Retinoic acid-induced alterations in the expression of growth factors in embryonic mouse palatal shelves

Spatial distribution of epidermal growth-factor transcripts and effects of exogenous epidermal growth factor on the pattern of the mouse dental lamina

The initiation of odontogenesis is characterized by the site-specific proliferation of mandibular epithelium in the formation of the dental lamina. The epidermal growth factor (EGF) gene is expressed in the developing mandible immediately before the appearance of the dental lamina. This expression is necessary for the formation of the dental lamina and subsequent development of teeth. Previous work has demonstrated that retinoids and EGF may interact in the establishment of the pattern of the dentition. In the present study explanted mandibles that were treated with exogenous EGF (40 ng/ml of medium) contained supernumerary buds of mandibular epithelium in the diastema region. These pattern changes were the same as in previous retinoid-treated explants. These results, in addition to the previously reported effects of retinoids on the expression of the EGF gene, support the hypothesis that retinoids and EGF interact in controlling, at least in part, the pattern of the dentition by affecting the pattern of the dental lamina. The spatial distribution of EGF transcripts was also characterized. The location of EGF transcripts in the mesenchyme adjacent to the mandibular epithelium suggests a paracrine mechanism in the stimulation of epithelial proliferation in the formation of the dental lamina.

Prenatal craniofacial development: new insights on normal and abnormal mechanisms

Technical advances are radically altering our concepts of normal prenatal craniofacial development. These include concepts of germ layer formation, the establishment of the initial head plan in the neural plate, and the manner in which head segmentation is controlled by regulatory (homeobox) gene activity in neuromeres and their derived neural crest cells. There is also a much better appreciation of ways in which new cell associations are established. For example, the associations are achieved by neural crest cells primarily through cell migration and subsequent cell interactions that regulate induction, growth, programmed cell death, etc. These interactions are mediated primarily by two groups of regulatory molecules: "growth factors" (e.g., FGF and TGFalpha) and the so-called steroid/thyroid/retinoic acid superfamily. Considerable advances have been made with respect to our understanding of mechanisms involved in primary and secondary palate formation, such as growth, morphogenetic movements, and the fusion/merging phenomenon. Much progress has been made on the mechanisms involved in the final differentiation of skeletal tissues. Molecular genetics and animal models for human malformations are providing many insights into abnormal development. A mouse model for the fetal alcohol syndrome(FAS), a mild form of holoprosencephaly, demonstrates a mid-line anterior neural plate deficiency which leads to olfactory placodes being positioned too close to the mid-line, and other secondary changes. Work on animal models for the retinoic acid syndrome (RAS) shows that there is major involvement of neural crest cells. There is also major crest cell involvement in similar syndromes, apparently including hemifacial microsomia. Later administration of retinoic acid prematurely and excessively kills ganglionic placodal cells and leads to a malformation complex virtually identical to the Treacher Collins syndrome. Most clefts of the lip and/or palate appear to have a multifactorial etiology. Genetic variations in TGF alpha s, RAR alpha s, NADH dehydrogenase, an enzyme involved in oxidative metabolism, and cytochrome P-450, a detoxifying enzyme, have been implicated as contributing genetic factors. Cigarette smoking, with the attendant hypoxia, is a probable contributing environmental factor. It seems likely that few clefts involve single major genes. In most cases, the pathogenesis appears to involve inadequate contact and/or fusion of the facial prominences or palatal shelves. Specific mutations in genes for different FGF receptor molecules have been identified for achondroplasia and Crouzon's syndrome, and in a regulatory gene (Msx2) for one type of craniosynostosis.(ABSTRACT TRUNCATED AT 400 WORDS)

The distribution of cellular retinoic acid-binding protein I (CRABPI) and cellular retinol-binding protein I (CRBPI) during molar tooth development and eruption in the rat

The distribution of cellular retinoic acid-binding protein (CRABPI) and cellular retinol binding protein (CRBPI) was studied in a series of prenatal and early postnatal rats, covering the main stages of development and eruption of the molar teeth. CRABPI positive cells were found in the mesenchymal cells of the dental follicle from the cap stage and in the dental papilla from the early bell stage. In the dental papilla, CRABPI positive cells were situated adjacent to the enamel organ in the cervical loop region and in the subodontoblastic region. Newly formed odontoblasts were CRABPI positive for a short period of time. The enamel organ was CRBPI and CRABPI negative, except for the presence of CRABPI positive cells in the internal enamel epithelium over the tip of cusps and in parts of the stratum intermedium. During root formation, CRABPI positive cells were found in the developing periodontal ligament, in the dental papilla adjacent to the epithelial root sheath and in the subodontoblastic zone. During crown formation, CRBPI positive cells were mainly localized to the mesenchymal cells of the dental papilla during the cap stage of crown development. The periosteum of the developing mandible contained CRABPI positive cells while some osteoclasts appeared to show a weak but positive reaction to CRBPI. The findings were considered in terms of the possible significance of retinoid-binding proteins during tooth and bone development.

