Plasticity and constraint in response to early‐life stressors among late/final jomon period foragers from Japan: Evidence for life history trade‐offs from incremental microstructures of enamel

This study evaluates two hypotheses that address how Late/Final Jomon period people responded to early‐life stress using linear enamel hypoplasia (LEH) and incremental microstructures of enamel. The first hypothesis predicts that Jomon people who experienced early‐life stressors had greater physiological competence in responding to future stress events (predictive adaptive response). The second hypothesis predicts that Jomon people traded‐off in future growth and maintenance when early investment in growth and survival was required (plasticity/constraint). High resolution tooth impressions were collected from intact, anterior teeth and studied under an engineer's measuring microscope. LEH were identified based on accentuated perikymata and depressions in the enamel surface profile. Age of formation for each LEH was estimated by summing counts of perikymata and constants associated with crown initiation and cuspal enamel formation times. The relationship between age‐at‐first‐defect formation, number of LEH, periodicity between LEH, and mortality was evaluated using multiple regression and hazards analysis. A significant, positive relationship was found between age‐at‐death relative to age‐at‐first‐defect formation and a significant, negative relationship was found between number of LEH relative to age‐at‐first‐defect formation. Individuals with earlier forming defects were at a significantly greater risk of forming defects at later stages of development and dying at younger ages. These results suggest that Late/Final Jomon period foragers responded to early‐life stressors in a manner consistent with the plasticity/constraint hypothesis of human life history. Late/Final Jomon period individuals were able to survive early‐life stressors, but this investment weakened responses to future stress events and exacerbated mortality schedules. Am J Phys Anthropol 155:537–545, 2014. © 2014 Wiley Periodicals, Inc.     

Macroscopic and microscopic analyses of linear enamel hypoplasia in Plio-Pleistocene South African hominins with respect to aspects of enamel development and morphology

This study uses macroscopic and microscopic methods to analyze the expression of linear enamel hypoplasia (LEH) in Plio-Pleistocene South African hominins. LEH is a developmental defect of enamel that is used in many anthropological contexts as a physiological stress indicator. Previous research has not settled the question as to whether differences in LEH expression exist between Paranthropus and Australopithecus and if they exist, to what extent these differences might be explained simply by taxonomic differences in enamel development and morphology rather than by differential stress experience. In this study, the analysis of LEH is conducted with respect to differences between Paranthropus and Australopithecus in aspects of enamel development and morphology that are thought to influence LEH expression. Two factors impacting LEH expression are considered: the duration of enamel formation, and the spacing of perikymata. It is predicted that if the first factor strongly influences the expression of LEH, then there should be fewer defects per tooth in Paranthropus because of its abbreviated crown formation spans (and fast extension rates) relative to Australopithecus. It is also predicted that because Australopithecus has more densely packed perikymata in comparable regions of the crown than Paranthropus, this taxon should, on average, have narrower defects than Paranthropus. To address these questions, 200 Australopithecus and 137 Paranthropus teeth were examined for LEH, and the analysis of defect width with respect to perikymata spacing was conducted on tooth impressions examined under a scanning electron microscope using INCA (Oxford Instruments) measurement software. Data support the first prediction: Australopithecus does have significantly more defects per canine tooth than Paranthropus. Data do not support the second prediction in large part because several Australopithecus specimens have wide groove defects in which perikymata are not visible and enamel is irregular. Such wide grooves are not predicted by perikymata spacing such that alternative explanations, including taxonomic differences in ameloblast sensitivity and the duration/severity of disruptions to enamel growth, must be considered.     

