Ultrastructural evidence of contractile systems in mouse palates prior to rotation

Characterizing cleft palate toxicants using ToxCast data, chemical structure, and the biomedical literature

Cleft palate has been linked to both genetic and environmental factors that perturb key events during palatal morphogenesis. As a developmental outcome, it presents a challenging, mechanistically complex endpoint for predictive modeling. A data set of 500 chemicals evaluated for their ability to induce cleft palate in animal prenatal developmental studies was compiled from Toxicity Reference Database and the biomedical literature, which included 63 cleft palate active and 437 inactive chemicals. To characterize the potential molecular targets for chemical-induced cleft palate, we mined the ToxCast high-throughput screening database for patterns and linkages in bioactivity profiles and chemical structural descriptors. ToxCast assay results were filtered for cytotoxicity and grouped by target gene activity to produce a "gene score." Following unsuccessful attempts to derive a global prediction model using structural and gene score descriptors, hierarchical clustering was applied to the set of 63 cleft palate positives to extract local structure-bioactivity clusters for follow-up study. Patterns of enrichment were confirmed on the complete data set, that is, including cleft palate inactives, and putative molecular initiating events identified. The clusters corresponded to ToxCast assays for cytochrome P450s, G-protein coupled receptors, retinoic acid receptors, the glucocorticoid receptor, and tyrosine kinases/phosphatases. These patterns and linkages were organized into preliminary decision trees and the resulting inferences were mapped to a putative adverse outcome pathway framework for cleft palate supported by literature evidence of current mechanistic understanding. This general data-driven approach offers a promising avenue for mining chemical-bioassay drivers of complex developmental endpoints where data are often limited.

Gene expression profiling in the developing secondary palate in the absence of Tbx1 function

BACKGROUND

Microdeletion of chromosome 22q11 is associated with significant developmental anomalies, including disruption of the cardiac outflow tract, thymic/parathyroid aplasia and cleft palate. Amongst the genes within this region, TBX1 is a major candidate for many of these developmental defects. Targeted deletion of Tbx1 in the mouse has provided significant insight into the function of this transcription factor during early development of the cardiac and pharyngeal systems. However, less is known about its role during palatogenesis. To assess the influence of Tbx1 function on gene expression profile within the developing palate we performed a microarray screen using total RNA isolated from the secondary palate of E13.5 mouse embryos wild type, heterozygous and mutant for Tbx1.

RESULTS

Expression-level filtering and statistical analysis revealed a total of 577 genes differentially expressed across genotypes. Data were clustered into 3 groups based on comparison between genotypes. Group A was composed of differentially expressed genes in mutant compared to wild type (n = 89); Group B included differentially expressed genes in heterozygous compared to wild type (n = 400) and Group C included differentially expressed genes in mutant compared to heterozygous (n = 88). High-throughput quantitative real-time PCR (RT-PCR) confirmed a total of 27 genes significantly changed between wild type and mutant; and 27 genes between heterozygote and mutant. Amongst these, the majority were present in both groups A and C (26 genes). Associations existed with hypertrophic cardiomyopathy, cardiac muscle contraction, dilated cardiomyopathy, focal adhesion, tight junction and calcium signalling pathways. No significant differences in gene expression were found between wild type and heterozygous palatal shelves.

CONCLUSIONS

Significant differences in gene expression profile within the secondary palate of wild type and mutant embryos is consistent with a primary role for Tbx1 during palatogenesis.

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.

