The life history of an embryonic signaling center: BMP-4 induces p21 and is associated with apoptosis in the mouse tooth enamel knot

The enamel knot, a transient epithelial structure, appears at the onset of mammalian tooth shape development. Until now, the morphological, cellular and molecular events leading to the formation and disappearance of the enamel knot have not been described. Here we report that the cessation of cell proliferation in the enamel knot in mouse molar teeth is linked with the expression of the cyclin-dependent kinase inhibitor p21. We show that p21 expression is induced by bone morphogenetic protein 4 (BMP-4) in isolated dental epithelia. As Bmp-4 is expressed only in the underlying dental mesenchyme at the onset of the enamel knot formation, these results support the role of the cyclin-dependent kinase inhibitors as inducible cell differentiation factors in epithelial-mesenchymal interactions. Furthermore, we show that the expression of p21 in the enamel knot is followed by Bmp-4 expression, and subsequently by apoptosis of the differentiated enamel knot cells. Three-dimensional reconstructions of serial sections after in situ hybridization and Tunel-staining indicated an exact codistribution of Bmp-4 transcripts and apoptotic cells. Apoptosis was stimulated by BMP-4 in isolated dental epithelia, but only in one third of the explants. We conclude that Bmp-4 may be involved both in the induction of the epithelial enamel knot, as a mesenchymal inducer of epithelial cyclin-dependent kinase inhibitors, and later in the termination of the enamel knot signaling functions by participating in the regulation of programmed cell death. These results show that the life history of the enamel knot is intimately linked to the initiation of tooth shape development and support the role of the enamel knot as an embryonic signaling center.

Expression patterns of bone morphogenetic proteins (Bmps) in the developing mouse tooth suggest roles in morphogenesis and cell differentiation

Bone morphogenetic proteins (BMP) are secretory signal molecules which have a variety of regulatory functions during morphogenesis and cell differentiation. Teeth are typical examples of vertebrate organs in which development is controlled by sequential and reciprocal signaling between the epithelium and mesenchyme. In addition, tooth development is characterized by formation of mineralized tissues: the bone-like dentin and cementum as well as epithelially derived enamel. We have performed a comparative in situ hybridization analysis of the expression of six different Bmps (Bmp-2 to Bmp-7) starting from initiation of tooth development to completion of crown morphogenesis when dentine and enamel matrices are being deposited. Bmps-2, -4, and -7 were frequently codistributed and showed marked associations with epithelial-mesenchymal interactions. Their expression shifted between the epithelium and mesenchyme starting from the stage of tooth initiation. They were subsequently expressed in the enamel knot, the putative signaling center regulating tooth shape. Their expression domains prior to and during the differentiation of the dentine-forming odontoblasts and enamel-forming ameloblasts was in line with functions in regulation of cell differentiation and/or secretory activities of the cells. The expression of Bmp-3 was confined to mesenchymal cells, in particular to the dental follicle cells which give rise to the cementoblasts, forming the hard tissue covering the roots of teeth. Bmp-5 was expressed only in the epithelial ameloblasts. It was upregulated as the cells started to polarize and intense expression continued in the secretory ameloblasts. Bmp-6 was expressed only weakly in the dental mesenchyme during bud and cap stages. Our results are in line with regulatory functions of Bmps at all stages of tooth morphogenesis. Bmps-2, -4, and -7 are conceivably parts of signaling networks regulating tooth initiation and shape development. They as well as Bmp-5 may be involved in the induction and formation of dentine and enamel, and Bmp-3 in the development of cementum. The remarkable overlaps in the expression domains of different Bmp genes may implicate functional redundancy and/or formation of active heterodimers between different BMPs.