禄丰古猿牙齿釉质发育不全的观察研究

对出自禄丰石灰坝的26个禄丰古猿下颌齿列的246枚恒齿进行了观察研究,发现禄丰古猿具有普遍的带状釉质发育不全（LEH）现象,个体LEH比例为100%,恒齿LEH比例为85%。乳齿几乎没有LEH现象,第一恒臼齿的LEH比例也很低仅57%。根据牙齿萌出顺序及现代大猿的牙齿发育年龄特征,作者推断2—3岁之前的幼儿古猿很少出现釉质发育不全现象,这可能与母体的营养关照有关。禄丰古猿的LEH的发生频率具有明显的季节性,结合中新世晚期气候变化特征、古猿的生态环境、生活习性及食性特征分析,作者推测：季节性营养不良可能是造成禄丰古猿釉质发育不全的主要原因。     

A fine line: a comparison of methods for estimating ages of linear enamel hypoplasia formation

It is accepted that linear enamel hypoplasias (LEHs), a specific type of enamel thickness deficiency, are related to periodic physiological disruptions to enamel matrix secretion during times that teeth are developing. Thus, LEHs are treated as general indicators of metabolic stress. Because the disruptions that cause LEHs affect only the portion of the crown that is in the process of forming, determining their locations allows researchers to reconstruct chronologies of stressful events. It is widely held that the many of the commonly used macroscopic methods for estimating the timing of LEHs are imprecise and do not conform to our current understanding of the process of enamel formation. The goal of the present study is to compare estimated ages of LEH formation produced by two of the most commonly used macroscopic methods to those derived from data in recent histological studies that include more precise information about the timing of crown formation across diverse human populations. These approaches are compared in two ways: 1) by creating a theoretical model using simulated LEHs and 2) empirically, by analyzing data collected on a sample of ancient Nubians from Semna South (present-day Sudan). Results indicate that the approach derived from histological studies provides significantly higher age estimates than the commonly used methods and this difference is particularly marked in early forming LEHs. The magnitude of this difference is large enough to produce divergent interpretation of bioarchaeological datasets and suggests that reevaluation of the methods used to estimate ages of LEH formation may be justified.     

Analysis and significance of linear enamel hypoplasia in Plio-Pleistocene hominins

This study of linear enamel hypoplasia (LEH) in Plio-Pleistocene hominins builds on a previous study (Guatelli-Steinberg [2003] Am. J. Phys. Anthropol. 120:309-322) that focused on LEH in early South African hominins. The present study is more comprehensive, encompassing dental specimens of hominins from East Africa as well, including early Homo. As a developmental defect of enamel, LEH is used in anthropological contexts to reveal information about physiological stress. However, intrinsic aspects of enamel development and morphology can affect the expression of LEH, complicating efforts to understand the significance of these defects. In this study, the analysis of LEH is conducted with respect to enamel development and morphology. It is predicted that Paranthropus should have fewer defects on its canine teeth than Australopithecus and Homo, owing to its abbreviated period of enamel formation. This prediction is supported: Paranthropus has statistically significantly fewer defects per canine than Australopithecus and Homo. The previous study demonstrated that despite the wider spacing of perikymata on the teeth of South African Paranthropus, defects on the canine teeth of this genus were not wider than those of Australopithecus. A multiple linear regression analysis in that study, as well as a separate analysis in the present study, indicate that the number of perikymata within defects is a better predictor of defect width than perikymata spacing. In this study, it was additionally found that the average number of perikymata within Australopithecus defects is statistically significantly greater than it is in Paranthropus, thus explaining why Paranthropus defects are not wider than those of Australopithecus. The biological significance of this difference in the number of perikymata within the defects of Australopithecus and Paranthropus is considered in light of several factors, including: 1) the possibility that other intrinsic attributes of enamel morphology may be involved (specifically the faster extension rates of Paranthropus that result in shallower defects), 2) generic differences in the canalization of enamel development, and 3) generic differences in the duration of disruptions to enamel growth.     

Age of linear enamel hypoplasia formation based on Massler and colleagues’ and Reid and Dean's standards in a Polish sample dated to 13th–18th century CE

In the study of enamel hypoplasia formation ages basically two standards of dental development are being applied. According to some authors, different methods produce different results, thus it is critical to decide which method to use. This study focuses on the comparison of three methods for estimating ages of linear enamel hypoplasia (LEH) formation.     

