|Year : 2010 | Volume
| Issue : 3 | Page : 167-172
Dentin comparison in primary and permanent molars under transmitted and polarised light microscopy: An in vitro study
N Chowdhary1, VV Subba Reddy2
1 Department of Pedodontics and Preventive Dentistry, Clinical Associate Professor, HKE's S. Nijalingappa Institute of Dental Sciences and Research, Gulbarga - 585 104, India
2 Principal, Head of the Department, College of Dental Sciences, Davengere - 577 004, India
|Date of Web Publication||11-Dec-2010|
Department of Pedodontics and Preventive Dentistry, Clinical Associate Professor, HKE's S. Nijalingappa Institute of Dental Sciences and Research, Gulbarga - 585 103
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Dentin is the fundamental substrate of restorative dentistry and its properties and characteristics are key determinants of nearly all restorative, preventive and disease processes of the teeth. The intrinsic permeability of dentin is responsible for permitting bacterial or chemical substances to diffuse across the dentin and irritate the pulpal and periradicular tissues. Improved understanding of the dentin structure and nature will have important consequences for today's dental procedures. The aims of the study were to observe the direction of dentinal tubules, interglobular dentin, incremental lines of dentin and the dead tracts. Materials and Methods: A total of 30 teeth (15 primary and 15 permanent molars), unrestored, noncarious, hypoplastic extracted molars were used. Longitudinal ground sections of teeth were obtained using hard tissue microtome. Results: Examination of ground sections of the primary teeth dentin showed "s"-shaped curvature in four (26.7%) specimens and a straight course of dentinal tubules in 11 (73.3%) specimens out of 15 teeth examined whereas in permanent teeth, all 15 (100%) specimens showed an "s"-shaped curvature. These results are statistically highly significant (P < 0.001). Conclusion: Dentinal tubules followed an "s"-shaped course in all the 15 (100%) permanent molars and in four (26.7%) primary molars. There was no significant difference in the occurrence of interglobular dentin of primary and permanent molars. But, they were at angles in the primary teeth.
Keywords: Dentinal tubules, incremental lines, interglobular dentin
|How to cite this article:|
Chowdhary N, Subba Reddy V V. Dentin comparison in primary and permanent molars under transmitted and polarised light microscopy: An in vitro study. J Indian Soc Pedod Prev Dent 2010;28:167-72
|How to cite this URL:|
Chowdhary N, Subba Reddy V V. Dentin comparison in primary and permanent molars under transmitted and polarised light microscopy: An in vitro study. J Indian Soc Pedod Prev Dent [serial online] 2010 [cited 2023 Feb 6];28:167-72. Available from: http://www.jisppd.com/text.asp?2010/28/3/167/73793
| Introduction|| |
Dentin is the fundamental substrate of restorative dentistry and its properties and characteristics are key determinants of nearly all restorative, preventive and disease processes of the teeth.  It is tough and resilient in nature, which is important for the proper functioning of the tooth. It provides flexibility and prevents fracture of the overlying enamel, which is very brittle and is the hardest tissue of the body. 
It is a hard tissue, ectomesenchymal in origin, and forms a part of the pulp dentine complex.  It is considered to be softer than enamel but harder than bone  and is characterized by the presence of multiple, closely packed dentinal tubules that traverse its entire thickness and contain cytoplasmic extensions of odontoblasts. The intrinsic permeability of dentin is responsible for permitting bacterial or chemical substances to diffuse across the dentin and irritate the pulpal and periradicular tissues. If the dentin was not permeable then the pulp would be spared a good deal of irritation.
Improved understanding of the dentin structure and nature will have important consequences for today's dental procedures. This will also lead to the development of new methods to preserve and protect the teeth and repair defects brought on by disease or trauma.
The aims of the study were to observe the
- direction of dentinal tubules
- interglobular dentin
- incremental lines of dentin
- dead tracts
| Materials and Methods|| |
A total of 30 teeth (15 primary and 15 permanent molars), unrestored, noncarious, hypoplastic extracted molars were mechanically cleaned using pumice and water slurry and stored in distilled water till further use for the study.
The roots of the selected teeth were removed 2 mm below the cementoenamel junction and the coronal portion of the tooth was mounted in a self-cure acrylic for longitudinal sectioning in such a way that two-thirds of the coronal portion was exposed for the sectioning.
Preparation of the ground section
A longitudinal ground section of the teeth was obtained using hard tissue microtome. During sectioning, the diamond edge of the blade was cooled with chilled distilled water, which was applied to both sides of the blade through a slotted tube, and the same procedure was repeated for all the teeth.
