A unique mouse strain expressing Cre recombinase for tissue-specific analysis of gene function in palate and kidney development

Mammalian palate development is a multistep process, involving initial bilateral downward outgrowth of the palatal shelves from the oral side of the maxillary processes, followed by stage-specific palatal shelf elevation to the horizontal position above the developing tongue and subsequent fusion of the bilateral palatal shelves at the midline to form the intact roof of the oral cavity. While mutations in many genes have been associated with cleft palate pathogenesis, the molecular mechanisms regulating palatal shelf growth, patterning, and elevation are not well understood. Genetic studies of the molecular mechanisms controlling palate development in mutant mouse models are often complicated by early embryonic lethality or gross craniofacial malformation. We report here the development of a mouse strain for tissue-specific analysis of gene function in palate development. We inserted an IresCre bicistronic expression cassette into the 3' untranslated region of the mouse Osr2 gene through gene targeting. We show, upon crossing to the R26R reporter mice, that Cre expression from the Osr2(IresCre) knockin allele activated beta-galactosidase expression specifically throughout the developing palatal mesenchyme from the onset of palatal shelf outgrowth. In addition, the Osr2(IresCre) mice display exclusive Cre-mediated recombination in the glomeruli tissues derived from the metanephric mesenchyme and complete absence of Cre activity in other epithelial and mesenchymal tissues in the developing metanephric kidney. These data indicate that the Osr2(IresCre) knockin mice provide a unique tool for tissue-specific studies of the molecular mechanisms regulating palate and kidney development.     

Site-Specific Expression of Gelatinolytic Activity during Morphogenesis of the Secondary Palate in the Mouse Embryo

Morphogenesis of the secondary palate in mammalian embryos involves two major events: first, reorientation of the two vertically oriented palatal shelves into a horizontal position above the tongue, and second, fusion of the two shelves at the midline. Genetic evidence in humans and mice indicates the involvement of matrix metalloproteinases (MMPs). As MMP expression patterns might differ from sites of activity, we used a recently developed highly sensitive in situ zymography technique to map gelatinolytic MMP activity in the developing mouse palate. At embryonic day 14.5 (E14.5), we detected strong gelatinolytic activity around the lateral epithelial folds of the nasopharyngeal cavity, which is generated as a consequence of palatal shelf elevation. Activity was concentrated in the basement membrane of the epithelial fold but extended into the adjacent mesenchyme, and increased in intensity with lateral outgrowth of the cavity at E15.5. Gelatinolytic activity at this site was not the consequence of epithelial fold formation, as it was also observed in Bmp7-deficient embryos where shelf elevation is delayed. In this case, gelatinolytic activity appeared in vertical shelves at the exact position where the epithelial fold will form during elevation. Mmp2 and Mmp14 (MT1-MMP), but not Mmp9 and Mmp13, mRNAs were expressed in the mesenchyme around the epithelial folds of the elevated palatal shelves; this was confirmed by immunostaining for MMP-2 and MT1-MMP. Weak gelatinolytic activity was also found at the midline of E14.5 palatal shelves, which increased during fusion at E15.5. Whereas MMPs have been implicated in palatal fusion before, this is the first report showing that gelatinases might contribute to tissue remodeling during early stages of palatal shelf elevation and formation of the nasopharynx.     

The pivotal role of CCN2 in mammalian palatogenesis

Mammalian palatogenesis is a complex process involving a temporally and spatially regulated myriad of factors. Together these factors control the 3 vital processes of proliferation, elevation and fusion of the developing palate. In this study, we show for the first time the unequivocally vital role of CCN2 in development of the mammalian palate. We utilized CCN2 knockout (KO) mice and cranial neural crest derived mesenchymal cells from these CCN2 KO mice to investigate the 3 processes crucial to normal palatogenesis. Similar to previously published reports, the absence of CCN2 inhibits proliferation of cells in the palate specifically at the G1/S transition. Absence of CCN2 also inhibited palatal shelf elevation from the vertical to horizontal position. CCN2 KO mesenchymal cells demonstrated deficiencies in adhesion and spreading owing to an inability to activate Rac1 and RhoA. On the contrary, CCN2 KO mesenchymal cells exhibited increased rates of migration compared to WT cells. The addition of exogenous CCN2 to KO mesenchymal cells restored their ability to spread normally on fibronectin. Finally, utilizing an organ culture model we show that the palatal shelves of the CCN2 KO mice demonstrate an inability to fuse when apposed. Together, these data signify that CCN2 plays an indispensible role in normal development of the mammalian palate and warrants additional studies to determine the precise mechanism(s) responsible for these effects.     

Ultrastructural evidence of contractile systems in mouse palates prior to rotation.

Previous studies have shown that the palatal shelves of mouse embryos synthesize the contractile proteins actin and myosin at a rate equal to that of tongue just prior to shelf movement (day 14.5). The purpose of this study was to examine the morphology of the palatal shelves for evidence of a contractile system. Myosin ATPase histochemistry was performed on frozen sections of day-14.5 fetal mouse heads. Three areas of the palatal shelves gave a positive reaction: 1) A reaction product typical of skeletal muscle on the oral side of the posterior palate (region 1); 2) a “heavy-diffuse” reaction product on the tongue side extending from the top mid-palate to the posterior end (region 2); and 3) a “light-diffuse” reaction product along the oral epithelium in the mid-palate (region 3). Electron microscopy of excised day-14.5 palates was carried out after fixation in glutaraldehyde or an acrolein-dichromate solution. Region 1 contained a large area of developing and adultlike skeletal muscle. In the area of region 2 a large population of filamentous-rich mesenchymal cells was observed. In addition, large neurons coursing through both contractile systems were noted. Preliminary observations in region 3 indicated the possibility of a primitive (nonmuscle) contractile system in that area. The contractile and nervous systems in the palate, prior to rotation, indicate the possibility that an innervated embryonic muscle system may provide the “intrinsic shelf force” to rotate the shelves.     