Under restrictive conditions,can the widths of linear enamel hypoplasias be used as relative indicators of stress episode duration?

Linear enamel hypoplasia (LEH), a type of enamel defect reflecting nonspecific physiological stress, has traditionally been used by bioarchaeologists to assess human health. Initially, measurements of defect width were used to estimate the duration of stress episodes. More recently, methods of counting within-defect perikymata (enamel growth increments) were developed to more accurately assess duration. Because perikymata are often not continuously visible within defects, while widths can usually be measured, the primary purpose of this article was to determine if, under restrictive conditions, the widths of LEH defects might be used as relative indicators of stress episode duration. Using a set of dental replicas from the prehistoric Irene Mound (1150-1400 A.D.), this study also investigated potential sources of variation in defect widths and how often defect widths could be measured and within-defect perikymata counted. Of 120 defects, only 47 contained both measurable defect widths and total within-defect perikymata, while 79 had measurable defect widths. Regression analysis revealed that, for these 47 defects, defect widths were more strongly related to the total number of within-defect perikymata than they were to crown region or tooth type. Although wide prediction intervals indicated that a defect's width could not be used to predict the number of within-defect perikymata for an individual, narrower confidence intervals associated with hypothetical mean population widths suggested that mean defect widths might be used to rank populations in terms of relative average stress episode duration.     

Prevalence and the duration of linear enamel hypoplasia: a comparative study of Neandertals and Inuit foragers

As a dental indicator of generalized physiological stress, enamel hypoplasia has been the subject of several Neandertal studies. While previous studies generally have found high frequencies of enamel hypoplasia in Neandertals, the significance of this finding varies with frequencies of enamel hypoplasia in comparative samples. The present investigation was undertaken to ascertain if the enamel hypoplasia evidence in Neandertals suggests a high level of physiological stress relative to a modern human foraging group, represented here by an archaeological sample of Inuit from Point Hope, Alaska. Unlike previous studies, this study focused specifically on linear enamel hypoplasia (LEH), emphasizing systemic over localized causes of this defect by considering LEH to be present in an individual only if LEH defects occur on two anterior teeth with overlapping crown formation periods. Moreover, this study is the first to evaluate the average growth disruption duration represented by these defects in Neandertals and a comparative foraging group. In the prevalence analysis, 7/18 Neandertal individuals (from Krapina and southern France) and 21/56 Neandertal anterior teeth were affected by LEH, or 38.9% and 37.5% respectively. These values do not differ significantly from those of the Inuit sample in which 8/21, or 38.1% of individuals, and 32/111, or 28.8% of anterior teeth were affected. For the growth disruption duration analysis, 22 defects representing separate episodes of growth disruption in Neandertals were compared with 22 defects in the Inuit group using three indicators of duration: the number of perikymata (growth increments) in the occlusal walls of LEH defects, the total number of perikymata within them, and defect width. Only one indicator, the total number of perikymata within defects, differed significantly between the Inuit and Neandertal groups (an average of 13.4 vs. 7.3 perikymata), suggesting that if there is any difference between them, the Inuit defects may actually represent longer growth disruptions than the Neandertal defects. Thus, while stress indicators other than linear enamel hypoplasia may eventually show that Neandertal populations were more stressed than those of modern foragers, the evidence from linear enamel hypoplasia does not lend support to this idea.     

Brief communication: physiological stress in the Florida Archaic-enamel hypoplasia and patterns of developmental insult in early North American hunter-gatherers

We examined the prevalence and developmental timing of linear enamel hypoplasias (LEHs) in an early Archaic Floridian population from Windover (8,120-6,980 (14)C years B.P. uncorrected). Using digital images, mandibular and maxillary canines were analyzed for defect prevalence and timing of insults. Although overall prevalence was very weakly correlated with earlier defect timing, there were significant differences in defect prevalence that varied by sex and tooth type. The mean LEH count in male mandibular canines was far higher than in male maxillary canines or in female mandibular or maxillary canines. We examined defect timing as a possible predictor of the sex differences in LEH prevalence. There were no significant sex differences in the developmental timing of the earliest defects in either tooth class. Developmental timing is not responsible for the sex differences seen in defect prevalence in mandibular canines.     