Integration of FGF and TWIST in calvarial bone and suture development

Mutations in the FGFR1-FGFR3 and TWIST genes are known to cause craniosynostosis, the former by constitutive activation and the latter by haploinsufficiency. Although clinically achieving the same end result, the premature fusion of the calvarial bones, it is not known whether these genes lie in the same or independent pathways during calvarial bone development and later in suture closure. We have previously shown that Fgfr2c is expressed at the osteogenic fronts of the developing calvarial bones and that, when FGF is applied via beads to the osteogenic fronts, suture closure is accelerated (Kim, H.-J., Rice, D. P. C., Kettunen, P. J. and Thesleff, I. (1998) Development 125, 1241–1251). In order to investigate further the role of FGF signalling during mouse calvarial bone and suture development, we have performed detailed expression analysis of the splicing variants of Fgfr1-Fgfr3 and Fgfr4, as well as their potential ligand Fgf2. The IIIc splice variants of Fgfr1-Fgfr3 as well as the IIIb variant of Fgfr2 being expressed by differentiating osteoblasts at the osteogenic fronts (E15). In comparison to Fgf9, Fgf2 showed a more restricted expression pattern being primarily expressed in the sutural mesenchyme between the osteogenic fronts. We also carried out a detailed expression analysis of the helix-loop-helix factors (HLH) Twist and Id1 during calvaria and suture development (E10-P6). Twist and Id1 were expressed by early preosteoblasts, in patterns that overlapped those of the FGF ligands, but as these cells differentiated their expression dramatically decreased. Signalling pathways were further studied in vitro, in E15 mouse calvarial explants. Beads soaked in FGF2 induced Twist and inhibited Bsp, a marker of functioning osteoblasts. Meanwhile, BMP2 upregulated Id1. Id1 is a dominant negative HLH thought to inhibit basic HLH such as Twist. In Drosophila, the FGF receptor FR1 is known to be downstream of Twist. We demonstrated that in Twist(+/)(−) mice, FGFR2 protein expression was altered. We propose a model of osteoblast differentiation integrating Twist and FGF in the same pathway, in which FGF acts both at early and late stages. Disruption of this pathway may lead to craniosynostosis.

The enamel knot as a signaling center in the developing mouse tooth

Mammalian tooth forms are produced during development by folding of the enamel epithelium but the molecular mechanisms involved in the formation and patterning of tooth cusps are not understood. We now report that several key signaling molecules found in well-known vertebrate signaling tissues such as the node, the notochord, the apical ectodermal ridge, and the zone of polarizing activity in the limb bud are specifically expressed in cells of the enamel knot, which is a transient cluster of dental epithelial cells. By comparing three-dimensional reconstructions of serial sections following in situ hybridization we localized Sonic hedgehog, Bone morphogenetic proteins-2, -4 and -7, as well as Fibroblast growth factor-4 in nested domains within the enamel knot. We suggest that the enamel knot acts as a signaling or organizing center, which provides positional information for tooth morphogenesis and regulates the growth of tooth cusps.

Cbfa1 is required for epithelial-mesenchymal interactions regulating tooth development in mice

Osteoblasts and odontoblasts, cells that are responsible for the formation of bone and dentin matrices respectively, share several molecular characteristics. Recently, Cbfa1 was shown to be a critical transcriptional regulator of osteoblast differentiation. Mutations in this gene cause cleidocranial dysplasia (CCD), an autosomal dominant disorder in humans and mice characterized by defective bone formation. CCD also results in dental defects that include supernumerary teeth and delayed eruption of permanent dentition. The dental abnormalities in CCD suggest an important role for this molecule in the formation of dentition. Here we describe results of studies aimed at understanding the functions of Cbfa1 in tooth formation. RT-PCR and in situ hybridization analyses show that Cbfa1 has a unique expression pattern in dental mesenchyme from the bud to early bell stages during active epithelial morphogenesis. Unlike that observed in osteoblast differentiation, Cbfa1 is downregulated in fully differentiated odontoblasts and is surprisingly expressed in ectodermally derived ameloblasts during the maturation phase of enamel formation. The role of Cbfa1 in tooth morphogenesis is further illustrated by the misshapen and severely hypoplastic tooth organs in Cbfa1−/− mice. These tooth organs lacked overt odontoblast and ameloblast differentiation and normal dentin and enamel matrices. Epithelial-mesenchymal recombinants demonstrate that dental epithelium regulates mesenchymal Cbfa1 expression during the bud and cap stages and that these effects are mimicked by the FGFs but not by the BMPs as shown by our bead implantation assays. We propose that Cbfa1 regulates the expression of molecules in mesenchyme that act reciprocally on dental epithelium to control its growth and differentiation. Taken together, our data indicate a non-redundant role for Cbfa1 in tooth development that may be distinct from that in bone formation. In odontogenesis, Cbfa1 is not involved in the early signaling networks regulating tooth initiation and early morphogenesis but regulates key epithelial-mesenchymal interactions that control advancing morphogenesis and histodifferentiation of the epithelial enamel organ.

