Like daily cross-striations, the proximate cause of SR is changes in ameloblast activity (reviewed in Risnes, 1998 ), but their ultimate cause is unknown (Dean ...
Misinterpretations, Misrepresentations and Misunderstandings Concerning Enamel. H. Warshawsky, Emeritus Professor Department of Anatomy and Cell Biology McGill University, Montreal, Quebec, Canada In following the development of “thoughts” concerning enamel structure and formation, I have discovered amazing and annoying patterns. The purpose of this note is to illustrate a few of these events. In a series of papers, mainly by anthropologist, I have followed their interest in time markers in enamel, namely cross striations and stria of Retzius. I continue to be amused that the only branch of science concerned with enamel is anthropology and not dentistry. My interests were again peaked by the recent paper of M. O’Hara (Am J Phys Anthropol. February, 2017,1–10) where perikymata and stria were being used to estimate linear enamel hypoplasia timing. One remark made me wondered if there had been a change in how these lines in enamel were being interpreted. “LEHs (Linear Enamel Hypoplasia) occur when an infant experiences an external systemic, physiological stress, such as febrile disease or malnutrition, that result in the abnormal slowing or cessation of enamel production in developing teeth (Dobney, 1991; Giro, 1947; Goodman and Rose, 1990; Guatelli-Steinberg, Ferrell, & Spence, 2012a; Hillson and Bond, 1997; Kreshover, 1960; Newman and Poole, 1974”. I checked several papers from researchers in that group and found in one paper, published in 2006, the standard explanation of enamel formation, “Daily cross-striations reflect the effects of a circadian rhythm on ameloblast activity during enamel matrix production (reviewed in FitzGerald, 1998). SR (stria of Retzius) occur at longer but regular time intervals in imbricational enamel, and reach the tooth surface as grooves called perikymata (Beynon, 1992 ). Like daily cross-striations, the proximate cause of SR is changes in ameloblast activity (reviewed in Risnes, 1998 ), but their ultimate cause is unknown (Dean, 1987).” (Donald J. Reid and Rebecca J. Ferrell, “The relationship between number of striae of Retzius and their periodicity in imbricational enamel formation”, Journal of Human Evolution 50, 195-202, 2006). They go on to describe the use of these markers as follows; “Despite their unknown etiology, it has been shown that SR occur with a constant periodicity in the teeth of an individual (FitzGerald, 1998; Reid et al., 1998a), and it is this feature that makes them useful for determining crown formation times. SR periodicity is determined by counting the number of daily cross-striations between adjacent SR in a longitudinal tooth
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section. The total number of SR in the tooth (‘‘total SR number’’) is determined by identifying each SR near its junction with the tooth surface. A tooth’s SR periodicity and total SR number are multiplied to calculate the tooth’s imbricational crown formation time (ICFT); this time can be added to an estimate of cuspal formation time to calculate a total crown formation time (see Dean, 1998 , for cuspal crown formation time estimation methods).” In another paper by some of the same authors, published 6 years later, in 2012, entitled, “Enamel extension rate patterns in modern human teeth: Two approaches designed to establish an integrated comparative context for fossil primates” by Debbie Guatelli-Steinberg, Bruce A. Floyd , M. Christopher Dean, and Donald J. Reid, Journal of Human Evolution 63, 475-486, (2012), we are dealing with an entity called “Enamel extension rates (EERs), the rates at which ameloblasts differentiate, determine how fast tooth crowns grow in height.” This is explained in the following legend of figure 1. Figure 1. Method by which enamel extension rates were calculated (by M.C. Dean) for first data set, illustrated in a high power image of lateral enamel at the EDJ in a human lower lateral incisor. A 200 mm line is drawn from the EDJ along the direction of the enamel prisms (line AC). In this tooth, the average time taken to form a 200 mm thickness of enamel from the EDJ (A to C) was w70 days. At point C, a second line is drawn, parallel to (or along) a stria of Retzius (or accentuated line), back to a lower position on the EDJ (point B). Striae of Retzius mark out the enamel forming front at any given time. Thus, the distance along the EDJ between points A and B was also formed in w70 days. The extension rate, the rate at which a cohort of new ameloblasts differentiates and covers this distance (A to B), is calculated by dividing this distance by 70 days.
We now see a new interpretation, “The extension rate, the rate at which a cohort of new ameloblasts differentiates and covers this distance (A to B).” This is new and not before seen in papers from this group. Where did it come from? It came from my work published in 1985 (Warshawsky, H., P. Bai and A. Nanci, Lack of evidence for rhythmicity in enamel development. In: Tooth Morphogenesis and Differentiation eds. A. B. Belcourt and J.-V. Ruch Inserm 125: 241-256, 1985) and 1988 (see below), but never cited in any of the papers from this group. Below is figure 1 from Guatelli-Steinberg et al, and I will return to it further down.