Interactions between the transforming growth factor beta (TGF beta) and retinoic acid signal transduction pathways in murine embryonic palatal cells

Regulation of expression of transforming growth factor-beta 3 (TGF-beta 3) and the cellular retinoic acid-binding proteins-I and II (CRABP-I, -II) by retinoic acid (RA) and TGF-beta was examined in primary cultures of murine embryonic palate mesenchymal (MEPM) cells. Northern blot hybridization revealed that RA and TGF-beta 1, beta 2 and beta 3 stimulated the expression of TGF-beta 3 mRNA within 24 hours of treatment. RA down-regulated the expression of CRABP-I mRNA and up-regulated the expression of CRABP-II mRNA in a time- and dose-dependent fashion. TGF-beta 1, beta 2 and beta 3 also down-regulated the expression of CRABP-I mRNA, while epidermal growth factor (EGF) and transforming growth factor alpha (TGF-alpha) were without effect. TGF-beta 1 also stimulated a dose-dependent increase in the expression of CRABP-II mRNA. Again EGF and TGF-alpha were without effect. Basic fibroblast growth factor (bFGF) elicited a slight inhibitory effect on CRABP-II and a slight stimulatory effect on CRABP-I mRNA expression. Thus, cells derived from the mammalian developing palate express CRABP-I and CRABP-II mRNAs, both of which may be regulated by RA and TGF-beta. These data constitute the first demonstration of an effect of TGF-beta on the expression of CRABP-I and CRABP-II and provide further evidence for cross-talk between RA and TGF-beta signal transduction pathways.

Spatial distribution of endogenous retinoids in the murine embryonic mandible

Retinoids play an important part in pattern formation during embryonic development. Exogenous retinoids alter the pattern of skeletal, neural and odontogenic tissues. Endogenous retinoids have been demonstrated previously in the murine embryonic mandible, reaching a concentration peak during the initiation of odontogenesis. It was now found that endogenous retinoids are present in a concentration gradient in the embryonic mouse mandible at the time of the initiation of the dental lamina. All-trans-retinoic acid was more concentrated in the incisor region and retinol in the molar region. These results, and the fact that exogenous retinoids produce supernumerary incisors and missing molars, suggest that all-trans-retinoic acid may instruct incisor morphology.

Temporal distribution of endogenous retinoids in the embryonic mouse mandible

Retinoids play an important part in embryonic pattern formation. They are necessary for normal differentiation of odontogenic tissues and, in excess, disrupt the pattern of tooth formation. Excess retinoids produce supernumerary buds of the dental lamina in the diastema region of the mouse embryonic mandible where teeth do not normally form. This effect is coincident with an increase in epithelial proliferation and an alteration in epidermal growth factor mRNA expression (a gene product necessary for tooth formation). It was found by high-performance liquid chromatography that endogenous retinoids are present in the developing murine mandible and that concentrations of some retinoids reach a peak at the time of the initiation of odontogenesis (dental lamina formation).

Cleft palate and open eyelids inducing activity of lorazepam and the effect of flumazenil, the benzodiazepine antagonist

Lorazepam (Ativan, Wyeth) at dosages of 20-36 mg/kg was used to test for developmental toxicity in the mouse embryo/foetus model. Two separate regions were considered: (1) the central nervous system and (2) the roof of the mouth and the eyelids. In the first case a single administration of lorazepam was applied at the very beginning of the 9th gestation day. In the second, it was administered in preliminary tests on two consecutive gestation days between the 11th and 14th days and in later experiments once only on the 13th or 14th gestation day. In the first part of investigations regarding the development of the central nervous system, lorazepam unlike many other neurotropic drugs, was found not to induce any aberrations in the process of the neural tube closure. In the second part, in which palate closure and the temporary closure of eyelids were monitored, it was found that lorazepam does interfere with these processes. In order to test whether lorazepam's neurocristopathic activity can be prevented, suggesting the presence of benzodiazepine receptors in the neural crest cells, we used the benzodiazepine antagonist, flumazenil (Anexate, Roche). The results of these experiments indicated the flumazenil was able to prevent cleft palate and open eyelids cases almost completely if it was administered 3 hr after administration of lorazepam. If the treatments were administered in the reverse order, the frequency of neurocristopathy cases was unaffected, i.e. flumazenil did not influence the teratogenic activity of lorazepam.