Apoptosis in the developing tooth: association with an embryonic signaling center and suppression by EGF and FGF-4

Apoptosis was localized in developing mouse teeth from initiation of morphogenesis to completion of cusp formation by using modified TUNEL method for serial sections and Nile Blue staining for whole mounts. Apoptosis was first detected at bud stage (E12-E13) in the central cells of the invaginating dental epithelium suggesting involvement of cell death in epithelial budding morphogenesis. During cusp development, apoptotic cells were located in the enamel knots, which are transient clusters of dental epithelial cells proposed to act as signaling centers directing the morphogenesis of tooth cusps. Apoptosis was also detected in other restricted epithelial cell populations including the dental lamina, ameloblasts, as well as stratum intermedium and stellate reticulum cells suggesting that the removal of these epithelial cells occurs by apoptosis. Apoptotic cells, presumably osteoclasts, were also located on the surfaces of the developing alveolar bone. When dissected E13 dental epithelium or mesenchyme were cultured in isolation, apoptotic cells were abundant throughout the tissues, whereas when cultured together, apoptosis was inhibited in both tissues close to their interface indicating that epithelial-mesenchymal tissue interactions prevent apoptosis. Epidermal growth factor (EGF) and fibroblast growth factor-4 (FGF-4) inhibited apoptosis in the dental mesenchyme when applied locally using agarose or heparin-coated acrylic beads, suggesting involvement of these or related growth factors in the prevention of apoptosis in dental tissues in vivo. The spatially and temporally restricted distribution patterns of apoptotic cells suggest multiple roles for programmed cell death in dental development. Of particular interest is the removal of the enamel knots by apoptosis which may terminate their tasks as regulators of the patterning of the tooth cusps. The apical ectodermal ridge (AER) of the limb bud has similar signaling characteristics as the enamel knot, and it also undergoes apoptosis. Hence, apoptosis may be a general mechanism for the silencing of embryonic signaling centers.

Epithelial-mesenchymal signaling during tooth development

Classic studies on experimental embryology have shown that organ development in an embryo is largely regulated by so called inductive tissue interactions which mostly take place between epithelial and mesenchymal tissues. Also in the developing tooth, both morphogenesis and cell differentiation are governed by such interactions. Characteristic features of epithelial-mesenchymal interactions are that they are sequential and reciprocal, i.e. "induction" appears to consist of a chain of signaling events between the tissues. During the last decade, the expression patterns of numerous molecules have been studied in developing organs by in situ hybridization and immunohistology. Many of them have been associated with epithelial-mesenchymal interactions, and it is apparent that same molecules participate in regulation of morphogenesis in a number of different organs. Transcription factors such as Msx-1, Msx-2 and Egr-1, growth factors, including TGF beta's, BMPs, and FGFs, and structural proteins such as syndecan and tenascin are expressed in transient, time and space-specific patterns in many organ rudiments, including the tooth. We have shown by tissue recombination studies that the expression of certain molecules is indeed regulated by epithelial-mesenchymal interactions in the early tooth germ. In particular, during the early stages of morphogenesis, when the dental epithelium induces the condensation of mesenchymal cells around the epithelial bud, the expression of many genes is upregulated in the condensed mesenchyme. Previous experimental tissue recombination studies have indicated that at the same time the capacity to instruct tooth morphogenesis shifts from the dental epithelium to the dental mesenchyme.(ABSTRACT TRUNCATED AT 250 WORDS)

