Phylogenetic memory of developing mammalian dentition

Structures suppressed during evolution can be retraced due to atavisms and vestiges. Atavism is an exceptional emergence of an ancestral form in a living individual. In contrast, ancestral vestige regularly occurs in all members of an actual species. We surveyed data about the vestigial and atavistic teeth in mammals, updated them by recent findings in mouse and human embryos, and discussed their ontogenetic and evolutionary implications. In the mouse incisor and diastema regions, dental placodes are transiently distinct being morphologically similar to the early tooth primordia in reptiles. Two large vestigial buds emerge in front of the prospective first molar and presumably correspond to the premolars eliminated during mouse evolution. The incorporation of the posterior premolar vestige into the lower first molar illustrates the putative mechanism of evolutionary disappearance of the last premolar in the mice. In mutant mice, devious development of the ancestral tooth primordia might lead to their revivification and origin of atavistic supernumerary teeth. Similarity in the developmental schedule between three molars in mice and the respective third and fourth deciduous premolar and the first molar in humans raises a question about putative homology of these teeth. The complex patterning of the vestibular and dental epithelium in human embryos is reminiscent of the pattern of "Zahnreihen" in lower vertebrates. A hypothesis was presented about the developmental relationship between the structures at the external aspect of the dentition in mammals (oral vestibule, pre-lacteal teeth, paramolar cusps/teeth), the tooth glands in reptiles, and the earliest teeth in lower vertebrates.     

Studies on dentin. 2. Vestigial lacteal incisor teeth of the rat.

The presence of a vestigial, lacteal incisor tooth is described in the laboratory rat. This tooth is felt to belong to the same dental generation as the other functional teeth. Accordingly, the rat is described as having a monophyodont, first dentition containing two incisor teeth in each quadrant. These vestigial teeth are then compared with other similar mammalian teeth and are defined as transient, partially formed and non-functional. As such, they are differentiated from other transient teeth. The examination of the fossil record suggests that tooth loss is a general phenomenon in rodents, but that this vestigial tooth probably represents a condition present in forms antecedent to rodents. A critical literature review strongly suggests that the teeth of the recent rat are members of the first dental generation. The presence of such a vestigial tooth and of the postincisive diastema in the rat is felt to be an example of phylogenetic reduction and progressive retardation in the sense of de Beer's concepts. These same two phenomena were analyzed with respect to the field theory of Butler and of the Zahnreihen theory of Edmund. Placed within the context of recent data on epithelioectomesenchymal interactions, both theories were supported, and both the vestigial teeth and anodontic diastema were shown to be explicable within these conceptual frameworks.     

Autocrine and paracrine Shh signaling are necessary for tooth morphogenesis, but not tooth replacement in snakes and lizards (Squamata)

Here we study the role of Shh signaling in tooth morphogenesis and successional tooth initiation in snakes and lizards (Squamata). By characterizing the expression of Shh pathway receptor Ptc1 in the developing dentitions of three species (Eublepharis macularius, Python regius, and Pogona vitticeps) and by performing gain- and loss-of-function experiments, we demonstrate that Shh signaling is active in the squamate tooth bud and is required for its normal morphogenesis. Shh apparently mediates tooth morphogenesis by separate paracrine- and autocrine-mediated functions. According to this model, paracrine Shh signaling induces cell proliferation in the cervical loop, outer enamel epithelium, and dental papilla. Autocrine signaling within the stellate reticulum instead appears to regulate cell survival. By treating squamate dental explants with Hh antagonist cyclopamine, we induced tooth phenotypes that closely resemble the morphological and differentiation defects of vestigial, first-generation teeth in the bearded dragon P. vitticeps. Our finding that these vestigial teeth are deficient in epithelial Shh signaling further corroborates that Shh is needed for the normal development of teeth in snakes and lizards. Finally, in this study, we definitively refute a role for Shh signaling in successional dental lamina formation and conclude that other pathways regulate tooth replacement in squamates.     

