There are no comprehensive data on the expected range of measurements for the spacing of short-period lines in ground sections of human dentine, or for that of other animals, at precise locations within the teeth. Kawasaki et al. (1980)found that concentric calcospheritic lines were 1.7 to 2.0 ?m apart in demineralized human dentine, and that other short-period markings were 4 ?m apart in both ground and demineralized sections of human incisors and premolars; the spacings of the calcospheritic markings were measured in dentine that had been block-stained with silver by the Bielschowsky technique before sectioning. Kobayashi (1984)measured the interval between ‘laminar lines’ in human dentine as between 2 and 4 ?m. Kodaka and Higashi (1995)described a new method of preparing dentine for scanning electron microscopy by etching a fractured surface with EDTA after treatment with NaOCl; they reported that the spacing between both spherical (calcospheritic) and ‘laminar lines’ was of the order of 1 ?m, and therefore considerably closer together than reported by Kawasaki et al. and Kobayashi. It is hard to relate any of these data to, for example, the earliest formed mantle dentine or root dentine, or to dentine that is forming fastest in cuspal regions, or to slower forming circumpulpal dentine, as little information about location is provided.

There is also some information on the average spacing of short-period markings in the dentine of other animals. Schour and Hoffman (1939a)made measurements of incremental markings in the dentine of 17 species and concluded that the average spacing of all of them was 16 ?m (range 11.25–19.25 ?m). However, these estimates were presumably based on measurements made from wherever possible in the roots or crowns of teeth. While Schour and Hoffman (1939b)also provide experimental evidence for the daily rate of dentine formation in some animals, including humans and macaque, Kawasaki et al. (1980)make the point that they never measured the spacing between the short-period incremental markings in more slowly forming, calcospheritic dentine in either human or non-human primates, nor in any of their experimental animals.

Some confusion exists in the literature on human dentine as short-period (von Ebner’s) lines have also been reported to be between 14 and 20 ?m apart, and this has become established in textbooks on dental histology suggest that such observations might have resulted from the misidentification of wider spaced, long-period Andresen lines in human dentine as short-period von Ebner’s lines in the sections used for making the measurements. The biggest problem, in fact, is that eponymous designations are an inappropriate way of describing any structural feature unless there is an anatomical definition of what they actually represent. Dental histology is replete with eponymous terms such as these that are not well defined. Frank and Nalbandian (1989)state that the eponymous descriptions of incremental markings in dentine are inadequate to distinguish between the numerous striae that can be revealed by various techniques. Part of the present purpose was to help clarify the description of short-period lines in dentine.

Given the similarity in the spacing of long-period lines in human dentine to that of short-period lines described for so many other animals by Schour, I. and Hoffman, M.M., 1939. Studies in tooth development. I. The sixteen microns calcification rhythm in the enamel and dentin from fish to man. J. Dent. Res. 18, pp. 91–102Schour and Hoffman (1939a), it is easy to see how confusion might arise in comparative studies. In fact, the short-period lines can sometimes be seen to occur between wider spaced, long-period lines in many sections of human and non-human primate dentine. Lines spaced between 14 and 20 ?m apart in human dentine are not likely, therefore, to be short-period lines at all.

Fig. 7.Both short- and long-period incremental lines in human dentine visible together in (A) a ground section viewed in polarized light, and (B) a demineralized silver-stained section viewed in transmitted light only. Both micrographs are of intercuspal circumpulpal dentine low in the crown. The long-period lines are approx. 20 ?m apart and there are seven or eight short-period lines between them in both of these sections (although they vary in number between seven and ten most commonly in human teeth). Fieldwidth 225 ?m. Original magnification ×500.

The principle aim now was to present some comparative data on the spacing of short-period (von Ebner’s) lines in human dentine and in the dentine of three other primates (a gibbon, a siamang, and a great ape, as well as in the dentine of a pig canine [as there are some good experimental data on the periodicity, but not the spacing, of von Ebner’s lines in pig dentine]. The exact locations of measurements made in dentine have not been well documented. Another aim, therefore, was to compare these data-sets at known distances from the root surface in order to describe the spacing of short-period lines close to the start of dentine formation here and, where possible, again in the axial plane of the tooth cusp, where rates of formation are likely to be maximal. A further aim was to compare data for the spacing of short-period lines in sections of demineralized human dentine with those from ground sections in order to judge the likelihood that any preparation artefact or shrinkage might affect measurements of the spacing of lines.

Several investigators have observed that incremental markings in dentine can be enhanced by either making ground sections anorganic, by demineralizing them (e.g. by observing sections of carious teeth), by heating them sufficiently to drive off CO₂ from carbonate-rich regions, by staining demineralized blocks or sections with silver, or by treating fractured blocks with NaOCl and then etching them with EDTA. In this study, sections of permanent human canine and premolar teeth were chosen from a large collection that showed good short-period markings. Two ground sections of permanent canines (made anorganic with NaOCl just before mounting with DPX) were chosen that showed good short-period markings close to the root surface and in the axial plane of the tooth, respectively. A further demineralized section of a premolar tooth stained with silver by the Bielschowsky technique, which showed good short-period markings close to the root surface and in the axial plane of the tallest cusp, was chosen.

Ground sections of some non-human primate teeth belonging to a gibbon (Hylobates moloch), a siamang (Hylobates (Syndactyles) symphalangus) and a great ape (Pongo pygmaeus), and of a pig canine (Sus) were also included in this study, primarily because they showed good short-period calcospheritic markings in the dentine close to the root surface. However, the data obtainable from these sections were much more limited than from the human sections. None of these sections had been made anorganic or was demineralized, nor had they been specially prepared to enhance lines in the dentine in any way but were simply routine ground sections from a large collection of comparative material in the Department of Anatomy and Developmental Biology at University College London. While the lack of special specimen preparation may account for their limited usefulness in this study, the data were included to broaden its scope.

Montages of photomicrographs were prepared from along the root surfaces of each of these sections that included the granular layer of Tomes and root dentine some 300 ?m deep to this layer. Sections of root were chosen in the cervical portion of the sections and the montages were constructed to extend at least 1000 ?m along the root. Montages of dentine in the axial plane of the tallest cusp were also prepared in regions where good short-period lines were present. Micrographs were taken with a ×50 objective in transmitted light using a Ziess Jenamed microscope (sections were not viewed in polarized light as this did not improve the quality of the image of the lines and if anything made them worse).