TNF signaling via the ligand-receptor pair ectodysplasin and edar controls the function of epithelial signaling centers and is regulated by Wnt and activin during tooth organogenesis

Ectodermal dysplasia syndromes affect the development of several organs, including hair, teeth, and glands. The recent cloning of two genes responsible for these syndromes has led to the identification of a novel TNF family ligand, ectodysplasin, and TNF receptor, edar. This has indicated a developmental regulatory role for TNFs for the first time. Our in situ hybridization analysis of the expression of ectodysplasin (encoded by the Tabby gene) and edar (encoded by the downless gene) during mouse tooth morphogenesis showed that they are expressed in complementary patterns exclusively in ectodermal tissue layer. Edar was expressed reiteratively in signaling centers regulating key steps in morphogenesis. The analysis of the effects of eight signaling molecules in the TGFbeta, FGF, Hh, Wnt, and EGF families in tooth explant cultures revealed that the expression of edar was induced by activinbetaA, whereas Wnt6 induced ectodysplasin expression. Moreover, ectodysplasin expression was downregulated in branchial arch epithelium and in tooth germs of Lef1 mutant mice, suggesting that signaling by ectodysplasin is regulated by LEF-1-mediated Wnt signals. The analysis of the signaling centers in tooth germs of Tabby mice (ectodysplasin null mutants) indicated that in the absence of ectodysplasin the signaling centers were small. However, no downstream targets of ectodysplasin signaling were identified among several genes expressed in the signaling centers. We conclude that ectodysplasin functions as a planar signal between ectodermal compartments and regulates the function, but not the induction, of epithelial signaling centers. This TNF signaling is tightly associated with epithelial-mesenchymal interactions and with other signaling pathways regulating organogenesis. We suggest that activin signaling from mesenchyme induces the expression of the TNF receptor edar in the epithelial signaling centers, thus making them responsive to Wnt-induced ectodysplasin from the nearby ectoderm. This is the first demonstration of integration of the Wnt, activin, and TNF signaling pathways.

The effect of metastasectomy: fact or fiction?

Metastasectomy in the treatment of solitary metastases has been recommended almost unanimously. The basis for this recommendation has been that 5-year survival after metastasectomy is around 30%, which is just as good as after operation for bronchial carcinoma. It has been assumed, implied, or claimed that the 5-year survival without operation is nil. Control material is, however, lacking. Seventy surgically treated patients were compared with a small, historical control group of 12 patients. There was no difference in 5-year survival. Because of these findings and after a study of the literature, we postulate that patients with lung metastases fulfilling the criteria for operation constitute a selected group with a favorable natural history. Five-year survival, therefore, is an insufficient way of describing the effect of metastasectomy. However, patients with 10-year survival are rare in the literature. In some patients with a favorable tumor-host relationship or with possibilities for effective chemotherapy, cure or prolongation of life has been achieved. Such patients should undergo operation. Randomized studies are needed in all groups for which we do not have sufficiently strong evidence that metastasectomy contributes to the longevity of the patient.

