The receptors and coding logic for bitter taste

The sense of taste provides animals with valuable information about the nature and quality of food. Bitter taste detection functions as an important sensory input to warn against the ingestion of toxic and noxious substances. T2Rs are a family of approximately 30 highly divergent G-protein-coupled receptors (GPCRs) that are selectively expressed in the tongue and palate epithelium and are implicated in bitter taste sensing. Here we demonstrate, using a combination of genetic, behavioural and physiological studies, that T2R receptors are necessary and sufficient for the detection and perception of bitter compounds, and show that differences in T2Rs between species (human and mouse) can determine the selectivity of bitter taste responses. In addition, we show that mice engineered to express a bitter taste receptor in 'sweet cells' become strongly attracted to its cognate bitter tastants, whereas expression of the same receptor (or even a novel GPCR) in T2R-expressing cells resulted in mice that are averse to the respective compounds. Together these results illustrate the fundamental principle of bitter taste coding at the periphery: dedicated cells act as broadly tuned bitter sensors that are wired to mediate behavioural aversion.

Zeb1 links epithelial-mesenchymal transition and cellular senescence

Overexpression of zinc finger E-box binding homeobox transcription factor 1 (Zeb1) in cancer leads to epithelial-to-mesenchymal transition (EMT) and increased metastasis. As opposed to overexpression, we show that mutation of Zeb1 in mice causes a mesenchymal-epithelial transition in gene expression characterized by ectopic expression of epithelial genes such as E-cadherin and loss of expression of mesenchymal genes such as vimentin. In contrast to rapid proliferation in cancer cells where Zeb1 is overexpressed, this mesenchymal-epithelial transition in mutant mice is associated with diminished proliferation of progenitor cells at sites of developmental defects, including the forming palate, skeleton and CNS. Zeb1 dosage-dependent deregulation of epithelial and mesenchymal genes extends to mouse embryonic fibroblasts (MEFs), and mutant MEFs also display diminished replicative capacity in culture, leading to premature senescence. Replicative senescence in MEFs is classically triggered by products of the Ink4a (Cdkn2a) gene. However, this Ink4a pathway is not activated during senescence of Zeb1 mutant MEFs. Instead, there is ectopic expression of two other cell cycle inhibitory cyclin-dependent kinase inhibitors, p15Ink4b (Cdkn2b) and p21Cdkn1a (Cdkn1a). We demonstrate that this ectopic expression of p15Ink4b extends in vivo to sites of diminished progenitor cell proliferation and developmental defects in Zeb1-null mice.

Death is the major fate of medial edge epithelial cells and the cause of basal lamina degradation during palatogenesis

During mammalian development, a pair of shelves fuses to form the secondary palate, a process that requires the adhesion of the medial edge epithelial tissue (MEE) of each shelf and the degeneration of the resulting medial epithelial seam (MES). It has been reported that epithelialmesenchymal transformation (EMT) occurs during shelf fusion and is considered a fundamental process for MES degeneration. We recently found that cell death is a necessary process for shelf fusion. These findings uncovered the relevance of cell death in MES degeneration; however, they do not discard the participation of other processes. In the present work, we focus on the evaluation of the processes that could contribute to palate shelf fusion. We tested EMT by traditional labeling of MEE cells with a dye, by infection of MEE with an adenovirus carrying the lacZ gene, and by fusing wild-type shelves with the ones from EGFP-expressing mouse embryos. Fate of MEE labeled cells was followed by culturing whole palates, or by a novel slice culture system that allows individual cells to be followed during the fusion process. Very few labeled cells were found in the mesenchyme compartment, and almost all were undergoing cell death. Inhibition of metalloproteinases prevented basal lamina degradation without affecting MES degeneration and MEE cell death. Remarkably, independently of shelf fusion,activation of cell death promoted the degradation of the basal lamina underlying the MEE (`cataptosis'). Finally, by specific labeling of periderm cells (i.e. the superficial cells that cover the basal epithelium), we observed that epithelial triangles at oral and nasal ends of the epithelial seam do not appear to result from MEE cell migration but rather from periderm cell migration. Inhibition of migration or removal of these periderm cells suggests that they have a transient function controlling MEE cell adhesion and survival, and ultimately die within the epithelial triangles. We conclude that MES degeneration occurs almost uniquely by cell death, and for the first time we show that this process can activate basal lamina degradation during a developmental process.