A new giant species of placented worm and the mechanism by which onychophorans weave their nets (Onychophora: Peripatidae)

Onychophorans, or velvet worms, are poorly known and rare animals. Here we report the discovery of a new species that is also the largest onychophoran found so far, a 22 cm long female from the Caribbean coastal forest of Costa Rica. Specimens were examined with Scanning Electron Microscopy; Peripatus solorzanoi sp. nov., is diagnosed as follows: primary papillae convex and conical with rounded bases, with more than 18 scale ranks. Apical section large, spherical, with a basal diameter of at least 20 ranks. Apical piece with 6-7 scale ranks. Outer blade 1 principal tooth, 1 accessory tooth, 1 vestigial accessory tooth (formula: 1/1/1); inner blade 1 principal tooth, 1 accessory tooth, 1 rudimentary accessory tooth, 9 to 10 denticles (formula: 1/1/1/9-10). Accessory tooth blunt in both blades. Four pads in the fourth and fifth oncopods; 4th. pad arched. The previously unknown mechanism by which onychophorans weave their adhesive is simple: muscular action produces a swinging movement of the adhesive-spelling organs; as a result, the streams cross in mid air, weaving the net. Like all onychophorans, P. solorzanoi is a rare species: active protection of the habitat of the largest onychophoran ever described, is considered urgent.     

Developmental origins and homologies of the hyracoid dentition

Understanding the origins of morphological specializations in mammals is a key goal in evolutionary biology. It can be accomplished by studying dental homology, which is at the core of most evolutionary and developmental studies. Here, we focused on the evolution and development of the specialized dentition of hyraxes for which dental homologies have long been debated, and could have implications on early placental evolution. Specifically, we analysed dental mineralization sequences of the three living genera of hyraxes and 17 fossil species using X‐ray computed microtomography. Our results point out the labile position of vestigial upper teeth on jaw bones in extant species, associated with the frequently unusual premolar shape of deciduous canines over 50 Ma of hyracoid evolution. We proposed two evolutionary and developmental hypotheses to explain these original hyracoid dental characteristics. (a) The presence of a vestigial teeth on the maxilla in front of a complex deciduous canine could be interpreted as extra‐teeth reminiscent of early placental evolution or sirenians, an order phylogenetically close to hyracoids and showing five premolars. (b) These vestigial teeth could also correspond to third incisors with a position unusually shifted on the maxilla, which could be explained by the dual developmental origin of these most posterior incisors and their degenerated condition. This integrative study allows discussion on the current evolutionary and developmental paradigms associated with the mammalian dentition. It also highlights the importance of nonmodel species to understand dental homologies.     

Time-related PDL: viscoelastic response during initial orthodontic tooth movement of a tooth with functioning interproximal contact-a mathematical model

Most anteroposterior orthodontic movements of posterior teeth have to overcome the "resistance" of adjacent teeth with functioning interproximal contacts. The aim of this study was to develop a mathematical model describing initial posterior tooth movement associated with functioning interproximal contacts in relation to the viscoelastic mechanical behavior of the human periodontal ligament (PDL). A linear viscoelastic 2D mathematical model was modified to depict tipping movement around the center of rotation (C(rot)) of a premolar where tipping is restrained by adjacent teeth. Equilibrium equations were applied taking into account the sagittal moment developed around the C(rot). The constants of the model were analyzed and applied to a numerical model that can simulate short-term tooth creep movement caused by a tipping force. Changes in force magnitude (0.5-3N) and crown length (6-10mm) were analyzed until no movement was observed (steady state). Premolar displacement in contact with adjacent teeth showed a non-linear progression over time with an initial sharp tipping movement followed by a transient period of 2.6-7.1min. As tipping force increased the transient period increased. A similar but smaller effect was observed with an increase in crown length. The premolar initial displacement within the arch (3.2-19.5microm) is about seven-fold smaller than retraction/protraction movement of an incisor. These suggest reduction in tooth displacement when functioning interproximal contact is present and clinically recommend establishing a space in the direction of tooth displacement before tooth movement.     

Measurement of tooth extraction forces in upper incisors

A new method, applied for the first time in this research, was used for measurement of tooth extraction forces. The research has been done in a group of 50 examinees to whom the tooth extraction has been done with lower premolar forceps - forceps "13" and in the control group of 54 examinees in whom the tooth extraction has been done with upper incisive forceps - forceps "1". The measurement instrument registered the extraction forces values in both types of forceps. There was no difference in any parameters in these two groups except in used pressure. While using the forceps "13", a significantly lower tooth extraction force was measured than the force measured while using the forceps "1" (p < 0.001). This means that in clinical work we can already apply noticeably less force using the lower premolar forceps for the extraction of the upper incisors (in the moments of rotation up to 70%). These results are meaningful, because they lead to better and improved instrument solutions and working techniques.     