Histochemical localization of glycosaminoglycans during morphogenesis of the secondary palate in mice

The hydration of hyaluronic acid (HA) accumulated in the secondary palatal processes is expected to exert an intrinsic tissue pressure that could, in part, provide the impetus for shelf reorientation. Glycosaminoglycans were histochemically localized in the A/J mouse palate during development (days 12 to 15) by specific enzymatic degradation followed by preferential staining with alcian blue under differential pH or MgCl2 concentration. The presence of HA and chondroitin sulphates A and C (CS) was demonstrated in proportions that differed regionally. At the time of reorientation (days 14 to 15) HA was the predominant staining component, being distributed according to the relative prominence of extracellular spaces (ECS). HA was present in higher concentration in the anterior than the posterior part of the palate, particularly in an area of low cell density adjoining the CS-rich mesenchyme of the maxillary process. This arrangement suggests that the maxillary process might provide a resilient incompressible structural base for the palate as its HA-rich ECS expands. Sulphated GAG, with CS being the predominant component, was localized for the most part on the oral-side mesenchyme both in the anterior and posterior palate. The most intense staining of sulphated proteoglycans occurred in association with the basal lamina along the presumptive oral-side. Mesenchymal cells along this region appeared condensed and may have been stabilized by these sulphated GAG providing structural constraints which might function in palate morphogenesis.

Developmental mechanisms in normal and abnormal palate formation with particular reference to the aetiology, pathogenesis and prevention of cleft palate

Palatal development was studied macroscopically, microscopically and ultrastructurally in foetuses of inbred Wistar rats and Alligator mississippiensis. In the rat, elevation of the palatal shelves from a vertical position lateral to the tongue to a horizontal position above the tongue, occurs very rapidly. This reorientation is postulated to be caused by an intrinsic turgor shelf force generated by the hydration of mesenchymal mucopolysaccharides (predominantly hyaluronic acid). Cleft palate was induced in rat foetuses using 5-fluoro-2-desoxyuridine and was associated with greatly decreased mucopolysaccharide synthesis. The alligator is the only animal which develops in an external egg and which possesses a true mammal-like secondary palate: it is therefore a useful animal model system because longitudinal studies and direct surgical and pharmacological manipulations can be performed. The palatal shelves of alligators grow horizontally above the dorsum of the tongue from their first appearance. This de novo horizontal shelf growth is associated with an increase amount of space in the alligator oronasal cavity due to the small, fatty, alligator tongue. It is postulated that the evolution of the large muscular mammalian tongue constrains the palatal shelves to grow vertically until sufficient space can be created to form the common nasal passage simultaneous with shelf elevation.

Palate morphogenesis. VI. Identification of stellate cells in culture

Mesenchymal cells from the palate of mouse embryos at day 14.5 of gestation produce a minor population of stellate cells in culture. These cells are often bipolar and spindle-shaped with long cytoplasmic processes similar to neural-crest cells. Culturing of explants of palatal mesenchyme enriched for this type of cell. Stellate cells were the first to migrate from explants, followed by fibroblast-like cells and then by squamous cells. The majority of the cells in the explant were fibroblast-like. Squamous cells were present mostly in the anterior and mid-palate and least frequently in those from the posterior palate. They may represent tooth-germ epithelium. When pieces of palate were dissected out and cultured for enrichment of non-muscle contractile systems, most of the migrating cells were stellate. These may represent the highly migratory cells that are, in part, responsible for elevation of the palate shelf. Serotonin was measured in cultured mesenchymal cells from the palate. Its occurrence is consistent with regulation of movement of palate cells.

Palate morphogenesis. I. Immunological and ultrastructural analyses of mouse palate