Programmed cell death is required for palate shelf fusion and is regulated by retinoic acid

The actual role of programmed cell death (PCD) in embryonic processes and the extrinsic signals that define the death fate in developing cells are still poorly understood. Here, we show that during secondary palate shelf fusion in the mouse, PCD appeared in the medial edge epithelia (MEE) of the anterior region only after shelf contact. Contact was necessary for efficient cell death activation in the MEE. However, exogenous all-trans-retinoic acid (RA) increased cell death independently of contact. Competence to induce cell death by contact or by RA exposure was obtained when the MEE were close to touch. Endogenous RA is a relevant regulator of the secondary palate PCD since this was reduced by a retinol dehydrogenase inhibitor and an RAR specific antagonist. Bmp-7 expression was positively regulated by RA. However, BMP-7 was unable to activate cell death within the palate tissue and NOGGIN, a natural BMP antagonist, did not block PCD. Reduction of PCD at the MEE directly with a caspase inhibitor or by inhibiting retinol dehydrogenase resulted in unfused palate shelves, but adhesion was not affected. In contrast, exogenous RA also blocked fusion, but in this situation the increased cell death within the MEE appeared to affect adhesion, thereby causing cleft palate in vivo.

TCDD alters medial epithelial cell differentiation during palatogenesis

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widely distributed, persistent environmental contaminant that is teratogenic in mice, where it induces hydronephrosis and cleft palate. The incidence of clefting has been shown to be dose dependent after exposure on either gestation Day (GD) 10 or 12, although the embryo is more susceptible on GD 12. TCDD-exposed palatal shelves meet but do not fuse, and programmed cell death of the medial epithelial cells is inhibited. The mechanism of action through which TCDD alters the program of medial cell development has not been examined in earlier studies, and it is not known whether the mechanism is the same regardless of the dose or developmental stage of exposure. In this study, C57BL/6N mice, a strain sensitive to TCDD, were dosed orally on GD 10 or 12 with 0, 6, 12, 24, or 30 micrograms/kg body wt, in 10 ml corn oil/kg. Embryonic palatal shelves were examined on GD 14, 15, or 16. The degree of palatal closure, epithelial surface morphology, and cellular ultrastructure, the incorporation of [3H]TdR, the expression of EGF receptors, and the binding of 125I-EGF were assessed. After exposure on GD 10 or 12, TCDD altered the differentiation pathway of the medial epithelial cells. The palatal shelves were of normal size and overall morphology, but fusion of the medial epithelia of the opposing shelves did not occur. TCDD prevented programmed cell death of the medial peridermal cells. The expression of EGF receptors by medial cells continued through Day 16 and the receptors were able to bind ligand. The medial cells differentiated into a stratified, squamous, keratinizing epithelium. The shift in phenotype to an oral-like epithelium occurred after exposure on either GD 10 or 12. At the lower dose (6 micrograms/kg), fewer cleft palates were produced, but those shelves which did respond had a fully expressed shift in differentiation. Whether the exposure begins on GD 10 or 12 or whether the dosing level produces only a few cleft palates within a litter, TCDD produced cleft palate by altering the differentiation program of the medial cells.

Etiology of retinoic acid-induced cleft palate varies with the embryonic stage

Retinoic acid (RA) has been shown to be teratogenic in many species, and 13-cis-RA is teratogenic in humans. Exposure to RA during embryonic morphogenesis produced a variety of malformations including limb defects and cleft palate. The type and severity of malformation depended on the stage of development exposed. The purpose of this study was to compare the effects of RA exposure in vivo on different stages of palate development. These results were compared to effects observed after exposure in organ culture. The vehicle used in RA dosing was also shown to be a major factor in the incidence of RA-induced cleft palate. For the in vivo studies, RA (100 mg/kg) in 10 ml corn oil/kg was given p.o. on gestation day (GD) 10 or 12, and the embryos were examined on GD 14 and 16. Exposure to RA in an oil:DMSO vehicle resulted in much higher incidences of cleft palate than were observed after dosing with RA in oil only. After exposure on GD 10, to RA, small palatal shelves formed which did not make contact and fuse on GD 14. The medial cells did not undergo programmed cell death. Instead, the medial cells differentiated into a stratified, squamous, oral-like epithelium. The RA-exposed medial cells did not incorporate 3H-TdR on GD 14 or 16, but the cells expressed EGF receptors and bound 125I-EGF. In contrast, RA-induced clefting after exposure on GD 12 did not involve growth inhibition. Shelves of normal size formed and made contact, but because of altered medial cell differentiation did not fuse. Medial cells differentiated into a pseudostratified, ciliated, nasal-like epithelium. This response was produced in vivo at exposure levels which produced cleft palate, and after exposure of palatal shelves to RA in vitro from GD 12-15. The medial cells exposed on GD 12 incorporated 3H-TdR on GD 14, expressed EGF receptors, and bound 125I-EGF. The responses to RA which lead to cleft palate differed after exposure on GD 10 or 12, and the pathways of differentiation which the medial cells followed depended on the developmental stage exposed.

