Transplantation of Tooth Germ to the Mandibular Diastema of Mouse Embryo by Exo Utero Surgery
One of the greatest advantages of using avian embryo as a developmental biology model is the accessibility of its embryos throughout the entire developmental process. In the mouse, however, accessing to the developing embryo is always difficult and poses a great challenge due to the complex architecture of the mouse gestation organ. The finding that mouse embryos can develop normally outside the uterine myometrium has substantially extended the accessibility of mouse embryo to a much earlier stage, and moreover, such an exo utero mouse embryo can survive some surgical manipulations to term (Muneoka et al.,1986a,b). Benefiting from these findings, exo utero surgery has evolved to become a powerful tool to study mouse embryonic development (Ngo-Muller and Muneoka,2000). It has been widely used in studies of nerve system development such as migration of the neural crest cells during early mouse embryogenesis (Serbedzija et al.,1992), limb development and regeneration (Han et al.,2003), as well as in teratology research where effects of chemicals on the developmental mouse embryos can be studied. A recent review has summarized the applications of mouse exo utero development system in biological studies (Hatta et al.,2004).
We were originally seeking to identify an ideal site for the best development and growth of ectopically grafted tooth germ or recombinant tooth germ after genetic modification in vitro (Song et al.,2006). Compared with the kidney capsule, the anterior eye chamber, or other ectopic loci, the oral cavity of the mouse appears to represent the most natural environment for tooth development. The diastema, a toothless region between the incisor and molar, has been demonstrated to host a full array of teeth before they were evolutionally lost. Teeth indeed develop in the mandibular diastema of some genetically modified mice (Mustonen et al.,2003; Tucker et al.,2004; Zhang et al.,2003a; Kassai et al.,2005). We, therefore, deduced that the mandibular diastema of the mouse embryo had an intrinsic ability to support tooth development.
Although exo utero surgery can be carried out on mouse embryos as early as embryonic day (E) 11.5, a better survival rate is achieved with surgery on E13.5 or older embryos (Muneoka et al.,1986a). On the other hand, tooth germs of E13.5 mouse embryos are at the bud stage, which provides an ideal size and solidity for surgical operation compared with that from earlier stages. Thus, we began with transplanting E13.5 mouse tooth germs into the mandibular diastema of E13.5 host embryos. Such an arrangement allows the endogenous tooth primordia to be the control, which provides standard developmental indexes side by side for the evaluation of the ectopic tooth growth. Here, we further took advantage of tooth germs from Rosa26-Egfp mouse embryos, of which tissue can be easily detected and distinguished from the host embryo under the fluorescent microscope by activation through fluorescein isothiocyanate (FITC) -filter set.
For exo utero surgery, the incisor tooth germ was isolated from the E13.5 Rosa26-Gfp mouse embryo. At this stage, the incisor primordia is much better for transplantation experiments due to their smaller size compared with the molar tooth germs. It can be easily grafted into a small incision made in the diastema region of host embryos. As described in detail in the Experimental Procedures section, the same stage embryos were properly exposed from the uterus of an anesthetized female mouse. As shown in Figure 1A, an E13.5 host embryo was exposed after opening the uterus and removing the extraembryonic membrane in a timed-pregnant mouse. Under adequate illumination, details of the embryo can be easily defined under the dissection microscope. Before grafting into the small incision made in the diastema region, the isolated incisor germ was carried on a thin copper wire to the embryonic mandible. The tooth germ was then carefully placed into the incision with the epithelial side face up. By piercing the neighboring host tissue with the copper wire, the transplanted tooth germ was fixed at the incision site. Figure 1B shows a transplanted Rosa26-Egfp incisor primordia fixed at the mandibular diastema region of an E13.5 wild-type host embryo. Under the activation light through the FITC-filter set, the ectopic tooth germ was emanating green fluorescent light.
Figure 1. Ex utero surgery for tooth germ transplantation into the mandibular diastema of embryonic day (E) 13.5 mouse embryo. A: An E13.5 mouse embryo is exposed after opening of the uterus and extraembryonic membrane. B: An E13.5 host embryo receives an incisor tooth germ from E13.5 Rosa26-Egfp embryo in the mandibular diastema region. The grafted tooth germ that was fixed by two copper wire pins showed fluorescence under the fluorescent microscope. C: A full-term host mouse carrying the grafted tooth germ, as indicated by fluorescence. AM, amnion membrane; CB, cotton ball; CW, copper wire pin; E, eye; FL, front limb; GT, GFP mouse embryo derived tooth germ; H, head; MD, mandible; MX, maxillary.