Association of developmental regulatory genes with the development of different molar tooth shapes in two species of rodents

While the evolutionary history of mammalian tooth shapes is well documented in the fossil record, the developmental basis of their tooth shape evolution is unknown. We investigated the expression patterns of eight developmental regulatory genes in two species of rodents with different molar morphologies (mouse, Mus musculus and sibling vole, Microtus rossiaemeridionalis). The genes Bmp-2, Bmp-4, Fgf-4 and Shh encode signal molecules, Lef-1, Msx-1 and Msx-2, are transcription factors and p21CIP1/WAF1 participates in the regulation of cell cycle. These genes are all known to be associated with developmental regulation in mouse molars. In this paper we show that the antisense mRNA probes made from mouse cDNA cross-hybridized with vole tissue. The comparisons of gene expression patterns and morphologies suggest that similar molecular cascades are used in the early budding of tooth germs, in the initiation of tooth crown base formation, and in the initiation of each cusp's development. Furthermore, the co-localization of several genes indicate that epithelial signalling centres function at the three stages of morphogenesis. The earliest signalling centre in the early budding epithelium has not been reported before, but the latter signalling centres, the primary and the secondary enamel knots, have been studied in mouse. The appearance of species-specific tooth shapes was manifested by the regulatory molecules expressed in the secondary enamel knots at the areas of future cusp tips, whilst the mesenchymal gene expression patterns had a buccal bias without similar species-specific associations.

Cerebral protection during open-heart surgery

To reduce the incidence of cerebral damage after open-heart surgery measures were undertaken based on physiological principles and consideration of the possible injury caused by microemboli. Intellectual function was measured by psychometric tests before and after operation. The results were compared with those of an earlier series. There was a striking reduction in the incidence of neurolocial complications. There was also a considerable reduction in the degree of impairment of intellectual function previously shown to develop after open-heart surgery. However, there are still signs that cardiopulmonary bypass brings about subclinical cerebral injuriies. The measures taken and their rationale are discussed. Psychometric testing is a useful method for evaluating the quality of cardiopulmonary bypass as it allows a quantitative assessment of postoperative cerebral function.

Vein invasion in renal cell carcinoma: impact on metastatic behavior and survival

PURPOSE

The development of a thrombus extending into the veins is well recognized in renal cell carcinoma. We investigated the hypothesis that vein invasion alone has no adverse impact on survival but is a highly negative factor in other tumors.

MATERIALS AND METHODS

In 200 consecutive patients invasion of the renal vein and vena cava was evaluated and compared with the clinical course.

RESULTS

A total of 26 patients had vena caval and 47 had renal vein invasion. Patients with venous invasion had a significantly shorter survival but no survival difference was demonstrated based on the level of involvement.

CONCLUSIONS

Our study indicates that vein invasion itself seems to be an important prognostic factor in renal cell carcinoma.

Gene expression patterns associated with suppression of odontogenesis in mouse and vole diastema regions

Rodents have a toothless diastema region between the incisor and molar teeth which may contain rudimentary tooth germs. We found in upper diastema region of the mouse (Mus musculus) three small tooth germs which developed into early bud stage before their apoptotic removal, while the sibling vole (Microtus rossiaemeridionalis) had only a single but larger tooth germ in this region, and this developed into late bud stage before regressing apoptotically. To analyze the genetic mechanisms of the developmental arrest of the rudimentary tooth germs we compared the expression patterns of several developmental regulatory genes (Bmp2, Bmp4, Fgf4, Fgf8, Lef1, Msx1, Msx2, p21, Pitx2, Pax9 and Shh) between molars and diastema buds of mice and voles. In diastema tooth buds the expression of all the genes differed from that of molars. The gene expression patterns suggest that the odontogenic program consists of partially independent signaling cascades which define the exact location of the tooth germ, initiate epithelial budding, and transfer the odontogenic potential from the epithelium to the underlying mesenchyma. Although the diastema regions of the two species differed, in both species the earliest difference that we found was weaker expression of mesenchymal Pax9 in the diastema region than in molar and incisor regions at the dental lamina stage. However, based on earlier tissue recombination experiments it is conceivable that the developmental arrest is determined by the early oral epithelium.