The effect of prostaglandin E2 and calcium gluconate on orthodontic tooth movement and root resorption in rats

Possible modifications in orthodontic tooth movement (OTM) and root resorption as a result of local injections of prostaglandin E2 (PGE2) alone and with calcium gluconate (Ca) formed the aim of the present study. Twenty-four 8-week-old male Wistar rats were selected and randomly divided into three groups of eight. Both quadrants of the upper jaws of the first group of animals were used; therefore this group comprised two groups: control and normal. The upper left first molars of these eight animals were not placed under orthodontic force and received no injection, to serve as the normal group, considered for root resorption comparison only. The control group had localized submucosal injections of normal saline on the buccal side of the upper right first molar. In the third group, 0.1 ml of 1 mg/ml PGE2 was injected at the same site and the fourth group received an intraperitoneal injection of 200 mg/kg Ca (10%) in addition to the PGE2. All the injections were performed on days 0 and 7. The orthodontic appliance consisted of a closed coil spring ligated to the upper right first molar and incisor, exerting a force of 60 g during the 21-day experimental period, after which the animals were sacrificed. Palatal halves were removed for histological examination and for calculation of the amount of root resorption. Statistical analysis of data showed a significant (P < 0.05) acceleration in OTM after PGE2 injection compared with the control group. The addition of Ca reduced OTM but a significant increase (P< 0.05) was still recorded. A significant difference (P < 0.05) in root resorption was only observed between the PGE2 and normal groups. The findings show the importance of calcium ions working in association with PGE2 in stabilizing root resorption while significantly increasing OTM.

Ah receptor in embryonic mouse palate and effects of TCDD on receptor expression

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most potent member of a family of halogenated aromatic hydrocarbons which are widespread environmental contaminants. In animals the adverse biological effects of TCDD include carcinogenesis, reproductive toxicity, immune function alteration, hyperkeratosis, hepatotoxicity, thymic involution, and teratogenesis. In the mouse embryo, TCDD induces cleft palate through a mechanism which involves altered differentiation and proliferation of the palatal cells, resulting in the failure of opposing shelves to fuse. Cleft palate induction by TCDD requires the Ah receptor. This study examines the expression of the Ah receptor in secondary palate of control and TCDD-exposed C57BL/6N embryos using in situ hybridization, Northern blots, and immunohistochemistry. Ah receptor protein expression was significantly higher in epithelial versus mesenchymal cells, and regional differences in expression within the epithelium were statistically significant. TCDD exposure was shown to downregulate Ah receptor mRNA and protein throughout the palatal shelf and this occurred at both the teratogenic dose and the dose which was not sufficient to produce cleft palate. This study represents the first demonstration of the tissue and cellular localization of the Ah receptor, raising questions about the extrapolation of results from cultured tumor cells to those observed in vivo.

Mesenchymal cells from the human embryonic palate are highly responsive to epidermal growth factor

An established line of mesenchymal cells from the human embryonic palate is highly sensitive to the stimulatory effect of epidermal growth factor on growth, labeled thymidine incorporation, and ornithine decarboxylase activity. The results suggest that epidermal growth factor may play a key role in development of various human embryonic and fetal tissues.

Involvement of GABA in palate morphogenesis and its relation to diazepam teratogenesis in two mouse strains

Previous studies have indicated that serotonin and acetylcholine stimulate palate shelf reorientation. The present studies were undertaken to determine whether gamma-aminobutyric acid (GABA) functions as an inhibitory neurotransmitter in the palate and whether diazepam mimics GABA to inhibit shelf reorientation and cause cleft palate. First, it was shown that 10(-4) M GABA inhibits palate shelf reorientation in day 14.5 AJ embryos cultured for 2 hours. Anterior palate reorientation stimulated by 10(-5) M serotonin was decreased by GABA; 10(-5) M picrotoxin (GABA antagonist) stimulated anterior shelf reorientation and reversed the effect of GABA. Diazepam (10(-4) M) partially inhibited palate shelf reorientation and that stimulated by 10(-5) M serotonin. Diazepam (400 mg/kg) was administered to AJ mice at day 13.5 of gestation and embryos were cultured at day 14.5. The inhibition produced by diazepam was significantly reduced by 10(-5) M picrotoxin. The teratogenic effect of diazepam was compared with AJ and Swiss-Webster Vancouver (SWV) inbred strains. Diazepam produced greater clefting in SWV mice (57% net) than in the AJ (18% net) when compared to their water- and food-starved controls. The greater sensitivity of the SWV strain than the AJ strain to diazepam, as well as to GABA, was also observed in embryo culture. GABA (10(-5) M) markedly inhibited posterior palate reorientation and reversed the stimulation produced by bethanechol in SWV mice. The inhibitory effects of GABA on the posterior palate were partially reversed by picrotoxin. Furthermore, diazepam inhibited palate reorientation either when administered to the pregnant dam or added in embryo culture.(ABSTRACT TRUNCATED AT 250 WORDS)

