Cellular organization in the subodontoblastic zone of the dental pulp. II. Period and mode of development of the cell-rich layer in rat molar pulps

Peripherin- and CGRP-immunoreactive nerve fibers in rat molars have different locations and developmental timing

Developing rat molars gain mature sensitivity to electric stimulation at 4-5 weeks after eruption, but the related mechanisms are incompletely understood. Preliminary studies showed weak co-localization of calcitonin gene-related peptide (CGRP) immunoreactivity (IR) with peripherin (PER) or neurofilament protein (NF) in rat molar nerve fibers, while the latter two co-localized extensively.

OBJECTIVE

Our goal was to compare timing and location of PER-IR and CGRP-IR innervation in rat first molars during tooth maturation.

METHODS

We used single and double immunocytochemistry to study molars of rats aged 10 days to 1 year. Neural patterns were compared with odontoblast maturation stages, dentinogenesis, formation of cell-free and cell-rich zones, and root closure.

RESULTS

Spatial and temporal patterns showed that most CGRP-IR and PER-IR have different terminal domains in teeth. PER-IR fibers were well established among immature odontoblasts prior to tooth eruption, but CGRP-IR fibers were absent. Two weeks after eruption of first molars, many CGRP-IR beaded fibers entered dentin, the larger PER-IR fibers began shifting away from odontoblasts towards the pulp, and the symmetrical PER-IR pulpal pattern was being established. The CGRP-IR fibers continued to increase their asymmetric dentinal innervation until root growth was completed, during which time odontoblasts matured, the cell-free and cell-rich zones appeared, and roots closed.

CONCLUSIONS

Sensory maturation of rat molars coincides with closed root apices, extensive innervation of dentin by CGRP-IR nerve fibers, and the appearance of the mature avascular odontoblast layer next to cell-free and cell-rich zones in the pulp horns.

A comparative histologic study of the pulp-dentinal interface in undemineralized and demineralized tooth sections

This study examines cellular differences between undemineralized and demineralized tooth sections. The material comprised unerupted and partly erupted third molars. After fixation for 24 h, 10- to 15-microns ground sections were prepared from half of the tooth, using a cutting-grinding system. The sections were compared with demineralized routine sections from the other half of the tooth. Undemineralized sections showed a better capacity for preserving the tissue integration. Especially between the odontoblast layer and the predentin a low frequency of disjunctions or tears were noted. At this level of examination fixation quality and vacuole frequency (intra-cellular spaces) did not show any differences between the two methods.

A method for light microscopy examination of cellular and structural interrelations in undemineralized tooth specimens

This study examines ameloblasts and odontoblasts in relation to mineralizing enamel and dentin in thin undemineralized tooth sections from unerupted and partly erupted human third molars. The material comprised 12 freshly removed third molars. After fixation for 24 h the specimens were dehydrated and embedded in hydroxyethylmethacrylate. Undemineralized sections 10-15 microns thick were prepared with a cutting-grinding system, and radiomicrographs were taken. Postsecretory ameloblasts were seen in different morphologic stages ranging from ruffle- and smooth-ended ameloblasts to cuboidal and squamous cells adhering to incomplete and fully mineralized enamel. The former occurred in the most coronal tooth part, whereas the latter were seen cervically. It was not possible to identify systematic modulation cycles as seen in postsecretory rat incisor ameloblasts. The well-defined subodontoblastic region with cell-free and cell-rich layers was established before tooth eruption. The function of the subodontoblastic region is not fully understood, but our findings suggest that the establishment of the region plays a role in maintaining the specific odontoblast phenotype in the coronal region possibly linked to progressive dentin mineralization.