Analysis of Fused Teeth in p53-deficient exencaphalic Mice

Fused midline upper incisor teeth are only very rarely encountered in mice, although they are relatively frequently observed in association with experimentally induced severe neural tube defects in this species. In one particularly large study, Knudsen (1965) exposed pregnant mice to a high dose of vitamin A and observed some degree of fusion of the upper incisor dentition in 48% of cases. These were subdivided into 3 categories depending on the degree of fusion, from partial fusion (fusio partialis) in which fusion was only observed in the central parts of the tooth germs, to subtotal fusion (fusio subtotalis) in which both the anterior (incisal) and central parts were fused while the most posterior (or basal) parts of the 2 tooth germs (i.e. the roots) were still present as separate units, through to the third category in which complete fusion (fusio totalis) was present and extended from the incisal to the basal end of the tooth germs. The incidence of the 3 types of fusion observed was 7.7%, 13.1% and 79.2%, respectively, and in a fourth category 'contact', but with no evidence of fusion, was observed between the 2 upper incisors in 2 additional cases.

Knudsen(1966) subsequently induced exencephaly by exposing pregnant mice to trypan blue in order to establish whether fused upper incisors were induced as a consequence of exposure to vitamin A, or there was an association between fused teeth and exencephaly. 47 exencephalic fetuses were induced,with only 21 % displaying evidence of fusion of the upper incisor teeth: 7 displayed evidence of fusio totalis, 3 of fusio partialis and no examples were seen of fusio subtotalis. Evidence was observed of 'contact' in 2 additional cases. In man there is also a frequent occurrence of dental anomalies in the presence of a variety of brain malformations.

We had previously noted the presence of fused upper incisor teeth in 5 out of a total of 19 exencephalic p53-1- embryos and newborn mice (Armstrong et al. 1995). Homozygous animals were found to be viable but predisposed to tumours with an early onset (Donehower et al. 1992), and closure of the cephalic neural tube was seen to be perturbed in a significant proportion of embryos.

The original fetuses and newborn analysed and described previously and 2 additional newborn animals were examined in the present study. Because of the complexity of the morphology of some of the fused teeth, we decided to undertake their 3-D reconstruction using Mouse Atlas Project reconstruction methods. Some of the fused teeth seen in these mice appeared on histological examination to have a more complex structure than the fused teeth formerly described by Knudsen (see fig 1).

Figure 1.

(a-e): Intermittent transverse serial sections through the maxillary region of a p53-deficient exencephalic newborn mouse with an example of fused maxillary incisor teeth associated with a dens in dente. Note the presence of 2 discrete basal ends (a, b) which merge together to form a single incisal end (d, e). At the base of the site of union a region of disorganised tissue (b, arrow) gives rise to an involuted element which has the form of a 'tooth within a tooth' (c, arrow). Both the 'crown' of the latter and the labial aspect of the crown of the fused teeth are covered by a very thin layer of preameloblast. A thin layer of dentine is present with an artefactual space separating it from the overlying layer of preameloblast. Definitive enamel has yet to form. The bottom of the sections is directed towards the anterior (or labial) surface of the face, while the top is directed towards the oral cavity. (f-j): Intermittent serial sections through the maxillary region of a p53-deficient newborn mouse with an example of fused maxillary incisor teeth associated on their labial aspect with a median supernumerary 'peg-like' tooth. Note the presence of 3 distinct basal ends (f, g, arrows) 2 of which merge to form a single crown (h-j). While the supernumerary tooth indents the labial surface of the fused teeth (h), it remains a distinct entity throughout its length. The orientation of these sections is similar to those in the previous series (a-e). Both series stained with haematoxylin and eosin.

The grey-level voxel (3-D array of grey values) image of the teeth of each embryo was reconstructed from images of the individual sections and the teeth in the resulting 3-D image were delineated using MAPaint which allows segmentation of arbitrary structures at arbitrary section orientations within the 3-D volume defined by the voxel image. This software was used to define the volumes occupied by the dentine and enamel for visualisation using the commercial 3-D rendering software AVS. The surfaces of each of the identified structures were displayed in 3-D using the isosurface and geometry-viewer modules on a Silicon Graphics Inidigo 2 workstation.

