An Improved Method for the Preparation of "Serial" Sections of Undecalcified Dental Tissues
INCE our previous report' various improvements on the existing methods bJof sectioning undecalcified dental tissues have come to our knowledge. Lefkowitz2 used a rapidly revolving large diameter phosphobronze disc, the circumference of which was charged with diamond powder. The width of the cuts produced by his apparatus is not given, but it cannot have been less than 0.150 mm., this being the thickness of the disc. As the thickness of the sections is stated to vary from 0.250 to 0.400 mm., this method can at best have yielded an average of 2 sections per mm. Sognnaes3 cuts a number of sections simultaneously with a set of circular steel saws each 0.250 mm. thick. Only one 0.750 mm. section is produced per mm. But, as in the case of Lefkowitz' apparatus, comparatively large sections can be obtained (e.g., complete longitudinal sections of human molars). His introduction of methyl methacrylate as an embedding medium constitutes an important improvement of the technic. In a very simple arrangement Atkinson4' 5 employed bonded carborundum discs from 0.8 to 0.125 mm. in thickness, but it is not stated how many sections per mm. can be obtained. In this case too, large sections can apparently be made. In 1946 we1 reported on a method producing small sections at the rate of 5 sections per 2 mm. The number of sections per mm. and the size of the sections seemed capable of improvement. A large and very thin circular saw blade is capable of cutting large sections and at the same time causing small losses of material as narrow saw cuts can be made with it. The unstability of its cutting edge calls for an allowance in the distance of the successive saw cuts (i.e., in the thickness of the slabs) which precludes an approach to serial sectioning.* On the other hand, as in our 1946 machine, the unwavering edge of a narrow and thick saw blade enables very thin sections to be cut, but there is a regrettable loss of material and moreover only small blocks of tissue can be cut. In general, a thin free saw blade can be given a firmness comparable to that of a narrow thick blade by subjecting it to a tension as is done in the ordinary bow saw. Our first apparatus built on this principle was a miniature reciprocating saw. The length of the saw blade (a narrow strip of phosphobronze 2 mm. in width and 0.120 mm. thick) was 3 cm. and the stroke 2 cm. It was driven at the rate of 150 strokes per minute. Diamond powder applied to the edge of the saw blade acted as a cutting agent. Owing to the relatively short stroke and the slowness of action only small pieces of dental tissue could be sectioned by this machine, but not less than four 0.075 mm. sections per mm. could be obtained. The saw cuts were 0.175 mm. in width.
Basic study on experimental rat caries. Serial observations of caries incidence and bacterial variation on rat molars in short-term experimental caries.
A short-term rat experiment was carried out in order to investigate the time course of the incidence of rat molar caries and the variation in the number of the oral cariogenic microorganisms or their biochemical properties. From the results obtained, an attempt was made to clarify the oral environment at the time of incidence of initial caries.In the time course of the caries incidence, carious lesions appeared as soon as rat were introduced to a sucrose diet and increased quickly. At day 15, carious lesions were found in about 80% of all the fissures. Moreover, the rate of caries incidence was unchanged for the first 6 days. According to the caries score, the mean number of carious lesions of the rat group which were fed a non-sucrose diet was lower than that of the experimental group, so that a difference in caries incidence was seen even in short-term experiments.As to the bacteria existing in the teeth, according to the incidence of the initial fissure caries, variation in the quantity of some microorganisms occured. Regarding S. mutans, the isolation rate and the number were not lower in the rats fed a non-sucrose diet than in the rats fed a sucrose diet.In biochemical tests, the strains of S. mutans derived from sucrose diet group strongly fermented sucrose, while a strain from the non-sucrose diet group had lower final acidities from glucose or starch than those of sucrose.From these aspects, regarding the incidence and progress of incipient caries, I suggest that biochemical factors of the various bacteria have stronger influence than their mere numbers, and it seems likely that the oral environmental factors, especially sucrose, have an important effect upon the biochemical behavior of oral microorganisms, and then the biochemical properties are changed to cariogenic. I believe that the early establishment and evaluation of oral environment factors such as diet and microorganisms are very important in caries prevention.
Concerning the "Vitality" of the Calcified Dental Tissues
The vitality of the enamel, dentin, and cementum is still an open question. Up to the end of the last century, dental tissues were believed to possess a low degree of vitality and hence were considered subject to systemic metabolic changes. Later, there was a complete reversal of opinion, resulting in the almost general belief that the enamel was an inert, dead structure; some authorities going so far as to include the dentin and the cementum. This point of view seems to carry the non-vitalist conception to the extreme. It is true that both dentin and cementum are composed principally of a matrix, nevertheless cells or cell processes are included in these tissues. The belief that enamel is an inert structure has its justification in the fact that its formative organ is destroyed upon the completion of the crown of the tooth; therefore, no new enamel can be formed. But this fact does not necessarily exclude the possibilities of physical and chemical changes occurring in the enamel after eruption of the tooth. The conception of non-vitality of the dental tissues is slowly undergoing a second reversal, brought about by the observation of the beneficial effect of diet on the teeth. It seems that the entire question of the vitality of the teeth rests upon an exact definition of terms. Churchill (1) presents his conception of the subject in a clear and concise manner. He states: In view of our lack of knowledge concerning vital processes in general, it seems difficult to offer an opinion in a controversy with regard to the vitality of the hard structures of the teeth. And yet, this question is so often the subject of discussion in professional circles that a commitment does not appear out of place.