TCDD exposure of human embryonic palatal shelves in organ culture alters the differentiation of medial epithelial cells

The highly toxic, polychlorinated aromatic compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) occurs as a contaminant throughout the environment. Epidemiology studies of populations accidentally exposed to TCDD have failed to identify TCDD as a human teratogen, but these studies are limited by the small numbers of exposed pregnancies and imprecise estimates of exposure. TCDD is highly teratogenic in mice, inducing cleft palate and hydronephrosis. TCDD exposure in vivo of embryonic mice alters the differentiation and expression of growth factors in the medial epithelial palatal cells. These alterations also occur in rat and mouse palates exposed to TCDD in organ culture. In the present study, human embryonic palatal shelves were cultured in the rodent organ culture system. In order to achieve in vitro the developmental stage at which fusion would normally occur, GD 52 shelves were cultured for 4 days, GD 53 shelves were cultured for 3 days, and GD 54 shelves were cultured for 3 days. Three of four palatal shelves exposed to 5 x 10(-11) M TCDD were identical to their homologous controls (right shelf cultured with control medium; left shelf cultured with TCDD-containing medium). TCDD at 1 x 10(-7) M produced cytotoxicity detected by transmission electron microscopy (TEM). Exposure to 1 x 10(-8) M TCDD resulted in continued incorporation of thymidine ([3H]-TdR detected autoradiographically) by palatal medial cells, failure of the medial peridermal cells to degenerate as observed by scanning electron microscopy (SEM), and differentiation into a stratified, squamous epithelium. These alterations are identical to those induced by TCDD in vitro in rat and mouse palatal cells. The main difference between these species is the level of TCDD required to elicit the responses. Cultured mouse palates respond to 5 x 10(-11) M TCDD with altered medial cell differentiation, and 1 x 10(-10) M TCDD is cytotoxic. The rat shelves respond with altered differentiation at 1 x 10(-8) M and cytotoxicity at 1 x 10(-7) M. All the human shelves respond at 1 x 10(-8) M TCDD with altered differentiation, 1 out of 4 responded at 5 x 10(-11) M, and cytotoxicity occurred at 1 x 10(-7) M. The present data suggest human embryonic palates are less sensitive than those of the C57BL/6N mouse, and that exposure to high levels of TCDD would be required to elicit altered differentiation in the palatal shelf.

RT-PCR quantification of AHR, ARNT, GR, and CYP1A1 mRNA in craniofacial tissues of embryonic mice exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin and hydrocortisone

C57BL/6N mouse embryos exposed to hydrocortisone (HC) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) develop cleft palate. An interaction between these agents produces clefts at doses which alone are not teratogenic. The glucocorticoid receptor (GR) and dioxin receptor (AhR) mediated these responses and their gene expression was altered by TCDD and/or HC in palates examined on gestation day (GD) 14 by Northern blot analysis and in situ hybridization. The present study quantifies AhR, AhR nuclear translocator (ARNT), and GR mRNA at 4, 12, 24, and 48 h after exposure (time 0 = dose administration at 8 A.M. on gestation day 12) on GD12 to TCDD (24 micrograms/kg), HC (100 mg/kg) or HC (25 mg/kg) + TCDD (3 micrograms/kg). The induction of CYP1A1 mRNA was also quantified at 2, 4, 6, 12, 24, and 48 h for control and TCDD-exposed samples. Total RNA was prepared from midfacial tissue of 4-6 embryos/litter at each time and dose. An RNA internal standard (IS) for each gene was synthesized, which included the gene's primer sequences separated by a pUC19 plasmid sequence. Reverse transcription-polymerase chain reaction (RT-PCR) was performed on total RNA + IS using a range of 5-7 IS concentrations across a constant level of total RNA. PCR products were separated in gels (mRNA and IS-amplified sequences differed by 30-50 bases), ethidium bromide-stained, imaged (Hamamatsu Photonics Systems, Bridgewater, NJ), and quantified with NIH Image. CYP1A1 mRNA was significantly induced in the TCDD-exposed samples at all time points examined (p = 0.005 at 2 h and 0.001 after 2 h). During palatal shelf outgrowth on GD12, AhR mRNA levels increased significantly and this was not affected by treatment with TCDD or HC + TCDD. A significant increase in GR was detected at 24 h (p < 0.05) and this was unaffected by any of the exposures. Expression of ARNT increased at 12 h (p < 0.001); however, treatment with HC or HC + TCDD blocked this increase (p < 0.05). At 24 h, the TCDD-treated embryos had significantly lower ARNT mRNA compared with controls (p < 0.001). The relative overall expression level of the genes was AhR > ARNT > GR. Within individuals, expression of AhR and/or ARNT was highly correlated with GR level. In conclusion, CYP1A1 mRNA was expressed in developing craniofacial tissue and was highly induced by TCDD exposure. AhR, ARNT, and GR mRNA are upregulated in early palatogenesis, although not on the same schedule. The TCDD-induced decrease in ARNT at 24 h after dosing and the HC and HC + TCDD-induced delay in upregulation of ARNT may affect the dynamics of heterodimer formation between AhR and ARNT. The changes in ARNT mRNA level could also affect availability of this transcriptional regulator to interact with other potential partners, and these effects, separately or in combination, may be involved in disruption of normal embryonic development.