From Embryo to Adult: The Complete Development and Unusual Replacement of the Dentition of the Tammar Wallaby (Macropus eugenii)

Unlike their reptile-like ancestors with continuous tooth replacement, mammals have evolved to replace each tooth either only once, or not at all. In previous large-scale comparative studies, it has been suggested that this tooth replacement only occurs from a successional dental lamina produced lingually to the primary tooth. This study aims to document the complete tooth development and replacement pattern of the tammar wallaby (Macropus eugenii). The tammar wallaby is a diprotodont marsupial, a group defined by their two procumbent lower incisors. To provide a comprehensive documentation of the spatio-temporal pattern of tooth development, we used Lugol’s Iodine staining and microCT scanning (diceCT) of embryos and pouch young into adulthood, resulting in high resolution 3D models for both soft and mineralised stages of development for all tooth positions. Our results reveal that the eponymous lower incisors are the successional generation at the third incisor locus, where the primary dentition initiates but never erupts. Furthermore, we track the development of the only replacement tooth, the permanent third premolar (P3), from initiation to eruption, and found it develops from the primary dental lamina, mesial to the dP3. This is contrary to the conventional view of lingual replacement from successional lamina in mammals. Our findings indicate that no functional tooth replacement occurs in the tammar wallaby, and expands the diversity of tooth replacement patterns found in mammals. We also conclude that since almost all marsupial and placental mammals produce replacement teeth from the distalmost deciduous premolar, this tooth should be considered homologous in these two groups.

     

Maxillary canine/third premolar transposition in a prehistoric population from Santa Cruz Island, California.

Canine/premolar transposition is rare in both historic and prehistoric Homo sapiens with a known occurrence of less than 0.10%. This report describes a prehistoric population sample from one site (SCrI-3) on Santa Cruz Island, California in which the rate of C/P3 transposition is greater than eight percent, based on nine of 106 adult crania which exhibit the anomaly either uni- or bilaterally. As a means of investigating the etiology of this anomaly, the location of the canine root in adult crania was studied. Root location should indicate tooth bud origin, a factor likely to be under genetic control. In crania with normally erupted canines, the superior portion of the root averages 4.43 mm from alare, while this distance is 8.96 mm for anomalous roots. This difference suggests that during ontogeny the tooth buds for the canine and premolar arose in the wrong (or reversed) places, causing the teeth to erupt anomalously. It is suggested that inbreeding in a small island community resulted in a short-lived appearance of this anomaly at a high frequency.     

The cytology of the dental follicle and adjacent alveolar bone during tooth eruption in the dog

Previous studies from our laboratories have shown that premolar eruption in dogs depends upon the presence of the dental follicle and is independent of root or crown growth or attachment to the oral epithelium. The present study is an analysis of the cellular composition of the dental follicle and the cellular investment of the adjacent walls of the bony crypt before and during eruption of the third and fourth mandibular permanent premolars in young beagle dogs. Four premolar follicles and their adjacent bony crypts were examined at 2-week intervals over 12 weeks before and during eruption of these teeth. Tissues were removed, fixed, processed, and oriented so that each follicle and the adjacent crypt wall could be reproducibly examined in vertical and horizontal planes. Mononuclear cells with abundant cytoplasm, euchromatic nuclei, and prominent nucleoli were present in juxtavascular location in the coronal part of the dental follicle; and these cells increased in number immediately preceding and during tooth eruption in parallel with an increase in osteoclasts on the adjacent crypt wall. These data are interpreted to mean that the coronal part of the dental follicle may coordinate the alveolar bone resorption required for tooth eruption by attracting and directing to the crypt wall a population of mononuclear cells, which either become osteoclasts and/or direct osteoclastic activity during tooth eruption.     