Mounting of the sections
The section of 75 μm thickness was mounted on a microscopic glass slide using mounting media, Dystrene Polystren Xylene, and covered with a cover slip. These sections were observed under transmitted and polarized light microscopes for the following histological features:
- direction of dentinal tubules
- intergluobular dentin
- incremental lines in dentin
- dead tracts
| Results|| |
Under a transmitted light microscope and polarized light microscope, the following histological features were observed:
Examination of ground sections of primary teeth dentin showed an "s"-shaped curvature in four (26.7%) specimens and a straight course of dentinal tubules in 11 (73.3%) specimens out of 15 teeth examined whereas, in permanent teeth, all 15 (100%) specimens showed an "s"-shaped curvature. These results are statistically highly significant (P < 0.001) [Table 1].
Examination of ground sections of primary teeth dentin showed the presence of interglobular dentin in three (20%) specimens and absence in 12 (80%) specimens out of 15 teeth examined, whereas in permanent teeth dentin, five (33.3%) specimens showed the presence of interglobular dentin and 10 (66.7%) specimens showed the absence of interglobular dentin out of the 15 teeth examined. These results are not statistically significant (P = 0.23) [Table 2].
Examination of ground sections of primary teeth dentin showed incremental lines, which are at an angle to dentinal tubules in 10 (66.7%) specimens and incremental lines, which are at right angles to dentinal tubules in five (33.3%) specimens out of 15 teeth examined, whereas in permanent tooth dentin, all the 15 (100%) specimens showed incremental lines that are at right angles to the dentinal tubules. These results were found to be statistically highly significant (P < 0.001) [Table 3].
Examination of ground sections of primary teeth dentin showed the presence of dead tracts in five (33.3%) specimens and absence of dead tracts in 10 (66.7%) specimens out of 15 teeth examined, whereas in permanent teeth dentin, seven (46.7%) specimens showed the presence of dead tracts and absence of dead tracts in eight (53%) specimens out of 15 permanent teeth examined. The results were found to be statistically not significant (P = 0.22) [Table 4].
| Discussion|| |
Dentin forms the bulk of the tooth, providing the tooth with the shape and rigidity necessary for functioning effectively during mastication. Along the crown, the dentin is covered by enamel and along the root, by cementum. It encloses the dental pulp with which it shares a common origin from the dental papilla. Indeed, the dentin and pulp can be considered as a single developmental and functional unit, often described as the pulpo-dentinal complex. 
Dentin in permanent and primary teeth has similar morphology and composition and it has been assumed that dentin of both kinds of teeth are similar in histological structure. The findings of teeth have been assumed to apply to primary teeth, but some evidence ,, suggests significant differences are present between them. Thus, an attempt to compare the histological features of dentin.
Polarized light microscope
Natural light vibrates in many planes and has different wavelengths, whereas polarized light vibrates in only one plane and has a single wavelength. Some structures like crystals and collagen fibers are capable of splitting a polarized light into two, this property being called birefringence. This property is observed in structures like enamel and dentin because of their inherent crystalline nature. Thus, some of the histological features of dentin can be better visualized under polarized microscopy, which may not be seen with transmitted light microscopy. 
Direction of dentinal tubules
Dentin is permeated by the dentinal tubule, which runs from the pulp surface toward the dentinoenamel junction. The tubules follow a curve sigmoid course. Their configuration indicates the course taken by odontoblasts during dentinogenesis. 
On examination of longitudinal ground sections of permanent teeth dentin and primary teeth dentin for the direction of dentinal tubules, there was a highly significant difference (P < 0.001). The permanent teeth dentin showed dentinal tubules following an "s"-shaped curve in all 15 (100%) [Figure 1], whereas in primary teeth dentin only four (26.7%) specimens showed an "s"-shaped curve and 11 (73.3%) specimens exhibited straight course dentinal tubules [Figure 2].
This implies that there is a definite difference in the direction of dentinal tubules in primary tooth dentin when compared with permanent tooth dentin. The "s"-shaped curvature of dentinal tubules indicates the course taken by the odontoblasts during dentinogenesis. This "s"-shaped curvature results from crowding of the odontoblasts as they move from the periphery toward the center of the pulp. , However, in primary tooth dentin, most of the dentinal tubules in our study did not show the "s"-shaped curve. The probable reason could be the lower difference between the surface area of dentin near the amelodentinal junction and that near the pulp. This is evinced by the fact that primary tooth have wider pulps than permanent teeth.  This factor could result in less crowding of the odontoblasts and, hence, straight course of the dentinal tubules in the coronal dentin. This structural appearance of the dentinal tubule in the primary tooth dentin may be one of the factors contributing to the faster progression of caries. 