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Transplanted Tooth Germ Develops Normally in the Diastema
Histological analysis of a full-term host embryo shows a well-developed tooth germ in the transplanted site (Fig. 2A). Both epithelial and mesenchymal structures of the transplanted incisor are comparable to that of the endogenous incisors (Fig. 2B). To confirm the supportive role of host tissue in the ectopic tooth development, we further examined blood vessel invasion and innervation of the transplanted tooth. By immunohistochemical staining using antibody against laminin, a molecular marker for blood vessel endothelial cells, we were able to detect blood vessels in the dental pulp of both the endogenous and the transplanted teeth (Fig. 2C,D), clearly demonstrating the presence of blood vessels in the transplanted tooth. The invasion of blood vessels provides the transplanted tooth with nutrients through the blood supplies from the host. Moreover, using antibodies against neurofilament, we have detected nervous termini around the transplanted tooth, indicating that the transplanted tooth will have the similar potential as the endogenous teeth to be innervated (Fig. 2E,F). Thus, the transplanted tooth germ develops synchronically with the endogenous tooth in the host embryo. Based on the facts that blood vessels are not present in the developing mouse tooth before the cap stage (E14.5; Luukko et al.,2005) and that tooth innervation does not occur until birth (Tsuzuki and Kitamura,1991; Kettunen et al.,2005), we conclude that these blood vessel cells and neuron cells within or surrounding the transplanted tooth are derived from the host.
Figure 2. Development of transplanted tooth germ in the term host mouse. A: A coronal section through a host mouse head shows a grafted incisor germ (GI) in the mandibular diastema adjacent to the endogenous incisor germ (EI). B: Higher magnification of A shows a well developed ectopic incisor (arrow), comparable to the adjacent endogenous one. C,D: Innmunohistochemical staining shows cells positive for laminin, a molecular marker for blood vessel endothelial cells, in the dental pulp of a transplanted incisor in a host mouse at birth (C), compared with the endogenous incisor at the same age (D). E,F: Immunostaining for neurofilament shows nerve termini (arrow) around a transplanted incisor (E), and in the endogenous control tooth (F). AB, alveolar bone; AM, ameloblast; BV, blood vessel; D, dentin; DP, dental pulp; EI, endogenous incisor; GI, grafted incisor; OB, odontoblast; T, tongue; N, nerve.
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Of interest, when E13.5 embryos were used as hosts for tooth transplantation, all the hosts (21 in total) that were monitored died within 1 day after caesarian section. Examination of gross morphology identified the cleft palate phenotype in all the surgical embryos, explaining the postnatal lethality. It is known that the palate shelves elevated from a perpendicular to the horizon position and fused to each other above the tongue between E13.5 and E14.5. Therefore, our surgical operation in the oral cavity of E13.5 embryo must have disturbed such an elevation and/or fusion process of the palate shelves, resulting in the cleft palate formation.
Because the palate shelves have already fused in E14.5 mouse embryos, we decided to use E14.5 mouse embryos as host for tooth transplantation, in hope of getting living pups with an ectopic tooth in their mouth. As expected, none of the pups (25 in total) that received tooth germ transplantation at E14.5 developed a cleft palate phenotype. The mice were fed by a foster mother immediately after C-section, and grew up as normal mice. Figure 3 shows such a weaned mouse carrying an ectopic incisor growth in the mandibular diastema. These results clearly demonstrate that the transplanted tooth germ can develop normally in the diastema region of mouse embryo and can eventually erupt and form as a normal tooth in the host mouse oral cavity.
Figure 3. Growth of a transplanted incisor in the mandibular diastema of a host mouse. The grafted tooth erupted and grew at the comparable size as the endogenous ones. LI, lower incisor; GI, grafted incisor; UI, upper incisor; N, nasal
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The mechanism of how some vertebrates lost their teeth during evolution remains elusive. It could be a result of loss of gene function that is critical for tooth development. In avian, the remnant of developing tooth primordia can be found in the oral cavity too, and the abortion of tooth development is at least partially due to the loss of odontogenic Bmp4 expression (Chen et al.,2000). On the other hand, tooth agenesis could also be a result of an active inhibitory mechanism that is present in the oral cavity to repress tooth development. It is evidenced that Gas1, a diffusible protein acting as Shh antagonist, is able to sequester Shh and, therefore, to block Shh function in the mouse diastema region. Such an antagonistic mechanism may be used to delineate the border of tooth forming sites and the diastema region at the beginning of tooth development, or required to constantly inhibit tooth development within the diastema (Cobourne et al.,2004). Nevertheless, we show here that, at or after E13.5, the mandibular diastema of mouse embryo serves as an excellent site to support tooth development. Our results suggest that the grafted tooth germ is able to override any inhibitory mechanism or can develop independent of the inhibitory mechanism. Alternatively, the constant inhibitory mechanism discussed above may have disappeared in the diastema region at the time when tooth germ is grafted.
Similar to many other ectopic sites in living animals, particularly the mouse kidney capsule, the embryonic diastema region appears to be excellent for grafted tooth development and differentiation. However, the approach we describe here also has several advantages, such as (1) it makes it possible to study tooth eruption of embryonic lethal mutant mice, and (2) it permits studies on tissue interactions between grafted tooth germ and the host supportive tissues in the oral cavity. On the other hand, this approach has its limitation as well. The size of a graft is a concern. For example, an E13.5 molar germ was proven too big to be grafted properly. In conclusion, while the diastema region of the mouse mandible does not grow endogenous teeth, it does fully support tooth formation when ectopic tooth germ is grafted.