Spatial relations in the oral cavity of cortisone-treated mouse fetuses during the time of secondary palate closure

The control and importance of hyaluronan synthase expression in palatogenesis

Development of the lip and palate involves a complex series of events that requires the close co-ordination of cell migration, growth, differentiation, and apoptosis. Palatal shelf elevation is considered to be driven by regional accumulation and hydration of glycosoaminoglycans, principally hyaluronan (HA), which provides an intrinsic shelf force, directed by components of the extracellular matrix (ECM). During embryogenesis, the extracellular and pericellular matrix surrounding migrating and proliferating cells is rich in HA. This would suggest that HA may be important in both shelf growth and fusion. TGFβ3 plays an important role in palatogenesis and the corresponding homozygous null (TGFβ3^−/−) mouse, exhibits a defect in the fusion of the palatal shelves resulting in clefting of the secondary palate. TGFβ3 is expressed at the future medial edge epithelium (MEE) and at the actual edge epithelium during E14.5, suggesting a role for TGFβ3 in fusion. This is substantiated by experiments showing that addition of exogenous TGFβ3 can “rescue” the cleft palate phenotype in the null mouse. In addition, TGFβ1 and TGFβ2 can rescue the null mouse palate (in vitro) to near normal fusion. In vivo a TGFβ1 knock-in mouse, where the coding region of the TGFβ3 gene was replaced with the full-length TGFβ1 cDNA, displayed complete fusion at the mid portion of the secondary palate, whereas the anterior and posterior regions failed to fuse appropriately. We present experimental data indicating that the three HA synthase (Has) enzymes are differentially expressed during palatogenesis. Using immunohistochemistry (IHC) and embryo sections from the TGFβ3 null mouse at days E13.5 and E14.5, it was established that there was a decrease in expression of Has2 in the mesenchyme and an increase in expression of Has3 in comparison to the wild-type mouse. In vitro data indicate that HA synthesis is affected by addition of exogenous TGFβ3. Preliminary data suggests that this increase in HA synthesis, in response to TGFβ3, is under the control of the PI3kinase/Akt pathway.

The etiology of cleft palate: a 50-year search for mechanistic and molecular understanding

Dates of special, historical significance, such as the 50th anniversary of the founding of the Teratology Society, prompt a desire to pause and look back and contemplate where we began, how far we have come, and consider the future for our scientific endeavors. The study of the etiology of cleft palate extends many years into the past and was a subject of interest to many of the founding members of the Teratology Society. This research area was intensively pursued and spawned a vast portfolio of published research. This article will look back at the state of the science around the time of the founding of the Teratology Society, in the 1950s and 1960s, and track the emergence and pursuit of an interest in an etiology for cleft palate involving failure of palatal fusion. Studies of medial epithelial cell fate and induction of cleft palate by interference with adhesion or fusion span the period from the 1960s to the present time. Teratology Society members have been and continue to be key players in cleft palate research. In this retrospective article, seminal research published by Teratology Society members will serve as a platform to launch the discussion of the emergence of our current understanding of medial epithelial cell differentiation and fate and the potential for these processes to be targets of teratogenic action.

Mesenchymal cell remodeling during mouse secondary palate reorientation

The formation of mammalian secondary palate requires a series of developmental events such as growth, elevation, and fusion. Despite recent advances in the field of palate development, the process of palate elevation remains poorly understood. The current consensus on palate elevation is that the distal end of the vertical palatal shelf corresponds to the medial edge of the elevated horizontal palatal shelf. We provide evidence suggesting that the prospective medial edge of the vertical palate is located toward the interior side (the side adjacent to the tongue), instead of the distal end, of the vertical palatal shelf and that the horizontal palatal axis is generated through palatal outgrowth from the side of the vertical palatal shelf rather than rotating the pre-existing vertical axis orthogonally. Because palate elevation represents a classic example of embryonic tissue re-orientation, our findings here may also shed light on the process of tissue re-orientation in general.

