Enamel microstructure in lemuridae (mammalia,primates): Assessment of variability

This study describes the molar enamel microstructure of seven lemurid primates: Hapalemur griseus, Varecia variegata, Lemur catta, Lemur macaco, Lemur fulvus rufus, Lemur fulvus fulvus, and Lemur fulvus albifrons. Contrary to earlier accounts, which reported little or no prism decussation in lemurid enamel, both Lemur and Varecia molars contain a prominent inner layer of decussating prisms (Hunter-Schreger bands), in addition to an outer radial prism layer, and a thin, nonprismatic enamel surface layer. In contrast, Hapalemur enamel consists entirely of radial and, near the surface, nonprismatic enamel. In addition, for all species, prism packing patterns differ according to depth from the tooth surface, and for all species but Varecia (which also has the thinnest enamel of any lemurid), average prism area increases from the enamel-dentine junction to the surface; this may be a developmental solution to the problem of accommodating a larger outer surface area with enamel deposited from a fixed number of cells. Finally, contradicting some previous reports, Pattern 1 prisms predominate only in the most superficial prismatic enamel. In the deeper enamel, prism cross-sections include both closed (Pattern 1) and arc-shaped (Pattern 2 or, most commonly, Pattern 3). This sequence of depth-related pattern change is repeated in all taxa. It should also be emphasized that all taxa can exhibit all three prism patterns in their mature enamel. The high degree of quantitative and qualitative variation in prism size, shape, and packing suggests that these features should be used cautiously in phylogenetic studies. Hapalemur is distinguished from the other lemurids by unique, medially constricted or rectangular prism cross-sections at an intermediate depth and the absence of prism decussation, but, without further assessment of character polarity, these differences do not clarify lemurid phylogenetic relations. Some characters of enamel microstructure may represent synapomorphies of Lemuridae, or of clades within Lemuridae, but homoplasy is likely to be common. Homoplasy of enamel characters may reflect functional constraints. © 1994 Wiley-Liss, Inc.     

Molar development in common chimpanzees (Pan troglodytes)

Numerous studies have reported on enamel and dentine development in hominoid molars, although little is known about intraspecific incremental feature variation. Furthermore, a recent histological study suggested that there is little or no time between age at chimpanzee crown completion and age at molar eruption, which is unlikely given that root growth is necessary for tooth eruption. The study presented here redefines growth standards for chimpanzee molar teeth and examines variation in incremental features. The periodicity of Retzius lines in a relatively large sample was found to be 6 or 7 days. The number of Retzius lines and cuspal enamel thickness both vary within a cusp type, among cusps, and among molars, resulting in marked variation in formation time. Daily secretion rate is consistent within analogous cuspal zones (inner, middle, and outer enamel) within and among cusp types and among molar types. Significantly increasing trends are found from inner to outer cuspal enamel (3 to 5 microns/day). Cuspal initiation and completion sequences also vary, although sequences for mandibular molar cusps are more consistent. Cusp-specific formation time ranges from approximately 2 to 3 years, increasing from M1 to M2, and often decreasing from M2 to M3. These times are intermediate between radiographic studies and a previous histological study, although both formation time within cusps and overlap between molars vary considerably. Cusp-specific (coronal) extension rates range from approximately 4 to 9 microns/day, and root extension rates in the first 5 mm of roots range from 3 to 9 microns/day. These rates are greater in M1 than in M2 or M3, and they are greater in mandibular molars than in respective maxillary molars. This significant enlargement of comparative data on nonhuman primate incremental development demonstrates that developmental variation among cusp and molar types should be considered during interpretations and comparisons of small samples of fossil hominins and hominoids.     