Enamel hypoplasias and physiological stress in the Sima de los Huesos Middle Pleistocene hominins

This study presents an analysis of linear enamel hypoplasias (LEH) and plane-form defects (PFD) in the hominine dental sample from the Sima de los Huesos (SH) Middle Pleistocene site in Atapuerca (Spain). The SH sample comprises 475 teeth, 467 permanent and 8 deciduous, belonging to a minimum of 28 individuals. The method for recording PFD and LEH is discussed, as well as the definition of LEH. The prevalence of LEH and PFD in SH permanent dentition (unilateral total count) is 4.6% (13/280). Only one deciduous tooth (lower dc) showed an enamel disruption. Prevalence by individual ranges from 18.7-30%. The most likely explanation for the relatively low LEH and PFD prevalence in the SH sample suggests that the SH population exhibited a low level of developmental stress. The age at occurrence of LEH and PFD was determined by counting the number of perikymata between each lesion and the cervix of the tooth. Assuming a periodicity of nine days for the incremental lines, the majority of LEH in the SH sample occurred during the third year of life and may be related to the metabolic stress associated with weaning.     

Missing defects? A comparison of microscopic and macroscopic approaches to identifying linear enamel hypoplasia

Linear enamel hypoplasia (LEH), the presence of linear defects of dental enamel formed during periods of growth disruption, is frequently analyzed in physical anthropology as evidence for childhood health in the past. However, a wide variety of methods for identifying and interpreting these defects in archaeological remains exists, preventing easy cross‐comparison of results from disparate studies. This article compares a standard approach to identifying LEH using the naked eye to the evidence of growth disruption observed microscopically from the enamel surface. This comparison demonstrates that what is interpreted as evidence of growth disruption microscopically is not uniformly identified with the naked eye, and provides a reference for the level of consistency between the number and timing of defects identified using microscopic versus macroscopic approaches. This is done for different tooth types using a large sample of unworn permanent teeth drawn from several post‐medieval London burial assemblages. The resulting schematic diagrams showing where macroscopic methods achieve more or less similar results to microscopic methods are presented here and clearly demonstrate that “naked‐eye” methods of identifying growth disruptions do not identify LEH as often as microscopic methods in areas where perikymata are more densely packed. Am J Phys Anthropol 153:463–472, 2014. © 2013 Wiley Periodicals, Inc.     

Linear enamel hypoplasia as an indicator of physiological stress in great apes: reviewing the evidence in light of enamel growth variation

Physiological stress, such as malnutrition or illness, can disrupt normal enamel growth, resulting in linear enamel hypoplasias (LEHs). Although ecological factors may contribute to LEH expression, other factors, such as surface abrasion and enamel growth variables, are also likely to be involved. Attention to these other factors is necessary before we can begin to understand what LEH might signify in terms of ecological sources of physiological stress in non-human primates. This study focuses on assessing the contribution of these other factors to variation in LEH expression within and across great ape taxa. Here, we present LEH data from unabraded crown regions in samples of seven great ape species. We analyze these data with respect to lateral enamel formation time and the angles that striae of Retzius make with the enamel surface, as these variables are expected to affect variation in LEH expression. We find that although the duration of enamel formation is associated with sex differences in LEH expression, it is not clearly related to taxonomic variation in LEH expression, and does not explain the low frequency of LEH in mountain gorillas found in this and a previous study. Our data on striae of Retzius angles suggest that these influence LEH expression along the tooth crown and may contribute to the consistently high frequencies of LEH seen in Pongo in this and previous studies. We suggest that future work aimed at understanding species variation in these angles is crucial to evaluating taxonomic patterns of LEH expression in great apes.     