A comparison of the position of the mental foramen in Chinese and British mandibles

The antero-posterior position of the mental foramen was studied in 68 Chinese and 44 British skulls of known or calculated age at death. All skulls showed low pre-mortem tooth loss and had a good occlusion. The position of the foramen was related to the body of the mandible as well as to the standing mandibular teeth using two previously published methods. There was no significant difference in the size of the Chinese and British mandibles. There was a significant difference between the two groups when measurements relating the foramen to the body of the mandible (symphysis menti) were considered, the foraminal position being more distal in the Chinese group. The modal position of the foramen in the Chinese sample was along the long axis of the second premolar, whereas in the British sample it lay between the apices of the first and second premolar. The foraminal position apparently moved distally in both groups with age and this was likely to be associated with mesial tooth drift and age-related attrition.     

Oligodontia and its inheritance in Kerry blue terrier

Defects of the premolar tooth formula (oligodontia, tooth number reduction) were studied in dogs of the Kerry blue terrier breed. For this purpose, a database including 480 individuals of 96 litters was constructed. The occurrence of oligodontia was investigated in pedigree groups with inbred and outbred crosses. No selective mating choice for the anomaly under study was found in the sample. The results indicate that oligodontia is inherited, which requires comprehensive study of its genetic control and search for corresponding genes.     

Cemented annulation as an age criterion in the common marmoset (Callithrix jacchus)

Teeth from 18 marmosets, seven months to 5.8 years of age, were studied histologically. A minimum of one dark and one light band comprised one cemental annulation. Age was estimated by counting the number of annulations and adding four months to premolar and eight months to canine teeth for tooth development. Cemental annulations were most consistent near the gingival border on the labial aspect of the tooth. The method will be valuable in estimating the ages of marmosets and other nonhuman primates.     

Inheritance of Oligodontia in Kerry Blue Terrier Dogs

Defects of the premolar tooth formula (oligodontia, tooth number reduction) were studied in dogs of the Kerry Blue Terrier breed. For this purpose, a database including 480 individuals of 96 litters was constructed. The occurrence of oligodontia was investigated in pedigree groups with inbred and outbred crosses. No selective mating choice for the anomaly under study was found in the sample. The results indicate that oligodontia is inherited, which requires comprehensive study of its genetic control and search for corresponding genes.     

Mandibular premolar and second molar root morphological variation in modern humans: What root number can tell us about tooth morphogenesis

This investigation of modern human mandibular premolar root variation tests the hypothesis that population-specific mandibular single-rooted premolar root size can predict a predisposition to root morphological complexity. Mandibular postcanines were examined and quantified from dental radiographs in a globally spread sample of 1,615 modern humans. Multirooted premolars and a fused molar root phenotype were investigated as probes into greater than, and less than, the normative root number. Twelve questions were addressed concerning root structure of mandibular premolars and second molars. A direct correlation was found between single-rooted mandibular premolar size and the predisposition to multirootedness. This correlation infers the following: 1) that postcanine primordia size during root formation predisposes to the development of more, or less, than the normative postcanine root number; and 2) that the epigenetic effect of tooth primordium size per se influences the induction of interradicular processes, which divides the root during its development. This simple developmental model helps explain the following observations: 1) population-specific variation in postcanine root number; 2) sexual dimorphism for multirooted mandibular premolar prevalence; 3) why microdont teeth are single-rooted; 4) the hierarchy of developmental canalization of interradicular processes; 5) megadont-hominin to late-hominin mandibular premolar root number transition; and 6) the fluctuation of mandibular premolar root number in primate evolutionary history.     

Suppressors of the vestigial mutant phenotype increase the level of expression of the gene

Suppressor genes of the vestigial phenotype have been isolated in a wild-type population. These suppressors have an effect on different wing mutants and are allele-specific in the case of vestigial . In a vg^BG background they produced overgrowth of the imaginal wing disc. They also induce cell death in the wild-type strain and alter the distribution of cell death in the mutant strain. Expression of vestigial is increased in the wing disc only. Hypotheses formed to determine the nature of these suppressors are in favor of a direct interaction between these genes and vestigial .     