Histomorphologic analysis of the soft palate musculature in prenatal cleft and noncleft A/Jax mice

The two specific aims of this study were as follows: to evaluate the appropriateness of the A/Jax mouse model in the investigation of the key cellular stages in prenatal soft palate morphogenesis and myogenesis; and to describe structural differences in the histomorphology of the soft palate anatomy from cleft and noncleft mice prior to, during, and after palatogenesis. Cleft-induced and control groups of A/Jax mouse embryos from timed pregnancies were harvested sequentially on gestational days 15 to 19. Embryos were weighed and staged for external body morphology. The heads were removed and fixed for light microscopy, sectioned serially in the frontal plane at 10 microns and stained with hematoxylin-eosin to characterize and compare the soft palate musculature. All observations were made at the head depth of the trigeminal ganglion in both age- and stage-matched embryos. The following findings were made: (1) the A/Jax mouse is a suitable animal model for the study of soft palate myogenesis; (2) there were no discernible morphologic differences between the soft palate muscles in cleft and noncleft A/Jax mice when viewed under light microscopy; (3) the soft palate and related muscles were identifiable as muscle fields, in both the cleft and noncleft fetuses, as early as gestational day 15 and as specific muscles at gestational day 18; (4) in both the cleft and noncleft A/Jax fetuses, the soft palate muscles appeared in a sequential anatomic fashion (the palatine aponeurosis appeared first, next the tensor palatini, and then the levator palatini muscles); and (5) in the cleft palate fetuses, both pterygoid plates were angulated and displaced laterally.

Effects of the Ay gene on susceptibility to hydrocortisone fetotoxicity and teratogenicity in mice

Effects of the Ay gene, a coat color gene, on susceptibility to hydrocortisone fetotoxicity and teratogenicity were investigated by using the congenic strain of C57BL/6-Ay (Ay/a) which had been maintained by repeated back-crosses of the Ay gene to the C57BL/6 (a/a) background. Matings were conducted as follows (female x male): group I, a/a; group II, a/a x Ay/a; and group III, Ay/a x a/a. Pregnant females were subcutaneously given daily doses of 0, 12.5, 25, or 50 mg/kg of hydrocortisone on days 10-13 of pregnancy. On day 18 of pregnancy, fetuses were sexed, weighed, and examined for external abnormalities. In group I, the mean fetal weight was significantly decreased at a dose of 25 mg/kg or more. The incidences of cleft palate were 3.2 and 22.7% at 25 and 50 mg/kg, respectively. In group II, in which half of the fetuses were expected to carry the Ay gene, the mean fetal weight was decreased significantly at 12.5 mg/kg or more. The incidence of cleft palate in group II at 50 mg/kg was 44.2%, which was significantly higher than that in group I. In group III, in which maternal mice as well as half of their fetuses carried the Ay gene, a decrease in the mean fetal weight was greater than in group II. In addition, the mean percentage of fetal resorptions was significantly increased at 50 mg/kg. The incidence of cleft palate in group III was significantly increased at 25 mg/kg (10.5%) when compared with those in groups I and II. These results indicate that the Ay gene may be associated with susceptibility to hydrocortisone fetotoxicity and teratogenicity in mice.

Characteristics of growth and palatal shelf development in ICR mice after exposure to methylmercury

A single dose of 25 mg/kg methylmercuric chloride (MeHg) was given orally to gravid ICR mice. Cleft palate was induced in 100% of the offspring, with the critical treatment period ranging from day 10/8 hours (10/8) to 12/16 of gestation. Dose-dependent body weight reduction was observed in day 18 fetuses from both the day 10/8 and 12/16 groups. However, fetal weight reduction was greater in the day 12/16 group for all the MeHg treatments investigated. The relative potency of the induction of cleft palate by MeHg was slightly but significantly higher in the fetuses of the day 12/16 group (1.044-1.197-fold in 95% limits) than in the day 10/8 group. The results showed that when 25 mg/kg of MeHg was given to the fetuses in the day 10/8 group, palatal shelf growth was delayed at a more primitive stage than in the day 12/16 fetuses. Moreover, disharmony of development between the overall fetus and palatal shelf was noticed. Furthermore, in the day 12/16 fetuses, a delay of palatal shelf growth occurred just prior to shelf elevation. Prior to shelf elevation, coordination was probably lost in the development between the fetus and the palatal shelves. Normal palatal closure in ICR fetuses occurs about 1 day and 10 hours earlier (P less than 0.05) than in the A/J fetuses (Biddle, '80). Normal palatal shelves in ICR fetuses moved rapidly, with 3.0 to 5.7 hours (in 95% limits) required for all fetuses to achieve elevation, while, in MeHg-treated groups, palatal shelf elevation did not occur. The results suggest that the cause of the failure in palatal shelf elevation may be understood by examining the disharmonious development of the fetus after exposure to MeHg.

