Innervation of the human upper primary dentition: Implications for understanding tooth initiation and rethinking growth and eruption patterns

Recent comparisons of humans with apes and early fossil hominids have prompted renewed interest in the study of sequences of dental growth and development. Such comparisons, however, rely on certain assumptions about tooth development and dental homology and the biological reality of distinguishing “deciduous” from “permanent” teeth. In light of earlier suggestions by Schwartz that there might be a correlation between nerves and the stem progenitors of tooth classes, and thus between nerve branch number and number of tooth classes, we studied a large sample of ～ 3 month fetuses to elucidate the nature of nerve branching patterns and the development of the primary dentition (i.e., the “deciduous” incisors, canine, and molars, and the first “permanent” molar). Contrary to expectation, variation in nerve branch patterning was the rule. If nerve fibers do have a role in tooth development, it can only be at the time of initiation, with definitive innervation occurring late in tooth development. In taking into consideration the entire span of tooth development—from initiation to innervation to eruption—and the process by which successional teeth arise (each from the external dental epithelium of a predecessor tooth), we suggest that dividing tooth growth and eruption into patterns of the “deciduous” teeth vs. those of the “permanent” is artificial and that a more meaningful approach would be the study of the entire dentition.     

Development of the dentition in Alligator mississippiensis: upper jaw dental and craniofacial development in embryos, hatchlings, and young juveniles, with a comparison to lower jaw development

Development of the upper dentition in Alligator mississippiensis was investigated using a close series of accurately staged and aged embryos, hatchlings, and young juveniles up to 11 days posthatching, as well as some young and old adult specimens. Studies from scanning electron microscopy, light microscopy, acetate and computer reconstructions, radiography and macroscopy were combined to elucidate the details of embryonic dental development, tooth initiation pattern, dentitional growth, and erupted functional dentition. The results were compared with those from the lower jaw and related to the development of other craniofacial structures. Approximately 17 early teeth in each jaw half develop as surface teeth, of which 13 project for 1 to 12 days before sinking into the mesenchyme. The first three teeth initiate directly from the oral epithelium at Ferguson stages 14-15 (days 15-19 after egg laying), before there is any local trace of dental lamina formation. All other teeth develop from a dental prolamina or lamina; and with progressive lamina development, submerged teeth initiate from the aboral end leading to the formation of replacement teeth. All teeth form dentin matrix, but 12 early teeth do not form enamel. Approximately 20 embryonic teeth are resorbed, 6 are transitional, and 42 function for longer periods after hatching. The embryonic tooth initiation pattern (illustrated by defining a tooth position formula) does not support the previous models of Odontostichi, Zahnreihen, and Tooth Families, each of which postulates perfect regularity. Up to three interstitial tooth positions develop between sites of primary tooth initiation, and families with up to five generations at hatching are at first arbitrarily defined.     

First record of an upper deciduous molar in Desmanella (Uropsilinae,Talpidae, Mammalia)

The dP4 of Desmanella engesseri Ziegler, 1985 from a maxillary which includes the P2 and P3 in crypt, the P4 in eruption and fully erupted dP4 and M1, is described from Petersbuch 28 (Germany; Lower Miocene, MN3/4). The maxillary was studied using X-ray microtomography to make detailed images of the internal and external features of the sample. This is the first record of a dP4 from Desmanella Engesser, 1972. Its shape is unique for fossil talpids, possessing the apomorphic feature of replacing the protocone with a large lingual cingulum. The functional use of deciduous teeth and the process that led to the complete loss of milk teeth in extant talpids are discussed.     

Evolutionary and developmental dynamics of the dentition in Muroidea and Dipodoidea (Rodentia, Mammalia)

SUMMARY When it comes to mouse evo‐devo, the fourth premolar–first molar (P4–M1) dental complex becomes a source of longstanding controversies among paleontologists and biologists. Muroidea possess only molar teeth but with additional mesial cusps on their M1. Developmental studies tend to demonstrate that the formation of such mesial cusps could result from the integration of a P4 germ into M1 during odontogenesis. Conversely, most Dipodoidea conserve their fourth upper premolars and those that lost these teeth can also bear additional mesial cusps on their first upper molars. The aim of this study is to assess this developmental model in both Muroidea and Dipodoidea by documenting the morphological evolution of the P4–M1 complex across 50 Ma. Fourteen extinct and extant species, including abnormal and mutant specimens were investigated. We found that, even if their dental evolutionary pathways strongly differ, Dipodoidea and Muroidea retain common developmental characteristics because some of them can present similar dental morphological trends. It also appears that the acquisition of a mesial cusp on M1 is independent from the loss of P4 in both superfamilies. Actually, the progressive decrease of the inhibitory effect of P4, consequent to its regression, could allow the M1 to lengthen and mesial cusps to grow in Muroidea. Apart from these developmental explanations, patternings of the mesial part of first molars are also deeply constrained by morpho‐functional requirements. As there is no obvious evidence of such mechanisms in Dipodoidea given their more variable dental morphologies, further developmental investigations are needed.     