Runx2 (Cbfa1) inhibits Shh signaling in the lower but not upper molars of mouse embryos and prevents the budding of putative successional teeth

Heterozygous mutations in the RUNX2 (CBFA1) gene cause cleidocranial dysplasia, characterized by multiple supernumerary teeth. This suggests that Runx2 inhibits successional tooth formation. However, in Runx2 knockout mice, molar development arrests at the late bud stage, and lower molars are more severely affected than upper ones. We have proposed that compensation by Runx3 may be involved. We compared the molar phenotypes of Runx2/Runx3 double-knockouts with those of Runx2 knockouts, but found no indication of such compensation. Shh and its mediators Ptc1, Ptc2, and Gli1 were down-regulated only in the lower but not the upper molars of Runx2 and Runx2/Runx3 knockouts. Interestingly, in front of the mutant upper molar, a prominent epithelial bud protruded lingually with active Shh signaling. Similar buds were also present in Runx2 heterozygotes, and they may represent the extension of dental lamina for successional teeth. The results suggest that Runx2 prevents the formation of Shh-expressing buds for successional teeth.

Release of adenylate kinase into cerebrospinal fluid during open-heart surgery and its relation to postoperative intellectual function

In 33 of 36 patients having open-heart surgery, the levels of adenylate kinase in cerebrospinal fluid (CSF-AK) were raised. No such increase was seen in 8 patients who had had lung operations without cardiopulmonary bypass. A significant increase in CSF-AK was recorded in 18 patients whose CSF was examined both preoperatively and postoperatively, and this increase was correlated with change in an index of intellectual function. Since there was no evidence of damage to the blood-brain barrier, these findings point to a causal relation between brain-cell injury during cardiopulmonary bypass and release of AK into CSF. Measurement of CSF-AK may therefore prove useful in research to improve the quality of open-heart surgery.

Quality of survival in patients with surgically treated bronchial carcinoma

Quality of survival was studied in 69 surgically treated bronchial carcinoma patients (25% of a total 273 patients in an unselected epidemiological sample). The Carlens vitagram index was used. The quality of survival in patients cured by pulmonary resection was excellent. It was poor in patients who underwent resection and subsequently died, and especially in patients who had non-resectional thoracotomies. The patients who were operated on and later died did not have a better quality of survival than non-surgically treated patients in the same stage. Thus pulmonary resection has no palliative effect in bronchial carcinoma patients who are not cured. The possible benefit of "removing the tumour burden" in patients treated with operation alone can, therefore, be dismissed. The only aim of the operation at present must be to cure.

Argyrophil carcinoid tumors of the lung. Incidence, clinical study, and follow-up of 46 patients

Forty-nine patients with lung tumors initially diagnosed as carcinoids from their cellular appearance and arrangement were originally included in the study. The tumors of 46 of these patients showed an argyrophil reaction with the Grimelius silver stain. The three nonargyrophil tumors were reclassified as noncarcinoid tumors at further examination. The use of the argyrophil technique in characterization of the lung carcinoid tumors is discussed. The incidence of lung carcinoid tumors in the County of Uppsala was 0.7 patients per 100,000 inhabitants per year. Eleven patients with detected disease were symptom-free. The doctor's delay in 9 patients was 3 to 13.8 years. Yet, the prognosis was good, with 5-year survival in 91 percent, 10-year survival in 91 percent, and 15-year survival in 86 percent of the 46 surgically-treated patients. It is concluded that the surgical treatment should at least comprise a lobectomy.

A graphical WWW-database on gene expression in tooth

We have constructed a WWW-site that presents gene expression in developing dental tissues (Gene expression in tooth, http://honeybee.helsinki.fi/toothexp). Our aim is to provide a tool for learning and for research, in particular to facilitate comparisons of the expression patterns of different genes and detection of their coexpression as a starting point for experimental studies. For this reason, we have included schematic illustrations of the gene expression in different stages of development when adequate information has been available. The site also contains a comprehensive list of references. Additionally phenotypic consequences of defective gene expression are indicated. The pages also provide links to other biological databases on the Internet, both as context-dependent and as general links. We welcome submissions from researches elsewhere to include their information of gene expression and function.