The distribution of cellular retinoic acid-binding protein I during odontogenesis in the rat incisor

Retinoids are important molecules in various aspects of embryological development. Here the distribution of cellular retinoic acid-binding protein I (CRABPI) was studied in the continuously growing incisor of adult rats using an affinity-purified rabbit polyclonal antibody. CRABPI was present throughout the presecretory and secretory ameloblast layer. The protein disappeared from that layer during its maturation phase. The adjacent dental mesenchyme of the developing pulp stained positively for CRABPI, especially in the layer immediately beneath the fully differentiated odontoblasts. Little CRABPI was present in the odontoblast layer itself. The distribution of CRABPI, both in the undifferentiated basal region of the incisor tooth and associated with the cells during hard-tissue formation, suggests a role for this molecule during differentiation and hard-tissue genesis.

Patterns of cellular proliferation and airway branching in cultured fetal hamster lung explants

Lungs were removed from hamsters at gestational day 12 and cultured in BGJb medium under the following conditions: (1) alone, (2) with 5% fetal bovine serum (FBS), (3) with defined additives (with and without vitamin A), and (4) with defined additives but lacking both epidermal growth factor (EGF) and vitamin A. Patterns of cell proliferation were determined by immunochemical labeling for bromodeoxyuridine (BrdU) and airway branching was evaluated in each explant. After 4 days in BGJb alone, an orderly but limited branching pattern occurred, and labeling was greater in the epithelium than in the connective tissue. With FBS a relatively normal branching pattern occurred, and labeling was nearly equivalent in connective tissue and epithelium. With defined additives the connective tissue was heavily labeled in the compact region supporting the distorted bronchioles and in the loose flange at the periphery; removing only vitamin A did not alter these patterns. Removing both vitamin A and EGF caused an extensive but distorted epithelial branching pattern that extended to the periphery of the explant (a flange of loose connective tissue did not form); BrdU labeling was sparse. These results indicate that EGF played a major role in contributing to alterations in the growth and development of fetal hamster lung.

Cellular retinoic acid binding protein in the periodontal ligament

As retinoic acid is an important signaling molecule during embryological development. Since periodontal tissues are thought to have fetal-like properties, the present study was to determine the presence and distribution of cellular retinoic acid-binding protein I(CRABPI) in the periodontal tissues of the rat. Following demineralization, wax sections of the molar teeth of four adult male rats were cut in a plane parallel to the molar tooth row. Five representative sections from each jaw were then processed and stained to reveal the presence of CRABP. This protein was identified by immunochemistry utilizing an affinity-purified rabbit polyclonal antibody. The IgG antibody was used at a dilution of 1 microgram ml-1 and immunoreactivity detected with a kit according to protocol. Immunoreactivity was detected with diaminobenzidine and sections counterstained with hematoxylin. Results showed that virtually all the cells of periodontal ligament, the vast majority representing fibroblasts, contained CRABP. Fibroblasts in the gingiva above the level of the alveolar crest also stained positively for CRABP. The periodontal tissues of all three molar teeth were similarly affected. Fibroblasts in other regions of the oral cavity, such as the dermis of the skin of the cheek or the lamina propria of the buccal mucosa or of the diastema region, exhibited little or no staining for CRABP. Because of its properties as a signaling molecule and its ability to interact with other regulatory molecules, retinoic acid and cellular retinoic acid-binding protein may play an important role in the biology of the periodontal tissues.

The effects of epidermal growth factor, transforming growth factors alpha and beta and platelet-derived growth factor on murine palatal shelves in organ culture

Palatal shelves isolated from day-13 embryonic mice were explanted on to the surfaces of collagen gels either singly or in pairs with their medial edges in contact, and cultured submerged in a 1:1 mixture of Dulbecco's modified Eagle's medium/Ham's F12 medium. The medium was supplemented with either 10 ng/ml epidermal growth factor (EGF), 10 ng/ml transforming growth factor alpha (TGF alpha), 1 ng/ml transforming growth factor beta (TGF beta 1) or 2 ng/ml platelet-derived growth factor (PDGF) all in the presence or absence of 2.5% donor calf serum (DCS). Cultures were terminated after 0, 24, 48 or 72 h and processed for histological and immunocytochemical examination. In serum-free medium and medium supplemented with 2.5% DCS the palatal epithelia differentiated in a manner similar to that seen in vivo (oral, keratinization; nasal, pseudostratified, ciliated columnar cells and medial edge, epithelial degeneration). A similar pattern was obtained in serum-free medium supplemented with either EGF or TGF alpha. However in cultures with either EGF or TGF alpha plus 2.5% DCS present in the medium, medial-edge epithelial degeneration was inhibited and the oral epithelia were more heavily keratinized. The mesenchyme of such cultures stained more intensely for various extracellular matrix molecules. In TGF beta 1-supplemented cultures (with, but especially without, serum supplementation) the epithelia were thin, medial-edge epithelial degeneration was marked, and the fibronectin content of the mesenchyme was increased. PDGF prevented medial-edge epithelial degeneration in the presence, but not in the absence, of serum; mesenchymal extracellular molecules were not as prevalent as with the EGF treatment. These results indicate that exogenous growth factors (including those present in serum) exert effects on organ-cultured mouse palatal shelves in a fashion similar to their effects in cell culture and that controlled physiological levels of such factors may be important in mouse palatal development.