Out of a total of 21 exencephalic p53-deficient embryonic and newborn mice, 6 (28.6 %) possessed fused maxillary incisor teeth. On histological analysis of the 5 examples seen on day 19.5 of gestation and newborn mice, 3 varieties were observed: an example of 'simple' fusion, 3 examples of simple fusion each of which contained a 'dens in dente' ('tooth within a tooth'), and a single example in which the fused teeth were associated with a median supernumerary incisor tooth which, while deeply indenting the labial surface of the fused teeth, was in all locations a completely separate unit. 3-D reconstructions of the fused teeth demonstrated that they were all of the fusio subtotalis variety. No gross abnormalities were observed in the other dentition in these mice, It is noted that in mice fused maxillary incisor teeth are relatively commonly associated with both hypervitaminosis A-induced and trypan blue-induced exencephaly. It is believed that the presence of dens in dente within fused maxillary incisor teeth has only once been reported in mice, and the association between fused maxillary incisor teeth and a median supernumerary incisor tooth has not previously been reported in this species.

Figures 2-6.
2	3-D reconstructions of labial and oral surfaces, respectively, of a normal pair of maxillary incisor teeth from a control newborn mouse (2 a, b, ) and a selection of fused maxillary incisor teeth (3-6). In I example (3 a, b, ), 'simple' fusion between 2 distinct basal ends is observed with the formation of a single crown. In 2 examples (4ci-d and 5 a,b, ), the fused teeth were associated with an involuted element, giving rise in each case to a dens in dente. This is not shown in these reconstructions. An example of fused teeth associated with a supernumerary median 'peg-like' tooth is shown (6 a, b, ). These reconstructions confirm that in each of the cases of fused teeth, these are of the fusio subtotalis variety. In all the examples illustrated in Figures 2-6, the core of the tooth is covered by a thin layer of dentine (shown in black) which is itself covered over most of its surfaces by a very thin layer of preameloblast (shown in pale grey). In the case of I of these teeth (Fig. 4), there are considerable deficiencies in the covering of preameloblast on the outer aspects of the fused teeth (Fig. 4c, d, right and left lateral views, respectively). In Figure 6, only the very thin covering of dentine of the supernumerary tooth is shown (in dark grey).
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Knudsen (1965)
Congenital Malformations of Upper Incisors in Exencephalic Mouse Embryos, Induced by Hypervitaminosis a I. Types and Frequency P. A. Knudsen Acta Odontologica Scandinavica Jan 1965, Vol. 23, No. 1, Pages 71-89: 71-89.

Congenital Malformations of Upper Incisors in Exencephalic Mouse Embryos, Induced by Hypervitaminosis a II. Morphology of Fused Upper Incisors P. A. Knudsen Acta Odontologica Scandinavica Jan 1965, Vol. 23, No. 4, Pages 391-409: 391-409.

Fusion of Upper Incisors at Bud or Cap Stage in Mouse Embryos with Exenceph-ALY Induced by Hypervitaminosis A P. A. Knudsen Acta Odontologica Scandinavica Jan 1965, Vol. 23, No. 5, Pages 549-565: 549-565.

Knudsen 1966
Incisor Germs in Mouse Embryos with Exencephaly Induced by Riboflavin Deficiency P. A. Knudsen Acta Odontologica Scandinavica Jan 1966, Vol. 24, No. 5, Pages 555-562: 555-562.

Congenital Malformations of Lower Incisors and Molars in Exencephalic Mouse Embryos, Induced by Hypervitaminosis A P.A. Knudsen Acta Odontologica Scandinavica Jan 1966, Vol. 24, No. 1, Pages 55-71: 55-71.

Congenital Malformations of the Jaws and Related Structures in Exencephalic Mouse Embryos with Anomalous Molar Germs, Induced by Hypervitaminosis A P. A. Knudsen Acta Odontologica Scandinavica Jan 1966, Vol. 24, No. 6, Pages 677-707: 677-707.

Malformations of Upper Incisors in Mouse Embryos with Exencephaly, Induced by Trypan Blue P. A. Knudsen Acta Odontologica Scandinavica Jan 1966, Vol. 24, No. 6, Pages 647-675: 647-675.

Donehower 1992
Mice deficient for p53 are developmentally normal but susceptible to spontaneous tumours. Donehower LA, Harvey M, Slagle BL, McArthur MJ, Montgomery CA Jr, Butel JS, Bradley A. Nature. 1992 Mar 19;356(6366):215-21.