The adaptation of a two-compartment cell renewal system to external demands

Summary The inner enamel epithelium (IEE) covers the labial tooth aspect as a one cell layer which, when cut sagittally, appears as a longitudinal cell column extending from the tooth origin toward the periphery. Following sudden tooth shortening, the IEE responds by an increased cell production which later declines below normal values. The perturbation affects all cell kinetic parameters; the progenitor compartment, which initially increases, diminishes in size toward end of the experiment. The cell cycle transition times, which initially decline, rise toward the end of the experiment. The mean normal daily cell production rate of 70 cell % (i.e. 70 cells are produced by 100 progenitors) increases to 111 cell % and then declines to a low of 51 cell %. The IEE response typifies the behavior of other cell renewal systems such as intestinal epithelium and epidermis.     

Carbon isotope fractionation between diet and bioapatite in ungulate mammals and implications for ecological and paleoecological studies

 The isotope enrichment ɛ* of ¹³C between tooth enamel of large ruminant mammals and their diet is 14.1 ± 0.5‰. This value was obtained by analyzing both the dental enamel of a variety of wild and captive mammals and the vegetation that comprised their foodstuffs. This isotope enrichment factor applies to a wide variety of ruminant mammals. Non-ruminant ungulates have a similar isotope enrichment, although our data cannot determine if it is significantly different. We also found a ¹³C isotope enrichment ɛ* of 3.1 ± 0.7‰ for horn relative to diet, and 11.1 ± 0.8‰ for enamel relative to horn for ruminant mammals. Tooth enamel is a faithful recorder of diet. Its isotopic composition can be used to track changes in the isotopic composition of the atmosphere, determine the fraction of C₃ or C₄ biomass in diets of modern or fossil mammals, distinguish between mammals using different subpathways of C₄ photosynthesis,and identify those mammals whose diet is derived from closed-canopy habitats. Received: 1 July 1998 / Accepted: 9 February 1999     

Phagocytosis and elimination of amelocyte debris by stratum intermedium cells in the transitional zone of the enamel organ of the rat incisor

Summary In the transitional zone of the enamel organ (rat) some of the amelocytes perish. Their debris is phagocytized and digested by stratum intermedium cells and macrophages. These two cell types also seem to remove cytosegresomes expelled from those amelocytes which survive and redifferentiate into transporting amelocytes. Digestion of the amelocyte debris in the stratum intermedium cells is effected rapidly and completely. Degeneration of stratum intermedium cells was not observed in the transitional zone.This work was supported by grants from The Danish Medical Research Council (512-149/69 and 512-1008/71) and The Danish Science Research Council (512-1009/71).     

Hypoplastic area method for analyzing dental enamel hypoplasia

Most analyses of dental enamel hypoplasia compare frequencies of disturbed tooth types, which do not account for variability in the area of affected enamel. An alternate methodology, hypoplastic area, is presented here that accounts for this variability by combining acute and continuous enamel hypoplasia into an interval-level variable. The method compares samples based on individuals, by multiple tooth type variables, or by a single value rather than by tooth types. Use of the hypoplastic area method is illustrated by analyzing human skeletal dentitions in three archaeological samples: Meroitic Nubians from Semna South, Sudan; Anasazi from Navajo Reservoir, New Mexico; and Mogollon from Grasshopper Pueblo, Arizona. Both univariate and multivariate statistical tests are employed to assess variation in defects between individuals and samples. By incorporating measurements of continuous defects, the hypoplastic area method provides information beyond that of frequency data in comparing levels of stress. Flexibility of the method is also discussed. © 1995 Wiley-Liss, Inc.     

What molars contribute to an emerging understanding of lateral enamel formation in Neandertals vs. modern humans