Review of the interaction between TCDD and glucocorticoids in embryonic palate

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant that produces adverse biological effects including developmental toxicity and teratogenesis. In the mouse embryo, TCDD induces cleft palate and hydronephrosis. The synthetic glucocorticoid, hydrocortisone (HC), induces cleft palate and a potent, synergistic interaction has been observed between TCDD and HC in C57BL/6N embryonic mice. The morphology and etiology of TCDD- and HC-induced clefts are distinctly different with formation of small palatal shelves following HC exposure and failure of normally-sized shelves to fuse after TCDD treatment. Each exposure also alters expression of several growth factors. When EGF, TGF alpha, EGF receptor, and the TGF beta's are considered as a combinatorial, interacting set of regulators, TCDD and HC each produce a unique pattern of increased and/or decreased expression across the set. The interaction of HC and TCDD results in a cleft palate whose etiology most closely resembles that observed after HC exposure, i.e. small palatal shelves. HC+TCDD-exposure also produces a pattern of growth factor expression which closely resembles that seen after HC. Both TCDD and HC act through receptor-mediated mechanisms and each compound has its own receptor. The Ah receptor (AhR) binds TCDD and the glucocorticoid receptor (GR) binds HC. On gestation day (GD) 14, in the embryonic palate exposed to TCDD, the AhR was downregulated and the GR expression increased. Conversely, following HC exposure, the GR was downregulated and AhR levels were elevated. HC+TCDD produced increased expression of both receptors and this pattern would be predicted to produce HC-like clefts as the GR-mediated responses would result in small palatal shelves. The observed cross-regulation of the receptors is believed to be important in the synergistic interaction between TCDD and HC for the induction of cleft palate.

Comparisons of the effects of TCDD and hydrocortisone on growth factor expression provide insight into their interaction in the embryonic mouse palate

Cleft palate (CP) can be induced in embryonic mice by a wide range of compounds, including glucocorticoids and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Hydrocortisone (HC), a glucocorticoid, retards embryonic growth producing small palatal shelves, while TCDD exposure blocks the fusion of normally sized shelves. TCDD induction of CP involves altered differentiation of the medial epithelial cells. Recent studies indicate that growth factors such as EGF, TGF-alpha, TGF-beta 1, and TGF-beta 2 are involved in palatogenesis, regulating proliferation, differentiation, and extracellular matrix production. A synergism has been observed between HC and TCDD in which doses too low to induce CP alone are able to produce greater than 90% incidence when coadministered. In the present study a standard teratology protocol was performed in C57BL/6N mice to examine the synergism at doses lower than those previously published. Data from this study indicate synergistic interactions at doses as low as 3 micrograms TCDD/kg + 1 mg HC/kg. This extreme sensitivity suggests the involvement of a receptor-mediated mechanism possibly resulting in altered regulation of gene expression. Mechanisms of interaction were further studied by comparing growth of the shelves, fate of the medial epithelium, and expression of growth factor mRNAs and peptides. Pregnant mice were dosed on GDs 10-13 with HC (100 mg/kg sc) or with HC (25 mg/kg sc) + TCDD (3 micrograms/kg orally), doses producing 30% and 99% CP, respectively. The interaction between HC and TCDD results in a small HC-like palate, rather than the morphology typical of TCDD-induced clefting. Both compounds inhibited programmed cell death of the medial epithelium, which instead differentiated into an oral-like epithelium. The alterations in growth factor expression after HC or HC + TCDD were similar. Expression of EGF, TGF-beta 1, TGF-beta 2, and EGF receptor increased in specific palatal regions. Increased levels of mRNA were observed only for TGF-beta 1. The effect of TCDD alone on growth factor expression differ from those seen with HC or HC + TCDD. These divergent effects on growth factor expression may contribute to the differences in shelf size and thus to the different mechanisms of HC and TCDD clefting. Thus the synergism between HC and TCDD may involve similar and potentially additive effects on regulators of proliferation and differentiation in the palate, but additional contributing factors cannot be excluded.