Life history, enamel formation, and linear enamel hypoplasia in the Ceboidea

Linear enamel hypoplasia (LEH), a developmental defect of enamel, increases in frequency from prosimian to monkey to lesser ape to great ape grades (Guatelli-Steinberg 2000 Am. J. Phys. Anthropol. 112:395-410, [2001] Evol. Anthropol. 10:138-151; Newell 1998 Ph.D. dissertation, Temple University). This taxonomic pattern in the distribution of LEH is closely related to maturation length across the primate order (Newell 1998 Ph.D. dissertation, Temple University, 2000 Am. J. Phys. Anthropol. [Suppl.] 30:236). Longer maturation periods are associated with higher LEH frequencies; they appear to provide greater opportunity for defects to form. The present study explores the relationship between maturation length and LEH frequency within the Ceboidea. Because of its prolonged period of growth, Cebus is predicted to manifest LEH at a higher frequency than the more rapidly maturing ceboid genera. To test this hypothesis, two separate researchers (E.A.N. and D.G.-S.) examined LEH in nonoverlapping museum series of ceboids. The results support the hypothesis: in 13 genera (n = 1,276), E.A.N. found that LEH frequencies ranged from 0% in Callicebus, Cebuella, and Saimiri to 20% in Cebus. D.G.-S. found similar frequencies among five genera (n = 107), from 0% in Saimiri to 32% in Cebus. Thus, the broad pattern of LEH distribution evident across major taxonomic groups of primates is repeated within the Ceboidea. We also examined a related hypothesis linking the spacing of perikymata, which is influenced by enamel extension rates (Shellis 1998 J. Hum. Evol. 35:387-400), to LEH. The most likely areas of tooth crowns to exhibit LEH in human teeth are those in which perikymata are most closely spaced (Hillson and Bond 1997 Am. J. Phys. Anthropol. 104:89-103). We hypothesized that the longer-maturing Cebus, with its elevated LEH frequency, will also exhibit more closely spaced perikymata than other ceboids. Analysis of a small microscopic subsample (n = 8) lends limited support to this second hypothesis.     

Human deciduous mandibular molar incremental enamel development

Quantitative studies of incremental markings retained within human enamel have reconstructed the duration and rate (crown and cusp formation times, initiation and completion, daily enamel secretion rates) of permanent tooth development. This approach has provided one way of estimating human age‐at‐death, and facilitated comparative dental studies of primate evolution. Similar applications from deciduous enamel are inhibited because developmental reconstructions from incremental markings for these teeth are less frequently reported in the literature. This study quantified the duration and rate of enamel development for mesial (protoconid, metaconid) and distal cusps (hypoconid, entoconid) for first (dm1) and second (dm2) deciduous mandibular molars from an archaeological sample of modern human juveniles. Crown formation time can be calculated from the dm1 protoconid because growth initiates and completes in this cusp, and from the dm2 protoconid combined with the final period of hypoconid growth. The dm1 postnatal crown formation time included the time taken for the tubercle of Zuckerkandl to develop, and differed slightly compared to radiographic methods. The majority of dm1 protoconid cuspal (occlusal region) enamel formed before birth. The dm2 entoconid enamel formed mainly after birth. Birth reduced daily enamel secretion rates, changed the visibility of incremental markings, and disrupted enamel growth for 3 to 8 days. Findings presented here can contribute to age‐at‐death estimates for human infants aged 13‐postnatal months or less, and should facilitate comparisons of primate deciduous incremental enamel development in an evolutionary context. Regression equations are included so that cuspal formation time can be estimated from enamel thickness. Am J Phys Anthropol, 2011. © 2010 Wiley‐Liss, Inc.     