Midpalate was analyzed for the presence of nonmuscle contractile systems. The results indicate that increased amounts of actin and myosin are present in cells of regions 2 and 3. A localization of the contractile proteins in cellular projections (filopodia) and in the peripheral cytoplasm of the cell body was confirmed by indirect immunofluorescence studies, using antibodies directed against smooth muscle myosin and against skeletal muscle actin. Specificity of the immunofluorescence reactions was ascertained by immunoabsorption studies using purified myosin and actin. Electron microscopic observations of the mesenchymal cells in region 2 revealed 70A microfilaments along the cell periphery and packed in fliopodia-like projections which course between the cells. These cells, which surround a small ossification center, show no orientation, but extend up to the cranial base perichondrium and down into the shelf between the tongue side epithelium and the ossification center. The cells and projections are attached to each other by adherens and tight-like junctions, forming a putative cohesive contractile network. Putative contractile cells in region 3 are strikingly aligned perpendicular to the oral epithelium and extend one-third of the distance into the shelf. Projections from region 3 cells are contiguous with basement membrane material of the oral epithelium. Axonal bundles and single axons were commonly observed coursing through regions 2 and 3, often seen in close association with the mesenchymal cells. Both clear and dense-core vesicles were found in the axons and cells of these regions. The possible role of these putative nonmuscle contractile cells in palate morphogenesis is discussed.

Palate morphogenesis: II. Contraction of cytoplasmic processes in ATP-induced palate rotation in glycerinated mouse heads

It has been previously shown that non-muscle contractile system(s) exist in mouse palate mesenchyme underlying the palatal epithelium before shelf rotation. In order to obtain evidence that the non-muscle contractile system(s) function to elevate the palate, glycerinated heads have been incubated with ATP. It was shown that 5 mM ATP and a 30 min incubation at 25 degrees C stimulated palate rotation optimally. Elevation of the anterior end of the palate was nearly complete (PSI = 3.90, p less than 10(-6)). Although rotation of the posterior end was significant (p less than 0.02), movement was limited (PSI = 1.70). Light microscopy of the palate revealed that ATP caused a marked condensation of the cytoplasmic processes of the mesenchymal cells. The contraction of the processes of the mesenchymal cells induced by ATP increased roughly with increased palate shelf rotation and was greater at the peripheral than at the internal mesenchyme. Cytochalasin B pretreatment at 40 microM completely blocked the ATP-induced rotation at the anterior end. The effect of other nucleotides on palate rotation was tested. GTP caused a significant stimulation of anterior shelf rotation (p less than 0.005), which was less than ATP, while ADP and CTP were ineffective. Low temperature (6 degrees C) prevented the ATP-induced shelf rotation. These results suggest that the non-muscle contractile cells in the mesenchyme play a role in palate elevation and that contraction of the actomyosin containing microfilaments supplies the motive force.

The american alligator (Alligator mississipiensis): a new model for investigating developmental mechanisms in normal and abnormal palate formation

Despite numerous investigations, there are still many unsolved problems concerning normal and abnormal palatal development. The American Alligator is here put forward as a new model for the investigation of a variety of developmental phenomena associated with palatogenesis. The structure of the palate of the adult Alligator is reported. This animal exhibits a unique combination of reptilian and mammalian features in its craniofacial anatomy and so its craniofacial development should be of considerable interest.

Palate morphogenesis. III. Changes in cell shape and orientation during shelf elevation

The process of palate shelf elevation has been analyzed by light microscopy in mouse embryos cultured in vitro. The observations presented correlate changes in cell shape and orientation in the palate with the morphogenetic movement of the shelf. These studies suggest that in addition to any physical-chemical force elevating the shelf an active contraction of specific palate cells could also aid the process. Contribution to elevation could be derived from masses of contracting cells from the previously described non-muscle contractile systems in posterior (region 2) and mid-anterior (region 3) palate as well as other peripheral mesenchymal cells. Finally, elongation and contraction of the tongue side epithelial cells may also play a role in palate elevation.

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.

The mechanism of palatal shelf elevation and the pathogenesis of cleft palate

Both normal Wistar rat fetuses and those with cleft palate induced by 5-Fluoro-2-Desoxyuridine were studied with a view to elucidating the mechanism of palatal shelf elevation and the pathogenesis of cleft palate. It was postulated that normal shelf elevation is brought about rapidly by an intrinsic turgor shelf force generated by binding of water to mucopolysaccharides. Interference with mucopolysaccharide synthesis would seem to be an important factor in the pathogenesis of some types of cleft palate.