2,3,7,8-Tetrachlorodibenzo-p-dioxin alters embryonic palatal medial epithelial cell differentiation in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is teratogenic in mice, inducing cleft palate and hydronephrosis. After exposure in vivo, TCDD specifically alters differentiation of embryonic palatal medial epithelial cells. In this study, the palatal epithelial cell response to TCDD is determined in vitro. C57BL/6N palatal shelves were placed in organ culture on gestation day (GD) 12 in Richter's improved modified Eagle's medium:Ham's F12 medium (1:1) with 1% fetal bovine serum for 3 or 4 days. Medium contained 0.1% dimethylsulfoxide and TCDD at 0, 10(-13), 10(-12), 10(-11), 10(-10), and 10(-9) M, with some doses at 5 x 10(-11), 7.5 x 10(-11), and 5 x 10(-12) M. Epithelial cell responses to TCDD occurred over a narrow range of concentrations, with maximal response at 5 x 10(-11) M. Cytotoxicity was detected at 1 x 10(-10) M. At a stage when control medial cells ceased proliferation and EGF receptors were not detected immunohistochemically. TCDD-exposed medial cells incorporated [3H]thymidine and high levels of epidermal growth factor receptors were detected. TCDD prevented programmed cell death of medial peridermal cells, and induced a shift in the differentiation of medial cells toward an oral-like phenotype. The responses to TCDD observed after exposure in vitro were indistinguishable from previously reported effects observed after exposure in vivo. In the present study, the distribution of TCDD in the fetus after exposure in vivo was examined. The levels of exposure to TCDD are similar for in vitro and in vivo exposure routes. The levels of TCDD in 1 x 10(-11) to 1 x 10(-10) M solutions (3 to 32 pg/ml) were comparable to levels observed in fetal tissues after in vivo exposure on GD 11 to 30 microns/kg [3H]TCDD, where the palatal shelf contained 1.4 to 3.5. pg TCDD, representing 0.0003% of the total dose. In vivo, TCDD was detected in the GD 11 embryo 3 hr postexposure and the TCDD was equally distributed between the embryonic head and body. At 72 hr postexposure, 0.035% of the total dose was in fetal tissues, and 1% of the TCDD in the fetus was found in the palatal shelf. The present study shows that the palatal epithelium responds to TCDD in vitro in a manner comparable to that observed after in vivo exposure, and that the response occurs at a concentration comparable to in vivo levels in the fetus. The availability of an in vitro system will facilitate studies of TCDD toxicity that are difficult or impossible to perform in vivo, such as comparisons of TCDD effects between species, including human tissues.

Cytological assessment of healing palatal donor site wounds and grafted gingival wounds after application of ozonated oil: an eighteen-month randomized controlled clinical trial

OBJECTIVE

The present study was undertaken to assess the therapeutic effects of topical ozonated oil on early healing of free gingival graft surgical sites.

STUDY DESIGN

Twenty subjects were entered into this triple-blinded, randomized, placebo-controlled clinical trial, designed to evaluate the efficacy of ozonated oil on free gingival graft surgical wounds. Subjects were assigned to either the ozone group, in which ozonated oil was applied to the surgical wound, or the control group, in which non-ozonated oil was used as a control. Patients were postoperatively evaluated by cytological analysis. Cytological analysis consisted of the keratinisation and superficial cell indices measured at baseline, after 24 h, on the 3rd, 7th, 14th and 21st day and 2, 3, 8 and 18 months postoperatively.

RESULTS

Cytological results showed that there was a significant (p < 0.001) improvement in epithelial healing by the 7th, 14th and 21st day and 2, 3 and 8 months postoperatively in the ozone group compared to the control group.

CONCLUSION

The present study showed significant improvement in epithelial healing and gingival health after topical application of ozone-treated plant oil to gingival surgical sites.