Biochemical mechanism of glucocorticoid-and phenytoin-induced cleft palate

The production of cleft palate by glucocorticoids and phenytoin is a complicated interference in a complex developmental program involving many genetic and biochemical processes. The H-2 histocompatibility region includes genes which affect (1) susceptibility to glucocorticoid- and phenytoin-induced cleft palate; (2) glucocorticoid receptor level in a variety of tissues including maternal and embryonic palates, adult thymuses, and lungs; and (3) the degree of inhibition of prostaglandin and thromboxane production by glucocorticoids and phenytoin in thymocytes. A gene linked to a minor histocompatibility locus (H-3) on the second chromosome also influences susceptibility to glucocorticoid- and phenytoin-induced cleft palate. Phenytoin is an alternate ligand for the glucocorticoid receptor affecting prostaglandin and/or thromboxane production. The capacity of glucocorticoids to induce cleft palate is correlated with their anti-inflammatory potency. At least some of the anti-inflammatory effects of glucocorticoids can be explained by the inhibition of prostaglandin and/or thromboxane release, which in turn could be caused by inhibition of arachidonic acid release from phospholipids. Similar mechanisms may be involved in cleft palate induction, as exogenous arachidonic acid injected into pregnant rats and mice at the same time as glucocorticoids reduces the teratogenic potency of the steroids, and indomethacin, an inhibitor of cyclooxygenase, blocks the corrective action of arachidonic acid. Glucocorticoids and phenytoin cause a delay in shelf elevation, and this delay is promoted by fetal membranes and the tongue. However, the cells of the medial edge epithelium are programmed to die whether contact is made with the apposing shelf or not. Glucocorticoids and phenytoin interfere with this programmed cell death, and this interference by both drugs seems to be glucocorticoid receptor mediated, to require protein synthesis, and to be related to arachidonic acid release.

Role of glucocorticoids and epidermal growth factor in normal and abnormal palatal development

The purpose of this chapter has been to discuss glucocorticoid and EGF involvement in normal and abnormal palatal development. It is to be hoped that we have made clear the important point that these hormone/growth factors and their receptors are present during normal embryonic palatal development to provide for regulation of growth and cellular differentiation. When these hormone/growth factors are administered in pharmacological or large doses that result in teratogenesis, these potent chemicals and their receptors then become inducers of cleft palate. The primary reason for this is that the hormone/growth factor receptors have unique and special areas of localizations in target (embryonic and fetal) tissues, e.g., glucocorticoids in the palate. Therefore, large amounts of these chemicals are specifically bound to receptors in these target tissues and these high levels of hormone/growth factor-receptor complexes result in aberrant development, e.g., glucocorticoids cause inhibition of palatal mesenchymal cell growth. These effects are distinct from the interactions of physiological levels of these hormone/growth factors with their receptors in these target tissues during development, e.g., glucocorticoids cause induction of key enzymes and modulation of EGF receptor levels. The exact molecular mechanism(s) by which high levels of hormone/growth factors--receptor complexes exert harmful effects on embryos or fetuses is (are) unknown and remain(s) a challenge for the future. Interaction of hormone/growth factors and their receptors certainly cannot provide an explanation for the mechanism of all types of craniofacial teratogenesis, but this concept certainly appears capable of providing important information relating to the mechanisms of many animal and human teratogens. The fact that these chemicals and their receptors are involved in normal development makes them all the more important since subtle alterations in their levels or activities could result in teratogenesis without an exposure to pharmacological levels of these hormone/growth factors. It seems that progress in this area will develop quickly since the techniques of recombinant DNA research are available in conjunction with responsive in vitro cell systems such as the established line of human embryonic palatal mesenchymal cells. Clearly, the future looks very exciting for understanding the role that these hormone/growth factors and their receptors play in normal and abnormal palate development.

Cortisone-induced cleft palate in A/J mice: failure of palatal shelf contact

Although cortisone treatment for induction of cleft palate in mice has been shown to delay the time of palatal shelf elevation, the effects of delayed elevation of shelf contact have not been critically evaluated in a cortisone-sensitive mouse strain. The objective of this study was to evaluated palatal development in cortisone-treated A/J mice in order to determine whether the shelves make contact upon elevation. Morphometric analysis of frozen sections revealed that cortisone-treated shelves were smaller than control shelves with apparent reductions in both the content of extracellular matrix and the number of cells. At a light microscopic level, thinning of medial epithelium in cortisone-treated palates appeared similar to that in untreated palates with spontaneous cleft lip and palate. Shelf elevation was delayed by approximately 12 hours and only half of the cortisone-treated palates achieved complete horizontal positioning of the shelves in all regions of the palate. Immediately after elevation, all control palates had extensive vertical contact along the complete length of the palate. In contrast, approximately 20% of the cortisone-treated fetuses had contact between the shelves in the middle palate region only, with the mean area of contact only 20% as large as in control fetuses. As result, the net shelf contact in all the cortisone-treated fetuses was only 4% of the potential contact shown in control fetuses. Therefore, failure of the palatal shelves to elevate and make extensive contact appeared to be the major factor contributing to cortisone-induced cleft palate in A/J mice.