Linear enamel hypoplasias as indicators of systemic physiological stress: evidence from two known age-at-death and sex populations from postmedieval London

Enamel hypoplasias are useful indicators of systemic growth disturbances during childhood, and are routinely used to investigate patterns of morbidity and mortality in past populations. This study examined the pattern of linear enamel hypoplasias in two different burial populations from 18th and 19th Century church crypts in London. Linear enamel hypoplasias on the permanent dentitions of individuals from the crypt of Christ Church, Spitalfields, were compared to enamel defects on the teeth of individuals from St. Bride's. The method used involves the identification of enamel defects at a microscopic level, and systemic perturbations are detected by matching hypoplasias among different tooth classes within each individual. The pattern of linear enamel hypoplasias was contrasted between individuals from the burial sites of Spitalfields and St. Bride's, between males and females, and between those aged less than 20 years of age and those aged over 20 years at death. Six different parameters were examined: frequency of linear enamel hypoplasias, interval between defects, duration of hypoplasias, age at first occurrence of hypoplasia, age at last occurrence of hypoplasia, and the percentage of enamel formation time taken up by growth disturbances. All individuals in the study displayed linear enamel hypoplasias, with up to 33% of total visible enamel formation time affected by growth disruptions. Multiple regression analysis indicated a number of significant differences in the pattern of enamel hypoplasias. Individuals from Spitalfields had shorter intervals between defects and greater percentages of enamel formation time affected by growth disturbances than did individuals from St. Bride's. Females had greater numbers of linear enamel hypoplasias, shorter intervals between defects, and greater percentages of enamel formation time affected by growth disturbances than males. There were also differences in the pattern of enamel hypoplasias and age at death in this study. Individuals who died younger in life had an earlier age at first occurrence of enamel hypoplasia than those who survived to an older age. The pattern of enamel hypoplasias detected in this study was influenced by tooth crown geometry and tooth wear such that most defects were found in the midcrown and cervical regions of the teeth, and greater numbers of defects were identified on the anterior teeth. Differences in sensitivity of the parameters used for the detection of enamel hypoplasias were found in this study. The percentage of visible enamel formation time affected by growth disturbances was the parameter that identified the greatest number of significant differences among the subgroups examined.     

Preferential expression of linear enamel hypoplasia on the sectorial premolars of rhesus monkeys (Macaca mulatta)

Three hundred and sixty rhesus macaque specimens at the Caribbean Primate Research Center were examined for evidence of linear enamel hypoplasia (LEH). A previously unreported intertooth pattern in LEH was observed. Defects occur preferentially on the sectorial premolar of both males and females. Relative to other teeth, the sectorial premolar exhibits more prominent defects and is more likely to exhibit multiple defects. This pattern is unlike the human intertooth LEH pattern and unlike patterns previously reported for monkeys and apes. These observations are discussed in the context of factors thought to influence the intertooth distribution of LEH in humans and in nonhuman primates. The authors reject crown height, the timing of crown development, and the duration of crown formation as factors contributing to the observed pattern and favor an explanation involving enamel thickness, perikymata spacing, and/or prism orientation. Am J Phys Anthropol 107:179–186, 1998. © 1998 Wiley-Liss, Inc.     

Brief communication: linear enamel hypoplasia and the shift from irregular to regular provisioning in Cayo Santiago rhesus monkeys (Macaca mulatta)

This study investigates changes in the prevalence of linear enamel hypoplasia (LEH) before and after the shift from irregular to regular provisioning in the Cayo Santiago rhesus monkey population. Prior to 1956, monkeys on this island colony did not receive consistent provisions, and were reported to be in poor health (Rawlins and Kessler [1986] The Cayo Santiago Macaques; Albany: State University of New York Press). A regular provisioning program, instituted in August 1956, resulted in the improved health of individuals and the growth of the population (Rawlins and Kessler [1986] The Cayo Santiago Macaques; Albany: State University of New York Press). LEH, a developmental defect of enamel, is a sensitive indicator of systemic physiological stress (Goodman and Rose [1990] Yrbk. Phys. Anthropol. 33:59-110). It was therefore hypothesized that the prevalence of LEH would be higher in monkeys who were irregularly provisioned than in monkeys who experienced regular provisioning. To test this hypothesis, teeth were examined for LEH in a sample of 181 female rhesus monkeys. The results support the hypothesis: the mean number of defects was statistically significantly higher in the preprovisioned group than it was in the postprovisioned one. When LEH prevalence was assessed using only defects occurring on antimeric pairs, the preprovisioned group again had a higher prevalence than the postprovisioned one, although the difference was not statistically significant, most likely because of the reduced sample size. The results of this study indicate that changes in LEH prevalence, at least in this population of rhesus monkeys, are associated with changes in nutritional status.     