Thalidomide for the treatment of recalcitrant oral Crohn's disease and orofacial granulomatosis

It has been suggested that thalidomide may be effective in the management of Crohn's disease, including the associated oral lesions. We detail the clinical response to low-dose thalidomide of 5 patients with clinical features of orofacial granulomatosis or oral Crohn's disease recalcitrant to recognized immunosuppressant therapy. All patients had clinical resolution of their symptoms and signs. Transient somnolence was the only reported adverse effect. Remission was maintained by extending the period between thalidomide doses. Thalidomide should be considered an effective therapy for the short-term treatment of severe orofacial granulomatosis in appropriately counseled patients.

Relationship between oral sensitivity and masticatory performance

The size of a bolus determines how it will be manipulated in the mouth and swallowed. We hypothesized that mucosal sensitivity would be important for masticatory function. The accuracy of solid object size perception, spatial acuity, and food particle size reduction during mastication were measured in 22 healthy adults with/without topical anesthesia of their oral mucosa. Topical anesthesia had no effect on the perception of sphere sizes, but significantly reduced spatial sensitivity. Without anesthesia, there was a correlation between an individual's ability to perceive the sizes of steel spheres (diameter, 4-9 mm) and the sizes of food particles chewed for 15 cycles and at swallowing. There was no correlation between spatial sensitivity and food particle size. We suggest that the stimuli used to test two-point discrimination stimulates only superficial receptors, which involve light touch and are easily anesthetized, while the spheres might excite more deeply-set receptors. The latter appear to be more important for masticatory performance and swallowing.

AhR, ARNT, and CYP1A1 mRNA quantitation in cultured human embryonic palates exposed to TCDD and comparison with mouse palate in vivo and in culture

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is developmentally toxic in many species and induces cleft palate in the C57BL/6N mouse embryo. Palatogenesis in mouse and human embryos involves homologous processes at the morphological, cellular, and molecular levels. In organ culture, mouse and human palates respond similarly to TCDD. The present study quantitates the expression of AhR, ARNT, and CYP1A1 mRNA in human embryonic palates in organ culture. Palatal tissues were exposed to 1 x 10(-10), 1 x 10(-9), or 1 x 10(-8) M TCDD or control medium and sampled at 0, 2, 4, and 6 hours for quantitative RT-PCR using a synthetic RNA internal standard. Similar measurements of CYP1A1 gene expression were collected for mouse palates cultured in this model. In human palates, AhR expression correlated with ARNT and CYP1A1 mRNA expression. TCDD induction of CYP1A1 was time- and concentration-dependent. The expression of these genes presented a uniform and continuous distribution across the group of embryos, with no subset of either high or low expressors/responders. The ratio of AhR to ARNT was approximately 4:1. AhR mRNA increased during the culture period in both treated and control subjects; however, ARNT expression was relatively constant. TCDD did not alter either AhR or ARNT expression in a consistent dose- or time-related manner. Comparison of human and mouse data showed a high correlation across species for the induction of CYP1A1. Human embryos expressed approximately 350 times less AhR mRNA than the mouse, and in earlier studies it was shown that human palates required 200 times more TCDD to produce the same effects. When the morphological, cellular, and molecular responses to TCDD between mouse and human are compared, it seems highly unlikely that human embryos could be exposed to sufficient TCDD to achieve changes in palatal differentiation that would lead to cleft palate.

Retinoic acid alters EGF receptor expression during palatogenesis

Various growth factors are necessary for normal embryonic development and EGF receptors are present in developing palatal shelves of embryonic/fetal mice at least from day 12 of gestation. The medial epithelium of the palatal shelf undergoes a series of developmental events which do not occur in the oral and nasal epithelia. In utero and in organ culture, the control palatal medial epithelium shows a developmental decline in EGF receptors, demonstrated both by a decrease in the binding of antibody to EGF receptors and a decrease in the binding of 125I-EGF; decreases which are not observed in cells of the adjacent oral or nasal epithelium. During this period, medial cells cease DNA synthesis and undergo programmed cell death. Medial epithelial cells exposed to all-trans-retinoic acid continue to express EGF receptors, bind EGF, proliferate, fail to undergo programmed cell death and exhibit a morphology typical of nasal cells. The data suggest that this disturbance by retinoic acid of EGF receptor localization and subsequent alterations in differentiation of the epithelial cells plays a role in the retinoic-acid-mediated induction of cleft palate.

