Distal cell membranes were folded, and mitochondria were scattered throughout the cytoplasm. In 4-day-old mice, the EFA epithelium was found to be formed of short columnar or cuboidal cells with distinct intercellular spaces. The cells of the stratum intermedium could no longer be detected, and cells of the EFA epithelium could not be distinguished from those of the stellate reticulum. Odontoblasts of the EFA were arranged and polarized parallel to the basal lamina, and odontoblastic processes extended toward the cusp tip. The orientation of thin and thick collagen fibers within predentin and dentin was also parallel to the basal lamina. Even after dentin mineralization, disrupted basal lamina and long, aperiodic, fine fibrils were found between the epithelium and the dentin. Following the disappearance of the basal lamina and fine fibrils, stippled material and crystals appeared on the dentin surface. The mineralized matrix, which x-ray microanalytical energy peaks identified as containing calcium and phosphorus, was continuous with enamel in the distal slope of the cusp at the cusp tip. Thus, the inner enamel epithelium of the EFA differentiated into secretory cells capable of enamel-like matrix formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical and biochemical detection of EMMPRIN in the rat tooth germ: differentiation-dependent co-expression with MMPs and co-localization with caveolin-1 in membrane rafts of dental epithelial cells

“…An important point concernes the EMMPRIN immunoreactivity detected at contact sites between the ameloblastic cell layer and the stratum intermedium cells. The evidence that preameloblasts cannot diVerentiate into secretory ameloblasts without stratum intermedium cells suggests that cell-cell-interactions play an essential role in the diVerentiation process of ameloblasts (Sakakura et al 1989). Several transmembrane proteins such as the gap junction protein connexin 43 and the cell adhesion receptor CD44 take part in the cross-talk between stratum intermedium cells and ameloblasts (Nakamura and Ozawa 1997;Joao and Arana-Chavez 2003).…”

Immunolocalization of CD44 and the Ezrin–Radixin–Moesin (ERM) Family in the Stratum Intermedium and Papillary Layer of the Mouse Enamel Organ

“…Furthermore, the co-localization of CD44, ezrin, and radixin was seen in the contact sites between preameloblasts and the stratum intermedium. The evidence that pre-ameloblasts in enamel-free areas of mouse molars cannot differentiate into secretory ameloblasts without the support of the stratum intermedium (Sakakura et al 1989) suggests that cell-cell communication and/or interaction is essential for differentiation of ameloblasts. Adhesion molecules and their signal transductions via cytoskeletons are believed to be important for the differentiation and the polarity of cells.…”

Crown morphology, enamel distribution, and enamel structure in mouse molars

“…Addison and Appleton (1921), Mellanby (1939), Gaunt (1956), and Cohn (1957) concluded that the cells of the inner enamel epithelium adjacent to enamel-free regions did not differentiate into secretory ameloblasts and consequently failed to produce enamel. However, more recent investigations have demonstrated that the epithelial cells over the enamel-free areas elaborate a thin, noncontinuous layer of organic matrix which contains amelogenin (Nakamura et al, 1991;Inai et al, 1992), which becomes mineralized by deposition of fine crystals (Sakakura et al, 1989) and which on demineralization leaves an organic residue devoid of collagen (Sakakura et al, 1989). Contrary to Sutcliffe and Owens (1980), who with the SEM were able to observe what seemed to be remnants of such an enamel layer on the EFA of rat molars, we could not find it on molars of 14-day-old mice.…”