Observations on tooth development and implantation in the upper pharyngeal jaws in Astatotilapia elegans (Teleostei,Cichlidae)

Prominent stages in the development of teeth, associated with the upper pharyngeal jaws in early postembryonic stages of the mouth brooding cichlid A statotilapia elegans were studied on semithin sections in relation to changes in the underlying endoskeletal parts and to the formation of the dentigerous bone. Because the pattern of tooth implantation on infrapharyngobranchial III-IV is constant, at least in early postembryonic stages, it is possible to trace the life history of a given tooth by tracing its homologue throughout the ontogenetic series. A probable causal relationship exists between tooth development and erosion of the underlying cartilage. Fully developed, though unerupted teeth, differentiate annular bone of attachment, which, depending on its position, is formed either outside the cartilage or within the previously induced erosion cavities. Attachment bone of adjacent teeth fuses to build up the dentigerous bone, which, as a result, may be situated within the area previously occupied by cartilage. As soon as the tooth has built up its bone of attachment, it may erupt. The collagenous matrix between tooth and attachment bone persists and gives rise to the movable connection between both. Differentiation of teeth on infrapharyngobranchial III-IV, together with enlargement of the dentigerous bone, proceeds from the lateral and the rostral border, where new germs constantly form. The appearance of new germs on infrapharyngobranchial II is more unpredictable.     

The deciduous dentition ofPercrocuta Kretzoi and the diphyletic origin of the hyaenas (Carnivora,Mammalia)

A juvenile mandible with complete deciduous dentition from the Middle Miocene locality Jianshan near Shilee (Xinan County, Honan Prov., China) belonging toPercrocuta hobeiensis Chen Guanfang & Wu Wenyu is described. The lower deciduous carnassial, which is by far the most informative element of the juvenile dentition, is distinguished from the D₄ of all other hyaenas by fundamental differences and exhibits principal similarities to that of the felids. Although these resemblances are very likely a result of parallel evolution, they can be taken as evidence of a considerable phylogentic distance betweenPercrocuta on one side and the Recent hyaenas with their forerunners on the other side. Judging from the special features of the deciduous carnassial and the high degree of hyaenid adaptation already attained in the Middle Miocene,Percrocuta probably derived from the neighbourhood of the Upper Oligocene and Lower Miocene generaStenoplesictis, Palaeoprionodon andProailurus, which in their turn take up a phylogenetically transitional position between felids and viverrids.     

Plate-like permanent dental laminae of upper jaw dentition in adult gobiid fish, Sicyopterus japonicus

Sicyopterus japonicus (Teleostei, Gobiidae) possesses a unique upper jaw dentition different from that known for any other teleosts. In the adults, many (up to 30) replacement teeth, from initiation to attachment, are arranged orderly in a semicircular-like strand within a capsule of connective tissue on the labial side of each premaxillary bone. We have applied histological, ultrastructural, and three-dimensional imaging from serial sections to obtain insights into the distribution and morphological features of the dental lamina in the upper jaw dentition of adult S. japonicus. The adult fish has numerous permanent dental laminae, each of which is an infolding of the oral epithelium at the labial side of the functional tooth and forms a thin plate-like structure with a wavy contour. All replacement teeth of a semicircular-like strand are connected to the plate-like dental lamina by the outer dental epithelium and form a tooth family; neighboring tooth families are completely separated from each other. The new tooth germ directly buds off from the ventro-labial margin of the dental lamina, whereas no distinct free end of the dental lamina is present, even adjacent to this region. Cell proliferation concentrated at the ventro-labial margin of the dental lamina suggests that this region is the site for repeated tooth initiation. During tooth development, the replacement tooth migrates along a semicircular-like strand and eventually erupts through the dental lamina into the oral epithelium at the labial side of the functional tooth. This unique thin plate-like permanent dental lamina and the semicircular-like strand of replacement teeth in the upper jaw dentition of adult S. japonicus probably evolved as a dental adaptation related to the rapid replacement of teeth dictated by the specialized feeding habit of this algae-scraping fish.     

New ontogenetic evidence on the septomaxilla ofTamandua andCholoepus (Mammalia,Xenarthra), with a reevaluation of the homology of the mammalian septomaxilla

An intramembranous ossification at the anterior end of the cartilaginous nasal capsule is described for the first time in prenatal specimens of the anteaterTamandua and the slothCholoepus and redescribed in prenatal specimens of the armadillosDasypus andZaedyus. From comparisons of this bone with the septomaxilla of monotremes and various Mesozoic mammals, it is concluded that (1) the bone inTamandua andCholoepus is homologous with the central part (processus ascendens) of the bone inDasypus, Zaedyus, and other armadillos and (2) the xenarthran processus ascendens, in turn, is homologous with the central part of the septomaxilla of monotremes and various Mesozoic mammals. Therefore, the bone in question in xenarthrans is a true septomaxilla. It is further concluded that the armadillo septomaxilla has two neomorphic components: a lamina palatina beneath the cartilaginous nasal floor and a processus intrafenestralis extending rostrally into the nasal fossa.     