Alteration of murine odontogenic patterning and prolongation of expression of epidermal growth factor mRNA by retinol in vitro

Retinoids alter the patterning of murine odontogenesis in vivo and stimulate epithelial proliferation. Because odontogenesis is dependent on proliferation of mandibular epithelium, the effects of retinol on the patterning of odontogenic epithelium were studied. These experiments control for developmental stage, applied retinoid concentration and duration of exposure. Explants exposed for 24 h to 0.1 micrograms/ml of retinol exhibited enhanced odontogenesis. Day-9 mandibles exposed to retinol at 1-5 micrograms/ml had altered epithelial patterns consistent with those in previous in vivo experiments, including supernumerary epithelial buds in regions associated with supernumerary incisors in vivo. These changes were associated with a dose-dependent increase in epithelial proliferation and a prolonged expression of epidermal growth factor (EGF) mRNA. Altered expression of EGF mRNA may be responsible for the disrupted pattern of the dental lamina. This is the first report of a retinoid-induced alteration in EGF mRNA expression.

The effects of transforming growth factor-beta 1 on protein production by mouse embryonic palate mesenchymal cells in the presence or absence of serum

Mouse embryonic palatal mesenchyme cells were cultured on a variety of substrata (plastic, on a collagen gel or within a collagen gel). On each substratum TGF-beta 1 (1 ng/ml) inhibited cell proliferation. Cells cultured within a collagen gel had the lowest rate of proliferation, but were metabolically the most active in terms of incorporation of [3H]-proline into both collagenous and non-collagenous proteins. TGF-beta 1, in the presence of 2.5% donor calf serum stimulated the production of fibronectin and the major collagen types I, III and V. However, in serum-free medium, TGF-beta 1 induced a large reduction in total collagen production, mainly due to an effect on type I collagen, whilst stimulating production of some non-collagenous proteins. Experiments involving combinations of TGF-beta 1 with other growth factors suggested that the different effects of TGF-beta 1 on collagen production, in the presence and absence of serum, may be due to an interaction with platelet-derived growth factor.

TCDD-induced altered expression of growth factors may have a role in producing cleft palate and enhancing the incidence of clefts after coadministration of retinoic acid and TCDD

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is teratogenic in mice, inducing cleft palate and hydronephrosis at doses which are not overtly maternally toxic or embryotoxic. After TCDD exposure the palatal shelves of normal size come into contact, but fail to fuse due to altered differentiation of the medial epithelial cells. These cells continue to express EGF receptors, proliferate, and differentiate into an oral-like stratified squamous epithelium. The present study examines the effect of TCDD on the expression of growth factors which are believed to regulate differentiation and proliferation in the palate. This study also examined the combined effect of TCDD and retinoic acid (RA), since in teratology studies coadministration of these agents results in an enhancement of cleft palate incidence. Embryos were exposed in vivo on Gestation Day (GD) 10 or 12 to TCDD ot TCDD + RA and the palatal shelves were dissected on GD 14-16. Growth factor expression was determined immunohistochemically using antibodies to TGF-alpha, EGF, TGF-beta 1, or TGF-beta 2. The growth factors displayed specific spatial and temporal expression in the palatal shelves. TCDD reduced the expression of TGF-alpha, EGF, and TGF-beta 1 in epithelial and mesenchymal cells. The degree of reduction was generally greater after exposure on GD 10 to TCDD alone or in combination with RA when compared to that on GD 12. The abnormal proliferation and differentiation of TCDD-exposed medial cells may be a response to reduced expression of EGF and TGF-alpha. Low levels of these factors may be related to the previously observed elevated levels of EGF receptors in medial cells. In other systems, low levels of ligand have resulted in upregulation of the EGF receptor. Continued proliferation and altered differentiation could also be attributable to decreased levels of TGF-beta 1, a factor inhibitory to epithelial proliferation. Since TGF-beta 1 stimulates mesenchymal growth and TGF-alpha and EGF stimulate epithelial proliferation, the formation of small shelves after exposure to TCDD + RA on GD 10 may be due to the severe reduction in these factors. Only a slight to moderate reduction in growth factor expression occurs after exposure to TCDD + RA on GD 12 and in this case shelves of normal size form. Since TCDD and RA appear to act in part through pathways that involve TGF-beta 1, in vitro experiments were designed to examine the involvement of TGF-beta 1 in TCDD teratogenicity.(ABSTRACT TRUNCATED AT 400 WORDS)