A tooth wear scoring scheme for age estimation of the Eurasian lynx (<Emphasis Type="Italic">Lynx lynx</Emphasis>) under field conditions

Within the framework of conservation actions for the Eurasian lynx (Lynx lynx), ageing of individuals is required to assess suitability for translocation and to investigate population dynamics and disease epidemiology. We aimed to develop a standardised ageing tool for free-ranging Eurasian lynx, which would be non-invasive, time- and cost-effective, and applicable under field conditions. We used tooth pictures of 140 free-ranging lynx of known age from Switzerland. Tooth colour, calculus, number of incisor teeth and canine, premolar and molar tooth wear were recorded according to pre-defined criteria. Statistical comparisons among the categories of each criterion revealed significant differences for all criteria. Tooth colour and canine tooth morphology showed obvious and consistent alterations with age. Together with the molar tooth shape, premolar tooth tips, incisor teeth and amount of calculus, they pictured the age-related changes in lynx dentition. Crown fractures, enamel flaking and open pulp cavities were observed with an increasing prevalence over age but were also sporadically seen in juveniles. Based on the obtained results, we developed a classification scheme distinguishing six age classes: <?1 year, 1–2 years, 3–6 years, 7–9 years, 10–13 years, ≥?14 years. The scheme was subsequently tested with the same lynx. Classification success rates of different readers ranged from 69 to 88% but errors did not exceed one age class. The homogenous tooth replacement pattern observed in lynx <?1 year allowed us to develop a separate ageing chart to age juveniles in months. The proposed scheme is a promising tool to objectively assign lynx to meaningful age categories.     

Bilateral "molarization" of teeth erupted in the region of the second mandibular premolars

A very large tooth, found in the region of the second mandibular premolar, in a subject, Jewish immigrant from Iraq, is in all likelihood an expression of a familial trait of disturbances in genesis of the shape of dental buds. The appearance of any primitive realization in this case can be conceivable and it was discussed.     

贵州六盘水发现褐腹长尾鼩鼱滇西亚种（Episoriculus caudatus umbrinus）

在贵州省六盘水市杨梅乡慕尼克村,利用陷阱法捕捉到褐腹长尾鼩鼱滇西亚种（Episoriculus caudatus umbrinus）雌性2号。本次采集的标本体型较小,头体长分别为57.2 mm和50.4 mm,尾长（53.2 mm和46.7 mm）略短于头体长。尾部背腹色差较小,腹面略呈现褐色。后足长分别为11.9 mm和12.4 mm。颅全长17.28 mm和17.23 mm,上颌骨较宽（5.02 mm和4.96 mm）与腭齿长（7.72 mm和7.56 mm）之比为65.31%。上门齿1枚,上单尖齿4枚,第1单尖齿和第2单尖齿大小近似,明显大于第3单尖齿,第4单尖齿微小,着生于第3单尖齿和前臼齿缝隙内侧。上颌1枚前臼齿,3枚臼齿,第3枚臼齿小。利用已报道的长尾亚洲鼩属检索表进行检索,鉴定上述采集物种为褐腹长尾鼩鼱滇西亚种。基于Cyt b基因全序列（1 140 bp）,采集标本与长尾亚洲鼩属中的褐腹长尾鼩鼱滇西亚种遗传距离最近,在0.030 ~ 0.062之间。系统发生树也显示,2号标本与褐腹长尾鼩鼱滇西亚种构成一个单系进化分支,进一步证实本次采集2号标本是褐腹长尾鼩鼱滇西亚种。     

Hereditary vestigial pulmonary arterial trunk and related defects in rabbits.

In a population of IIIVO/J rabbits at the Jackson Laboratory a vestigial pulmonary arterial trunk is the commonest expression of a malformation involving the heart and great vessels. There appears to be a series of effects from a completely absent pulmonary trunk through a vestigial but patent pulmonary trunk and/or pulmonary valve stenosis on the one hand to a vestigial or absent ascending aorta on the other. In a few cases the pulmonary trunk is bulbous and the ductus arteriosus is vestigial or absent. In almost all cases of this syndrome, there is also a high ventricular septal defect. The animals appear perfectly normal in every other respect. In its extreme forms this mutation is lethal, but those few animals that are somewhat less severely affected may live for a short time. No effect was seen on birth weight or litter size. Inheritance of vestigial pulmonary arterial trunk appears to be due to two autosomal recessive factors both of which must be homozygous for the expression of the condition. We propose the symbols vpt-1 and vpt-2 for the genes responsible for vestigial pulmonary arterial trunk and its related abnormalities.