Maria Hovorakova

Developmental disorders of the dentition: An update

Dental anomalies are common congenital malformations that can occur either as isolated findings or as part of a syndrome. This review focuses on genetic causes of abnormal tooth development and the implications of these abnormalities for clinical care. As an introduction, we describe general insights into the genetics of tooth development obtained from mouse and zebrafish models. This is followed by a discussion of isolated as well as syndromic tooth agenesis, including Van der Woude syndrome (VWS), ectodermal dysplasias (EDs), oral‐facial‐digital (OFD) syndrome type I, Rieger syndrome, holoprosencephaly, and tooth anomalies associated with cleft lip and palate. Next, we review delayed formation and eruption of teeth, as well as abnormalities in tooth size, shape, and form. Finally, isolated and syndromic causes of supernumerary teeth are considered, including cleidocranial dysplasia and Gardner syndrome.

Origin of the Deciduous Upper Lateral Incisor and its Clinical Aspects

The upper lateral incisor in humans is often affected by dental anomalies that might be explained developmentally. To address this question, we investigated the origin of the deciduous upper lateral incisor (i2) in normal human embryos at prenatal weeks 6–8. We used serial frontal histological sections and computer-aided 3D reconstructions. At embryonic days 40-42, two thickenings of the dental epithelia in an “end-to-end” orientation were separated by a groove at the former fusion site of the medial nasal and maxillary processes. Later, these dental epithelia fused, forming a continuous dental lamina. At the fusion site, i2 started to develop. The fusion line was detectable on the i2 germ until the 8th prenatal week. The composite origin of the i2 may be associated with its developmental vulnerability. From a clinical aspect, a supernumerary i2 might be a form of cleft caused by a non-fusion of the dental epithelia.

Regulation of tooth number by fine-tuning levels of receptor-tyrosine kinase signaling

Much of our knowledge about mammalian evolution comes from examination of dental fossils, because the highly calcified enamel that covers teeth causes them to be among the best-preserved organs. As mammals entered new ecological niches, many changes in tooth number occurred, presumably as adaptations to new diets. For example, in contrast to humans, who have two incisors in each dental quadrant, rodents only have one incisor per quadrant. The rodent incisor, because of its unusual morphogenesis and remarkable stem cell-based continuous growth, presents a quandary for evolutionary biologists, as its origin in the fossil record is difficult to trace, and the genetic regulation of incisor number remains a largely open question. Here, we studied a series of mice carrying mutations in sprouty genes, the protein products of which are antagonists of receptor-tyrosine kinase signaling. In sprouty loss-of-function mutants, splitting of gene expression domains and reduced apoptosis was associated with subdivision of the incisor primordium and a multiplication of its stem cell-containing regions. Interestingly, changes in sprouty gene dosage led to a graded change in incisor number, with progressive decreases in sprouty dosage leading to increasing numbers of teeth. Moreover, the independent development of two incisors in mutants with large decreases in sprouty dosage mimicked the likely condition of rodent ancestors. Together, our findings indicate that altering genetic dosage of an antagonist can recapitulate ancestral dental characters, and that tooth number can be progressively regulated by changing levels of activity of a single signal transduction pathway.

Three-dimensional analysis of the early development of the dentition.

Tooth development has attracted the attention of researchers since the 19th century. It became obvious even then that morphogenesis could not fully be appreciated from two-dimensional histological sections. Therefore, methods of three-dimensional (3D) reconstructions were employed to visualize the surface morphology of developing structures and to help appreciate the complexity of early tooth morphogenesis. The present review surveys the data provided by computer-aided 3D analyses to update classical knowledge of early odontogenesis in the laboratory mouse and in humans. 3D reconstructions have demonstrated that odontogenesis in the early stages is a complex process which also includes the development of rudimentary odontogenic structures with different fates. Their developmental, evolutionary, and pathological aspects are discussed. The combination of in situ hybridization and 3D reconstruction have demonstrated the temporo-spatial dynamics of the signalling centres that reflect transient existence of rudimentary tooth primordia at loci where teeth were present in ancestors. The rudiments can rescue their suppressed development and revitalize, and then their subsequent autonomous development can give rise to oral pathologies. This shows that tooth-forming potential in mammals can be greater than that observed from their functional dentitions. From this perspective, the mouse rudimentary tooth primordia represent a natural model to test possibilities of tooth regeneration.