禄丰古猿带状牙釉质发育不全的再观察

牙釉质发育不全是反映牙齿发育过程中遭遇生理性刺激的有效指标。本文对来自云南禄丰石灰坝的禄丰古猿(Lufengpithecus lufengensis)261枚单颗恒齿进行观察分析,发现223枚牙齿普遍罹患带状牙釉质发育不全(LEH),比例为85.4%,高于其它已报道过的中新世古猿、化石人科成员及现生大猿。另一方面,用扫描电镜和数码显微镜着重分析了12枚犬齿,均有多条LEH,且雄性犬齿的LEH条数多于雌性;相邻LEH间的釉面横纹数在14-25条之间。结合禄丰古猿釉面横纹的生长周期(9天),估算相邻LEH之间的时间间隔为4.8-6.6月,季节性营养不良可能是禄丰古猿釉质发育不全的主要原因。     

Molar tooth development in caspase-3 deficient mice

Tooth morphogenesis is accompanied by apoptotic events which show restricted temporospatial patterns suggesting multiple roles in odontogenesis. Dental apoptosis seems to be caspase dependent and caspase-3 has been shown to be activated during dental apoptosis.Caspase-3 mutant mice on different genetic backgrounds were used to investigate alterations in dental apoptosis and molar tooth morphogenesis. Mouse embryos at E15.5 were analyzed to reveal any changes in enamel knots, which are transient structures eliminated by apoptosis. In caspase-3(-/-) mice on the B57BL/6 background, disorganization of the epithelium was found in the original primary enamel knot area and confirmed by altered expression of Shh. Despite this early defect in molar tooth development, these mutants showed correct formation of secondary enamel knots as indicated by Fgf-4 expression. Analyses of adult molar teeth did not reveal any major alterations in tooth shape, enamel structure or pattern when compared to heterozygote littermates. In caspase-3(-/-) mice on the 129X1/SvJ background, no defects in tooth development were found except the position of the upper molars which developed more posteriorly in the oral cavity. This is likely, however, to be a secondary defect caused by a physical squashing of the face by the malformed brain. The results suggest that although caspase-3 becomes activated and may be essential for dental apoptosis, it does not seem fundamental for formation of normal mineralised molar teeth.     

Relationship of enamel hypoplasia to the pattern of tooth crown growth: A discussion

The defects of enamel hypoplasia can be related to the layered structure of enamel which represents the sequence of development in tooth crowns. From such studies, it is possible to see that furrow-type enamel defects (the most common form of hypoplasia seen with the naked eye) are just the most prominent expression of a continuum which extends ever smaller, down to a microscopic disturbance to a single layer in the crown formation sequence. Furthermore, the progressive decrease in spacing between development layers which occurs down the crown sides, from occlusal to cervical, affects both the prominence and apparent width of the defects. This makes it difficult to use measurements as a means of estimating the duration of the disturbance causing a particular defect. The difficulty is even greater for the less common pitted or exposed-plane-type defects, for which the apparent width bears very little relationship with the duration of the growth disturbance. The defects of enamel hypoplasia can therefore be understood clearly only when examined under the microscope in relation to the structures which mark the development sequence of the tooth crown. Am J Phys Anthropol 104:89–103, 1997. © 1997 Wiley-Liss, Inc.