Induction of Cell-Cycle Arrest by all-trans Retinoic Acid in Mouse Embryonic Palatal Mesenchymal (MEPM) Cells

all-trans retinoic acid (atRA), the oxidative metabolite of vitamin A, is essential for normal embryonic development. Also, high levels of atRA are teratogenic in many species and can effectively induce cleft palate in the mouse. Most cleft palate resulted from the failed fusion of secondary palate shelves, and maintenance of the normal cell proliferation is important in this process of shelf growth. To clarify the mechanism by which atRA causes cleft palate, we investigated the effect of atRA on proliferation activity and cell cycle distribution in mouse embryonic palatal mesenchymal (MEPM) cells. atRA inhibited the growth of MEPM cells by inducing apoptosis in a dose-dependent manner. atRA also caused a G1 block in the cell cycle with an increase in the proportion of cells in G0/G1 and a decrease in the proportion of cells in S phase, as determined by flow cytometry. We next investigated the effects of atRA on molecules that regulate the G1 to S phase transition. These studies demonstrated that atRA inhibited expression of cyclins D and E at the protein level. Furthermore, atRA treatment reduced phosphorylated Rb and decreased cdk2 and cdk4 kinase activity. These data suggest that atRA had antiproliferative activity by modulating G1/S cell cycle regulators and by inhibition of Rb phosphorylation in MEPM cells, which might account for the pathogenesis of cleft palate induced by retinoic acid.

Pathogenesis of cleft palate in mouse embryos exposed to 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces cleft palate in mouse embryos. It has been believed that TCDD inhibits palatal fusion by suppression of disappearance of medial edge epithelial (MEE) cells on palatal shelves. However, we found that exencephalic mouse embryos were resistant to the cleft palate-inducing action of TCDD. In the present study, we examined cell kinetics in MEE and palatal mesenchyme in embryos exposed to TCDD with or without exencephaly for elucidation of pathogenesis of cleft palate by TCDD. Pregnant Jcl:ICR mice were given TCDD orally at 40 microg/kg at gestation day (GD) 12.5. Embryos were harvested between GD 13.5 and GD 14.5 and examined for cell kinetics by bromodeoxyuridine (BrdU) and TUNEL methods. Exencephaly was induced by intraperitoneal injection of CdCl(2) at 6 mg/kg at GD 7.5. BrdU-positive cells were decreased in TCDD-treated embryos in MEE and mesenchymal cells. TUNEL-positive cells were detected in MEE both in TCDD-treated and untreated control embryos, as well as in embryos with or without exencephaly. We also measured the gap between shelves between GD 14. 0 and GD 14.5. There were no differences at GD 14.0 between control and TCDD-exposed embryos, but at GD 14.25 and GD 14.5, TCDD-exposed embryos had wider gaps than controls. These findings indicate that cleft palate by TCDD results from poor development of palatal shelves. Teratogenesis Carcinog. Mutagen. 20:73-86, 2000.

AH receptor, ARNT, glucocorticoid receptor, EGF receptor, EGF, TGF alpha, TGF beta 1, TGF beta 2, and TGF beta 3 expression in human embryonic palate, and effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Protein and mRNA for epidermal growth factor (EGF), transforming growth factor-alpha (TGF alpha), EGF receptor, transforming growth factor-beta 1 (TGF beta 1), TGF beta 2, TGF beta 3, glucocorticoid receptor (GR), the aryl hydrocarbon receptor (AhR), and the Ah receptor nuclear translocator (ARNT) were localized in gestational days (GD) 49-59 human embryonic secondary palates. The response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was determined for expression of these genes following palatal organ culture. Craniofacial tissues were shipped in medium from the Human Embryology Laboratory, University of Washington, Seattle, WA. Half of each specimen was cultured in control medium and half in medium containing TCDD at either 1 x 10(-8) or 1 x 10(-10) M. After fixation and paraffin-embedding, sections were examined either immunohistochemically or by in situ hybridization. Expression patterns were determined for each gene for the major stages of palatogenesis and in response to TCDD and compared to previously determined patterns of expression in the same developmental stages of palatogenesis for the mouse (GD49-59 in human palatogenesis corresponds to GD12-16 in the mouse). Human and mouse palates were dissimilar in particular spatiotemporal patterns of expression of these genes. Relative to patterns in mouse palatal development, human tissues demonstrated expression of EGF at early palatal stages, expression of EGF receptor and TGF alpha throughout fusion events, and uniform expression of TGF beta 3 in all epithelial regions without specifically higher levels in the medial cells. The responses to TCDD also differed in patterns of gene expression as well as in concentration required to induce hyperplasia of the medial epithelium. In summary, human palates expressed all of these regulatory genes, responses to TCDD were detected, and comparison between mouse and human palates revealed interspecies variation that may be a factor in each species' response to TCDD, as well as other teratogenic exposures.