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Please read the following from my review in Critical Reviews in Anatomical Sciences, Volume 1, Issue 4, 425-460, 1988, it contains a complete explanation of so-called extensions of enamel and is consistent with stria being interrod enamel layers separating cohorts of enamel rods produced by the generation of cohorts of ameloblasts during elongation of the tooth crown. “FORMATION OF ENAMEL AND DENTIN: A CRITICAL REVIEW Author: Hershey Warshawsky F. Paleoanthropology from Enamel Due to the survival of teeth as the major fossil tissue, they have been used extensively in paleotaxonomy. Recently, teeth have become prominent in attempts to unravel the hominid fossil record and to classify species as more or less ape-like or human-like. These studies used teeth to determine the length of the infancy period. A lengthening period of infancy is defined as more human-like. To determine the length of infancy, paleoanthropologists such as Bromage and Dean201 have seized on the periodicity found in enamel and have accepted the shaky assumption that cross-striations seen in ground sections correspond to daily accretion of enamel.
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FIGURE 6. Schematic representation of the growth pattern of a crown of limited eruption. As the small crown (represented by ameloblast cohorts labeled A to N in small letters) increases to the large crown, the same cell cohorts move to the outer positions (labeled A to N in large letters). Since no cell division occurs during secretion, the same number of cells are contained in the small- and large-letter cohorts. The position occupied by the cells at the start is preserved as the dentinoenamel junction. The movement of the cell cohorts is necessary to make room for the enamel rods secreted by each of the cells in the cohort, and, consequently, the entire cohort moves from the inner to the outer position. If the orientation of the rods in each cohort is the same, but the orientation of the rods in adjacent cohorts is different, then the interface between the two cohorts of rods would be marked by a line of separation which represents the position of a stria of Retzius. The crown lengthens as more cohorts are added beyond N. The separation of the new from the older cohorts is again marked by striae of Retzius.
Furthermore, since the intervals between striae of Retzius usually contain six to eight "cross striations", they have accepted the idea that each interval between striae corresponds to 1 week. Striae of Retzius at the occlusal tip do not reach the tooth surface, but the striae near the cervical portion of the crown emerge at the tooth surface as perikymata. These ridges and intervening grooves between them are readily seen on tooth surfaces, and it was assumed that counting the perikymata on developing teeth from young specimens would give the approximate age of the individual at the time of death. Since birth is recorded in teeth that form pre- and postnatally as a prominent stria of Retzius called the neonatal line (202) the number of cross-striations and the number of striae of Retzius and perikymata from the neonatal line would give the age at death. Since some striae never reach the surface of the crown, the number of subsurface striae was estimated and added as an additional developmental time. The assumption underlying all these counts of lines which supposedly represent time is expressed by Lewin (203) "Count the number of ridges in a tooth crown and you know how many weeks it took to grow, which then allows an estimate of infancy." Several articles have appeared using the assumption that cross-striations
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represent days and striae of Retzius weeks (204,205). The assumption was applied to fossil records and conclusions were drawn about the length of infancy, and on this basis individual fossils were classified as more or less human. Such pseudo-quantitative methods based on observations that have been proven to be erroneous are misleading and dangerous. The work of Weber and Glick (187) proved conclusively that the cross-striations seen and counted in ground sections were caused by cross sections of individual rods aligned to form what looked like longitudinal sections of single rods. Boyde cautioned that such alignment occurred (112) and the disregard for both his caution and the definitive work of Weber and Glick (187) is as great an anthropological hoax as the Piltdown man. However, groups of six or seven rods do make up bands that are cohorts of incremental growth based on ameloblast differentiation at the cervical loop (Figure 6), and this generation of cohorts can be timed in living species by thymidine labeling. Such an approach would put a true age value on the interval between striae of Retzius. In a critical review, it is the duty of the reviewer to single out items that require the most critical reevaluation. It is time to realize that cross-striations as seen in ground sections do not represent a diurnal growth pattern. Counting of these "units" only indicates the number of rods between striae. The number of rods by itself is no indication of time.” Although I am pleased that these authors now accept my explanation of “extensions”, I am disappointed that my papers were not cited and the work was treated as if it did not exist. So, do the authors now understand enamel formation? Unfortunately, not yet. Look at the figure 1 above; the legend states, “A 200 mm line is drawn from the EDJ along the direction of the enamel prisms (line AC).” However, the prisms, rods, do not run as indicated by line AC, they run parallel to the stria of Retzius and form the rod cohorts contained between adjacent stria. Finally, I enclose some shocking misrepresentation of our own work. The picture shown in Antonio Nanci’s text-book, Ten Cate’s Oral Biology, on page 183, was taken in my laboratory by Nanci when he was a graduate student under my supervision and used by us to show how images can be misinterpreted as cross striations; then I enclose the picture that I found when I Google cross striations.
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This is the photo and text from Nanci’s text book. See the legend of Fig. 7-49. “producing the impression of cross-striations along their length”. Two images used to show what could be misconstrued as periodicity. The irony is that A is from rat incisor and on these short segments of rods we see at least 15 “striations”, but we know from our work with thymidine labeling that it takes 7 days to product the entire enamel layer. Therefore, it cannot be diurnal variation in enamel production. Just imprints of one group of rods decussating over the next group. Figure b is human enamel and illustrates the repetitively gouged, ladder-like appearance produced during section preparation and has been dealt with in many of my papers.
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Finally, this picture from Google copied from the text-book using our pictures that were used to disprove cross striation, but here used to show the opposite. I wonder why others have not questioned where the 4 micrometers per day came from. It was made up a long time ago and it is now time to accept that it is only the diameter of an enamel rod. Hershey Warshawsky P.S. All my published articles are available at my ResearchGate profile.
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