Interglobular dentin is the term used to describe areas of unmineralized or hypomineralized dentin where globular zones of mineralization (calcospherites) have failed to fuse into a homogenous mass within the mature dentin.  Intergobular dentin is most frequently seen in the circumpulpal dentin because this irregularity of the dentin is a defect of mineralization and not of matrix formation, the normal architectural pattern of the tubules remains unchanged and they run uninterruptedly through the interglobular areas. ,
The irregular interglobular spaces appear dark in ground sections viewed by transmitted light.  These areas are especially prevalent in human teeth in which there has been a deficiency in Vitamin D, dentinal fluorosis and dentinogenesis imperfect. 
Examination of the longitudinal ground sections of primary teeth dentin showed the presence of interglobular dentin in three (20%) specimens [Figure 3] and absence of interglobular dentin in 12 (80%) specimens, whereas in permanent teeth dentin, interglobular dentin was present in five (33.3%) specimens [Figure 4] and absent in 10 (66.7%) specimens. These results were not statistically significant where P=0.23 [Table 2].
This lack of difference in occurrence could be explained by the fact that interglobular dentin is a hypomineralized area and dependent only on factors that are responsible for mineralization. Hence, there is no particular predilection for occurrence of interglobular dentin in either of the two dentitions.
Incremental lines in dentin
Dentinogenisis proceeds rhythmically, with alternating phases of activity and quiescence. These phases are represented in formed dentin as incremental lines and can be best seen in longitudinal ground sections of the teeth. These lines are also called as incremental growth lines. The incremental lines run roughly at right angles to the dentinal tubules and generally mark the normal linear pattern of dentin deposition.  However, controversy exists concerning the difference between the descriptions and explanations of these structures by different authorities. 
Lines related to disturbances in dentinogenesis or the rhythmic deposition of dentin is called Von Ebner lines. These Von Ebner lines run at roughly right angles to the dentinal tubules. Dentin matrix is laid down at a rate of 4 μm per day. More severe changes in orientation occur approximately every 5 days, accounting for the presence of the Von Ebner lines. 
Examination of ground sections of primary teeth dentin showed incremental lines at an angle to the dentinal tubules in 10 (66.7%) specimens [Figure 5] and, in five (33.3%) specimens, incremental lines were at right angles to the dentinal tubules, whereas in permanent teeth dentin incremental lines were roughly at a right angle to the dentinal tubules in all the 15 (100%) specimens [Figure 6]. This shows that there is a difference in the incremental pattern of primary tooth dentine to that of permanent tooth dentin. The incremental lines take up the shape of the mineralizing front at different stages of dentinogenesis. If the mineralizing front to odontoblastic process forms at an angle to the odontoblastic process, then the incremental lines will be at an angle.  It is a matter of anyones conjecture why mineralizing front to dentinal tubules of odontoblastic process are at an angle in some deciduous teeth. Further research may help us determine an answer to this question.
|Figure 5: Primary tooth showing incremental lines at an angle to the dentinal tubules|
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|Figure 6: Permanent tooth showing incremental lines at right angles to the dentinal tubules|
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Dead tracts are an area of dentin containing nonfunctional dentinal tubules. Severe irritation of the dentinal tubules will cause loss of contents with failure to become mineralized. Fish et al. stated that dead tracts appear to form as a result of severe injury to the terminal odontoblastic process.  Stainley et al. found that they occur under an area of attrition, erosive caries and also noted in teeth without clinical or microscopic evidence of disease or injury. 
A true dead tract may be recognized by area of sclerotic dentin bordering the tract laterally and pulpally. In disease-free teeth, dead tracts are possible due to atrophic degeneration of the odontoblast process due to excessive crowding during development. 
Because dead tracts occur as a result of death of adontoblasts due to any cause like crowding of odontoblast, attrition, erosion and abrasion, it is hardly surprising that there is no preferential occurrence of dead tract in either of two dentitions [Figure 7] and [Figure 8].
| Conclusion|| |
The following observations were noted:
- Dentinal tubules followed an "s"-shaped course in all the 15 (100%) permanent molars and in four (26.7%) primary molars.
- Dentinal tubules followed a straight course in 11 (73.3%) primary molars and none of the permanent molars showed a straight course tubules. This structural appearance of dentinal tubules in primary tooth dentin may be one of the factors contributing to the faster progression of caries.
- There was no significant difference in the occurrence of interglobular dentin of primary and permanent molars. But, they were at an angle in primary teeth.
- There was no significant difference in the occurrence of dead tracts of primary and permanent molars.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
[Table 1], [Table 2], [Table 3], [Table 4]
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