Sequence and age of eruption of deciduous dentition in the baboon (Papio sp)

A detailed eruption sequence and associated age of eruption for deciduous dentition in baboons (Papio sp) are presented in this paper. The sequence was determined by evaluation and comparison of the number and kinds of teeth present in nine age cohorts comprising the study sample of 88 males and 87 females who ranged in age from birth to 763 days. Eruption was assessed visually as present or absent. Several statistical methods used to derive the ages associated with the eruption sequence are described. The basic eruption sequence in the sample population is: i¹ i₁, i₂, i², \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm c}\limits_{\rm -} {\rm,}\mathop {\rm c}\limits^{\rm -} $\end{document} m¹ (m², m₂), M₁, M¹. Both sexes show the same pattern, with the exception of the second deciduous molar, where males show a sequence of m₂, m₂, while females show the opposite. Posterior dentition shows the greatest gender-specific variation in average age of eruption.     

Australian Oligo-Miocene Mystacinids (Microchiroptera): upper dentition, new taxa and divergence of New Zealand species

The upper dentition of two Australian early Miocene mystacinids, Icaropsparadox and I. aenae, from Riversleigh, Queensland, are described for the first time. Also recognised is a late Oligocene mystacinid from Lake Palankarinna, South Australia. The new fossils help refine understanding about the evolutionary history of mystacinids in Australia, including their temporal and geographical range, possible dietary and roosting habits, and likely separation time of New Zealand mystacinids.     

Spikelet structure and development in Cyperoideae (Cyperaceae): a monopodial general model based on ontogenetic evidence

Background and Aims

In Cyperoideae, one of the two subfamilies in Cyperaceae, unresolved homology questions about spikelets remained. This was particularly the case in taxa with distichously organized spikelets and in Cariceae, a tribe with complex compound inflorescences comprising male (co)florescences and deciduous female single-flowered lateral spikelets. Using ontogenetic techniques, a wide range of taxa were investigated, including some controversial ones, in order to find morphological arguments to understand the nature of the spikelet in Cyperoideae. This paper presents a review of both new ontogenetic data and current knowledge, discussing a cyperoid, general, monopodial spikelet model.

Methods

Scanning electron microscopy and light microscopy were used to examine spikelets of 106 species from 33 cyperoid genera.

Results

Ontogenetic data presented allow a consistent cyperoid spikelet model to be defined. Scanning and light microscopic images in controversial taxa such as Schoenus nigricans, Cariceae and Cypereae are interpreted accordingly.

Conclusions

Spikelets in all species studied consist of an indeterminate rachilla, and one to many spirally to distichously arranged glumes, each subtending a flower or empty. Lateral spikelets are subtended by a bract and have a spikelet prophyll. In distichously organized spikelets, combined concaulescence of the flowers and epicaulescence (a newly defined metatopic displacement) of the glumes has caused interpretational controversy in the past. In Cariceae, the male (co)florescences are terminal spikelets. Female single-flowered spikelets are positioned proximally on the rachis. To explain both this and the secondary spikelets in some Cypereae, the existence of an ontogenetic switch determining the development of a primordium into flower, or lateral axis is postulated.     

Variation of tooth number in mammalian dentition: connecting genetics,development, and evolution

A major question in modern biology is how gene mutations affect development and are translated into macroevolutionary changes in morphology. Variations in tooth number, a strategy used by many mammals to develop specialized dentitions, has been an important factor for species diversification. Changes in the number of teeth tend to occur in the reverse of the order teeth are formed during development, which also characterizes the general pattern of tooth loss observed during the evolution of placental mammals. To understand how changes at the molecular level affect the distinct stages of tooth development, we analyzed the ontogenesis of tooth growth arrest in sciurids and mice and in single and double knockout mutant mice. We show that the complexity of the genetic network that governs tooth development can change during ontogenetic trajectory, and these changes may be related to macroevolutionary changes. Furthermore, we show that the variation in tooth number in the affected members of human families bearing mutations in the MSX1 and PAX9 genes can help to understand how the genetic variations within a population can modulate evolutionary changes in dental patterning.     

Tooth eruption in Reeves' muntjac (Muntiacus reevesi) and its use as a method of age estimation (Mammalia: Cervidae)

The sequence of tooth eruption and replacement in Reeves' muntjac was determined from captive animals of known age. Pronounced sexual dimorphism is shown by the permanent upper canine which in the male is large, tusk-like and is used as a weapon. The upper canine was the first deciduous tooth to be replaced in males, at approximately 21 weeks of age, compared with 53–57 weeks in the female. The permanent mandibular teeth erupted in the order: molars, first and second incisors, premolars, third incisor and canine. The maxillary teeth erupted in the order: first molar, canine (in male), second and third molars, canine (in female), premolars. The full complement of 34 functional permanent teeth was attained by 83–92 weeks of age.     

A new genus (Chardinomys) of murid rodent (Mammalia,Rodentia) from the neogene of China,and comments on its biogeography

Three murid specimens from the late Neogene ofChina are described, one as the new taxon Chardinomys, which is most closely related to Orientalomys. The Orientalomys-Chardinomys complex was widely distributed over the Palearctic region during the late Neogene. Cf. Rattus occurring with the Orientalomys-Chardinomys complex in China suggests South Asian influence in the eastern Palearctic during the late Neogene.