Embryonic development of the head and neck: Part 3, The face

The embryology of the face is presented with respect to changes affecting the mandible, maxilla, upper and lower lips, palate, nose, and oral cavity. The embryonic development of the teeth and salivary glands is also included. Various facial clefts, including cleft lip and cleft palate, are discussed, in addition to some congenital anomalies affecting the nose and oral cavity.

The effect of hypervitaminosis A on rat palatal development

Retinoic acid or retinyl acetate was administered to pregnant rats in doses sufficient to induce a 90% incidence of cleft palate. In another study, a delay in the reorientation of the palatal shelves was observed to be longer with the more potent teratogen, retinoic acid. On day 16 of gestation, 24 hours after final dosage with vitamin A, the synthesis of DNA and protein was studied in fetal carcass, mandible, and palate, and that of sulfated mucopolysaccharides (S-MPS) and glycoproteins (GP) in fetal head, mandible, and palate. Increases in DNA synthesis in fetal palate and in GP synthesis in fetal palate were found; thus, the mechanism of action of vitamin A in inducing cleft palates in rats may be caused by interference with the normal biochemical synthetic pattern of the palatal shelves.

Palate development after fetal tongue removal in cortisone-treated mice

Morphological studies of cortisone-induced cleft palate have shown retardation in the rotation of palatine shelves from a sagittal to a transverse plane. Cortisone also reduces fetal muscular movements, which may explain why displacement of the tongue from between the palatine shelves is delayed. Previous work with extrauterine development of control fetuses demonstrated that fetal membranes and tongue were major obstacles to shelf rotation. Thus, removal of these obstacles might permit rotation and fusion of palatine shelves in cortisone-treated fetuses. In the present experiment, fetuses from cortisone-treated strain CD-1 mice were released from uterus and membranes and allowed to develop for eight hours in a fluid medium with the umbilical cord left intact. Compared to 4% fusion in utero, there was palatal fusion in 20% of fetuses released from membranes. When the fetal tongue was removed during extrauterine development, the frequency of fusions increased to 61%. Fusion appeared normal by the criteria applicable through light microscopy. Thus, cortisone induces cleft palate primarily through interference with shelf rotation. The palatine shelves of treated fetuses retain their ability to fuse when they can come in contact during the normal time for palate closure.

Effects of methyl mercury on murine fetal amino acid uptake, protein synthesis and palate closure

Methylmercury (MeHg: 5 mg Hg/kg maternal body weight) in 0.13 M NaCl, 0.01 M NaH2PO4-Na2HPO4, pH 7.4 (PBS) administered to gravid CFW mice on day 12, hour 6 (12(6)) of gestation induced a high incidence of cleft palate in fetuses examined on days 15(6) (72%), 16(6) (62%) and 17(6) (40%). Palate closure (100%) in PBS control animals occurred by 14(10). One day post MeHg administration, total fetal protein was decreased 22% while DNA content was unaltered. Protein was maximally decreased (28%) on 14(6) and, thereafter, returned toward control levels. Alterations in DNA content followed a similar pattern; but the maximal decrease (32%) occurred on 15(6). The rate of fetal protein synthesis was depressed 5% at 12(9) and between 20% to 26% from this time to 13(6) (end of observation). The agreement between the calculated decrease in protein synthesis (19%) and the measured decrease in protein content (22%) suggests that a reduction in protein synthesis is responsible for the decreased fetal protein content. Placental blood flow and fetal water space, measured with 3H--H2O at 12(18), were not affected by MeHg treatment. However, fetal free amino acid concentrations at 12(18) were generally decreased (alanine, 23.0%; valine, 9.7%; methionine, 22.6%; isoleucine, 12.0%; leucine, 18.2%) while uptake of the non-metabolizable amino acid, 14C-cycloleucine, was decreased 23%. From this, it is concluded that the growth inhibitory effects of MeHg are related, at least in part, to impaired placental/fetal transfer of amino acids.