Two hypotheses, based on previous work on Neandertal anterior and premolar teeth, are investigated here: (1) that estimated molar lateral enamel formation times in Neandertals are likely to fall within the range of modern human population variation, and (2) that perikymata (lateral enamel growth increments) are distributed across cervical and occlusal halves of the crown differently in Neandertals than they are in modern humans. To investigate these hypotheses, total perikymata numbers and the distribution of perikymata across deciles of crown height were compared for Neandertal, northern European, and southern African upper molar mesiobuccal (mb) cusps, lower molar mesiobuccal cusps, and the lower first molar distobuccal (db) cusp. Sample sizes range from five (Neandertal M(1)db) to 29 (southern African M(1)mb). Neandertal mean perikymata numbers were found to differ significantly from those of both modern human samples (with the Neandertal mean higher) only for the M(2)mb. Regression analysis suggests that, with the exception of the M(2)mb, the hypothesis of equivalence between Neandertal and modern human lateral enamel formation time cannot be rejected. For the M(2)mb, regression analysis strongly suggests that this cusp took longer to form in the Neandertal sample than it did in the southern African sample. Plots of perikymata numbers across deciles of crown height demonstrate that Neandertal perikymata are distributed more evenly across the cervical and occlusal halves of molar crowns than they are in the modern human samples. These results are integrated into a discussion of Neandertal and modern human lateral enamel formation across the dentition, with reference to issues of life history and enamel growth processes.     

The Neandertal lower right deciduous second molar from Trou de l'Abîme at Couvin, Belgium

A human lower right deciduous second molar was discovered in 1984 at the entrance of Trou de l'Abîme at Couvin (Belgium). In subsequent years the interpretation of this fossil remained difficult for various reasons: (1) the lack of taxonomically diagnostic elements which would support its attribution to either Homo (sapiens) neanderthalensis or H. s. sapiens; (2) the absence of any reliable chronostratigraphic interpretation of the sedimentary sequence of the site; (3) the contradiction between archaeological interpretations, which attributed the lithic industry to a transitional facies between the Middle and Early Upper Palaeolithic, and the radiocarbon date of 46,820 ± 3,290 BP obtained from animal bone remains associated with the tooth and the flint tools.Thanks to recent progress regarding these three aspects, the tooth from Trou de l'Abîme may now be studied in detail. Analyses of the morphology and enamel thickness of the fossil yielded diagnostic characters consistent with an attribution to Neandertals. Re-examination of the lithic industry of Couvin shows that it corresponds to the late Middle Palaeolithic rather than a transitional facies. Furthermore, a new analysis of the site stratigraphy indicates that the unit situated above the archaeological layer in which the tooth was found is probably a palaeosol of brown soil type. Comparison with the regional cave sequences as well as with the reference sequence from the Belgian loess belt tends to show that the most recent palaeosol of this type is dated between 42,000 and 40,000 BP. This is consistent with both a recently obtained AMS result at 44,500 BP and the published conventional date.     

Computed tomography and enamel thickness of maxillary molars of Plio-Pleistocene hominids from Sterkfontein, Swartkrans, and Kromdraai (South Africa): An exploratory study.

This paper is one in a series which explores the possibility of using the non-destructive CT technique to identify patterns in tooth enamel distribution and structure of hominid molars from Plio-Pleistocene sites in South Africa, notably Swartkrans, Sterkfontein, and Kromdraai. Whereas previous investigators have emphasised gross differences in absolute and relative or average enamel thickness between hominid taxa, the present study highlights differences in enamel thickness over functionally significant regions of the crown. Differences in the distribution of enamel in A. robustus, A. africanus, and Homo sp. are identified through the use of bivariate and multivariate analyses, and are interpreted in terms of dietary regimes.     

Crystal-associated matrix components in rat incisor enamel

Summary Sections of glutaraldehyde-OsO₄-fixed, plastic-embedded rat incisor enamel were left untreated, stained, decalcifed (1% formic acid in 10% sodium citrate), or decalcified-stained. The presence of apatite crystals was monitored with electron diffraction. After brief decalcification and staining, apatite crystals and matrix components were visualized in the same field. The ghost was continuous with crystal fragments, and the coat appeared as a dense line next to crystals and ghosts. Position of ghosts and crystals at the ameloblast-enamel junction (AEJ) of the secretion zone suggested that there may be a lag of no more than 1/5 min between the elaboration of ghost and crystal. A major change in enamel morphology occurs between the AEJ and the deep enamel of the secretion zone. The ghost becomes thinner, the coat more pronounced, and the crystal enlarges. There is only little change from the deep secretion to the maturation zone enamel.