Shh Expression in a Rudimentary Tooth Offers New Insights into Development of the Mouse Incisor

For teeth as for any organ, knowledge of normal development is essential for the proper interpretation of developmental anomalies in mutant mice. It is generally accepted that tooth formation is initiated with a single signaling center that, in the incisor region, is exclusively related to the development of the functional adult incisor. Here, using a unique combination of computer-aided three-dimensional reconstructions and whole mount in situ hybridization of mandibles from finely staged wild-type mouse embryos, we demonstrate that several Sonic hedgehog (Shh) expression domains sequentially appear in the lower incisor region during early development. In contrast to the single Shh expression domain that is widely assumed to be present in each lower incisor area at ED12.5-13.5, we identified two spatially distinct regions of Shh expression that appear in an anterior-posterior sequence during this period. The initial anterior, more superficially located Shh expression region represented the rudimentary (so-called deciduous) incisor, whereas only the later posterior deeper situated region corresponded to the prospective functional incisor. In the more advanced embryos, only this posterior Shh expression in the incisor bud was detectable as a precursor of the enamel knot. This study offers a new interpretation of published molecular data on the mouse incisor from initiation through ED13.5. We suggest that, as with Shh expression, other molecular data that have been ascribed to the progressive development of the mouse functional incisor at early stages, in fact, correspond to a rudimentary incisor whose development is aborted.

Three-dimensional analysis of molar development in the mouse from the cap to bell stage.

During four days of prenatal development in the mouse, the morphology of the first lower molar moves from the early cap to the bell stage. Five phenomena characterize this period: growth of the tooth germ; development of the cervical loop; histogenesis of the enamel organ; folding of the epithelial-mesenchymal junction associated with cusp formation; and change in cellular heterogeneity in the mesenchyme. All these processes are controlled by epithelial-mesenchymal interactions. These complex histo-morphogenetic events have been documented using histological sections and 3D reconstructions. When combined with functional tests in vitro, this approach allowed searching for possible relationships between simultaneous changes occurring in both the epithelial and ecto-mesenchymal compartments. Parallel changes that occur in the two tissues could result from different mechanisms, as illustrated by the increasing number of pre-odontoblasts and pre-ameloblasts during crown growth. Cell division was involved mainly in the ecto-mesenchyme, while proliferation and cell re-organization occurred in the inner dental epithelium. 3D reconstructions also raised still unsolved questions, such as the possible relationship between cusp size and spatial specification of cell kinetic parameters, changes in cell position within the inner dental epithelium, and tracing cell migration in the mesenchyme during development.

Prenatal Development of Crocodylus niloticus niloticus Laurenti, 1768

Prenatal development in crocodilians represents a very interesting model for comparative studies. As the speed of prenatal development of crocodilians varies depending on incubation conditions, the staging of embryos and fetuses is a very important prerequisite for data correlation. To establish a background for future developmental studies on Crocodylus niloticus, we characterized its prenatal development in a collection comprising 169 animals during embryonic/incubation days 9-70. The characteristics included external morphology, head morphometry, and wet body weight determined before fixation. We documented the external morphology of prenatal Nile crocodiles in a large collection of photographs and described landmarks during the morphogenesis of the head, face and limbs. In the development of the facial processes (medial nasal, lateral nasal, maxillary), three phases could be distinguished: union, separation, reunion. At the free jaw margin, a regular series of prominences was present. The outer aspect of a prominence gave rise to a labial scale, the inner aspect to a tooth. In contrast to mammals (humans and mice), the hindlimbs of C. niloticus developed faster than the forelimbs. We also determined changes in basic measures of the head and of the wet body weight. Both morphological and morphometric characteristics showed an apparent inter-individual variability among animals of the same age. This variability decreased among animals of a similar body weight (irrespective of their age). Body weight can be considered as the most representative and complex parameter for crocodile staging reflecting the overall growth of a whole embryo/fetus.