Graphic reconstructions of craniofacial structures during secondary palate development in rats

Lateral and ventral graphic reconstructions of coronally sectioned rat fetuses at four stages of secondary palate development were made to illustrate the size, form, and spatial relations of craniofacial structures at each stage, and to indicate changes between stages. The results illustrated extensive changes in the nasomaxillary and tongue-mandibular complexes and spatial relations in the oronasal cavity during this 2-day period. During closure of the palate the palatine processes and molar dental laminae moved medially, the vertical dimension between the cranial base and Meckel's cartilage increased, and the Meckel's cartilage changed in shape from a "U" to a "V". During the 2-day period extensive increases in anteroposterior and vertical dimensions and limited changes in lateral dimensions resulted in a change in shape of the complete orofacial region. More extensive investigations, preferably quantitative, of the changes shown are indicated to identify the relative contribution of various craniofacial components and to establish the role of differential growth in secondary palate closure.

Light and electron microscopic observations on hydrocortisone-induced cleft palate in hamsters

Palatal histogenesis in hydrocortisone-treated hamster fetuses was studied by both light and electron microscopy. At an early stage in the hydrocortisone-affected fetuses, when the palatal shelves hung vertically on either side of the tongue, necortic changes could be seen in some of the basal epithelial cells which lay adjacent to the fragmented basal lamina. The normal looking cells lay on an intact basal lamina and were attached to the contiguous necrotic cells by desmosomes. With horizontal reorientation of the palatal shelves and their approach to the midline, cellular necrosis and fragmentation of the basal lamina increased. When compared with normal cells, the hydrocortisone-affected ones were seen to be lighter, to contain fewer ribosomes and no lysosomes. At a later stage, when midline palatal fusion was lacking, the epithelium underwent stratification and keratinization while the necrotic debris was removed by mesenchymal macrophages. It appears that the normal process of protein synthesis is inhibited following hydrocortisone administration and that this, in turn, during palatogenesis, disrupts normal cellular differentiation and the integrity of the basal lamina, which are associated with the production of a cleft palate.

Teratogenic effects of chlorambucil on in vivo and in vitro organogenesis in mice

Pregnant ICR/DUB mice were each given a single oral injection of chlorambucil (14.2 or 20 mg/kg) on the 10th, 11th, 12th, or 13th day of gestation (plug day = 1st day). Fetuses examined on the 18th day were decreased in weight and had tail, cranial, and limb defects. They type and frequency of malformations differed according to the dosage and day of treatment. Limb defects resulted from treatment on the 11th or 12th days of gestation and tail defects from treatment on all days. Control limb buds from 12th day embryos cultured for 6 days in serum-supplemented BGJ medium containing 0.5-2 mug/ml chlorambucil were retarded in development and had cartilage abnormalities. The extent of the deformities was dose related. Limb buds were also taken from embryos 24 h after in vivo exposure to teratogenic doses of chlorambucil and cultured in control medium. After 6 days in culture these limbs also had growth impairment and cartilage abnormalities. The defects in limbs exposed in vitro were similar to those in limbs exposed in vivo.

Some aspects of corticosteroid-induced cleft palate: a review

Since the discovery 25 years ago that cortisone can produce cleft palate in mouse embryos investigations into possible mechanisms of this corticosteroid-induced defect have been many and varied. However, the teratogenic mode of action remains not fully clarified. It is with this thought in mind that we have reflected upon what is known concerning corticosteroids and cleft palate. The major metabolic pathways upon which glucocorticoids act as well as their intracellular mode of action are well known. Differential sensitivity of various mouse strains to cortisone treatment as well as recent results from interstrain blastocyst transfer experiments demonstrate that corticosteroid action is influenced by both the fetal and maternal genomes. Labeling experiments indicate that corticosteroid-induced cleft palate is the result of direct action of the steroid molecule on the fetus, whose own sensitivity to insult, perhaps owing to differences in binding of corticosteroids to tissue proteins, determines the final effect. Possible mechanisms that have been proposed by which corticoids may produce cleft palate include: disruption of glycosaminoglycan or collagen synthesis or both, intracellular lysosomal membrane stabilization, myopathy, weakened midline fusion, and loss of amniotic fluid. Also discussed is the role of stress and stress-induced corticosteroids and their possible role in the production of cleft palate.