A case of conjoined twin's cephalothoracopagus janiceps disymmetros

Conjoined twins are rare variants of monozygotic twins, which result from an incomplete late division of the embryonic disk. Here we report the rarest case of conjoined twins - the male cephalothoracopagus janiceps disymmetros - born in prenatal week 30, from the archive of the Department of Teratology of the Institute of Experimental Medicine AS CR in Prague. The crown-rump length of each twin, 21cm, corresponded to prenatal week 22 in a normal gravidity. The head, chest and upper portion of the abdomen of the twins were fused. The anatomical features of these extremely rare conjoined twins and the observed external anomalies as a narrow nose with a single nostril, male hypoplastic genitalia, partially duplicated sella turcica, spina bifida and further abnormalities are described and documented.

Sprouty gene dosage influences temporal-spatial dynamics of primary enamel knot formation

BackgroundThe mouse embryonic mandible comprises two types of tooth primordia in the cheek region: progressive tooth primordia of prospective functional teeth and rudimentary tooth primordia in premolar region – MS and R2. Mice lacking Sprouty genes develop supernumerary tooth in front of the lower M1 (first molar) primordium during embryogenesis. We focused on temporal-spatial dynamics of Sonic Hedgehog expression as a marker of early odontogenesis during supernumerary tooth development.ResultsUsing mouse embryos with different dosages of Spry2 and Spry4 genes, we showed that during the normal development of M1 in the mandible the sooner appearing Shh signaling domain of the R2 bud transiently coexisted with the later appearing Shh expression domain in the early M1 primordium. Both domains subsequently fused together to form the typical signaling center representing primary enamel knot (pEK) of M1 germ at embryonic day (E) 14.5. However, in embryos with lower Spry2;Spry4 gene dosages, we observed a non-fusion of original R2 and M1 Shh signaling domains with consequent formation of a supernumerary tooth primordium from the isolated R2 bud.ConclusionsOur results bring new insight to the development of the first lower molar of mouse embryos and define simple tooth unit capable of individual development, as well as determine its influence on normal and abnormal development of the tooth row which reflect evolutionarily conserved tooth pattern. Our findings contribute significantly to existing knowledge about supernumerary tooth formation.

Sequential Shh expression in the development of the mouse upper functional incisor

The mouse incisor is a frequently used model in studies of the molecular control of organ development. The appropriate interpretation of data on normogenesis is essential for understanding the data obtained in mutant mice. For this reason, we performed a very detailed investigation of the development of the upper incisor in wild-type mice from embryonic day (ED) 11.5 till 14.5. A combination of histology, whole mount in situ hybridization, computer-aided three-dimensional reconstructions, and fluorescent microscopy, has been used. Several sonic hedgehog (Shh) expression domains have been detected in the upper incisor region during early prenatal development. At ED11.5-13.5, there was a single Shh positive domain present in the anterior part of left or right upper jaw arches, corresponding to the epithelial thickening. More posteriorly, a new Shh expression domain appeared in the incisor bud in the developmentally more advanced ED13.5 embryos. At ED14.5, only this posterior Shh expression in the incisor germ remained detectable. This study brings new insights into the early development of the upper incisor in mice and completes the data on normal mouse incisor development. The temporal-spatial pattern of Shh expression reflects the development of two tooth generations, being detectable in two successive, antero-posteriorly located areas in the prospective incisor region in the upper jaw. The first, anterior and superficial Shh expression domain reflects the rudimentary tooth development suppressed during evolution. Only the subsequent, posterior and deeper Shh expression region, appearing at ED13.5, correlates with the prospective upper functional incisor in wild-type mice.