Temporal and Spatial Expression of Hoxa-2 During Murine Palatogenesis

1. Mice homozygous for a targeted mutation of the Hoxa-2 gene are born with a bilateral cleft of the secondary palate associated with multiple head and cranial anomalies and these animals die within 24 hr of birth (Gendron-Maguire et al., 1993; Rijli et al., 1993; Mallo and Gridley, 1996). We have determined the spatial and temporal expression of the Hoxa-2 homeobox protein in the developing mouse palate at embryonic stages E12, E13, E13.5, E14, E14.5, and E15.2. Hoxa-2 is expressed in the mesenchyme and epithelial cells of the palate at E12, but is progressively restricted to the tips of the growing palatal shelves at E13.3. By the E13.5 stage of development, Hoxa-2 protein was found to be expressed throughout the palatal shelf. These observations correlate with palatal shelf orientation and Hoxa-2 protein may play a direct or indirect role in guiding the palatal shelves vertically along side the tongue, starting with the tips of the palatal shelves at E13, followed by the entire palatal shelf at E13.5.4. As development progresses to E14, the stage at which shelf elevation occurs, Hoxa-2 protein is downregulated in the palatal mesenchyme but remains in the medial edge epithelium. Expression of Hoxa-2 continues in the medial edge epithelium until the fusion of opposing palatal shelves.5. By the E15 stage of development, Hoxa-2 is downregulated in the palate and expression is localized in the nasal and oral epithelia.6. In an animal model of phenytoin-induced cleft palate, we report that Hoxa-2 mRNA and protein expression were significantly decreased, implicating a possible functional role of the Hoxa-2 gene in the development of phenytoin-induced cleft palate.7. A recent report by Barrow and Capecchi (1999), has illustrated the importance of tongue posture during palatal shelf closure in Hoxa-2 mutant mice. This along with our new findings of the expression of the Hoxa-2 protein during palatogenesis has shed some light on the putative role of this gene in palate development.     

The anterior half of mouse palatal shelf is elevated by a remodeling movement

During the development of the mammalian secondary palate, the lateral palatine process (the palatal shelf) rises from the vertical plane beside the tongue to the horizontal plane above it. To determine the mode of elevation of the palatal shelf, we have cultured the whole ICR mouse fetus on day 14 (0-2 h) of gestation with a scratch as a marker at the distal edge of the anterior fourth of the vertical palatal shelf. After 6-18 h of culture, the survival ratio was 78.9%, and the palatal shelf rose horizontally above the tongue in 22.2% of the surviving fetuses. The scratch was found as a scar on the oral epithelium laterally to the medial edge of the elevated palatal shelf. These results indicate that the medial edge of the horizontal shelf was newly formed from the medial wall of the preceding vertical-stage shelf during elevation, and that the palatal shelf was elevated by a remodeling process in the anterior half of the shelf.     

Palatal shelf reorientation in hamster embryos following treatment with 5-fluorouracil

A study was undertaken to examine the issue of whether achieving a critical mass of cells and/or palatal shelf volume during vertical development of shelf is essential for reorientation to occur. In control and 5-fluorouracil (5FU)-treated hamster embryos' palatal shelves, at different times during gestation, the numbers of both epithelial and mesenchymal cells were counted and cross-sectional area was measured. DNA synthesis was measured by 3H-thymidine incorporation and was used as an index of growth by cell proliferation. The control data indicated that, unlike development during initial 24 hours, the later period of vertical palatal development was characterized by a steady level of mesenchymal and epithelial cell numbers and palatal shelf area. Following 5FU treatment all the measurements were reduced, and until they reached the equivalent of control values, the palatal shelves did not reorient. The density of mesenchymal cells in the developing palate did not seem to affect cell number. On the basis of the analysis of results of the present study, along with those reported in the literature, it is suggested that, in hamsters, acquisition and maintenance of both a specified number of mesenchymal cells and shelf area, at least 24 hours prior to reorientation, may be critical for ensuing mesenchymal differentiation to enforce palatal shelf reorientation on schedule. 5FU affected these features to delay reorientation of the palatal shelf.     

Ultrastructural and cytochemical observations on 5-fluorouracil-induced cleft-palate development in hamster

Sequential alterations in 5-fluorouracil-treated hamster fetal palate were studied by light and electron microscopy and by acid phosphatase cytochemistry. At an early stage in 5-fluorouracil-treated fetuses, when the palatal shelves were vertical, lysosomes first appeared in cells of the prospective fusion epithelium and then in the cells of subjacent mesenchyme. In contrast to controls, increasing numbers of both the epithelial and mesenchymal cells of the vertical palate showed lysosomal injury in 5-fluorouracil-treated fetuses as development progressed. Subsequently, the basal lamina in the vertical palate showed alterations, characterized initially by disturbances in lamina lucida, by fingerlike extensions of lamina densa, and ultimately by its complete breakdown. At a later stage, when shelves became horizontal, the lysosomes were absent in both the epithelial and mesenchymal cells, and the basal lamina continuity was restored. Unlike controls, however, 5-fluorouracil-treated horizontal shelves never contacted one another. Instead, the epithelia of the horizontal shelves underwent stratification. It appears that premature formation of lysosomes in palatal epithelial and mesenchymal cells following 5-fluorouracil treatment disrupts normal cytodifferentiation and affects the integrity of the basal lamina; both effects are associated with cleft-palate development.     

Pathogenesis of bromodeoxyuridine-induced cleft palate in hamster

In the present study, the morphological, histochemical, biochemical, and cellular aspects of the pathogenesis of bromodeoxyuridine (BrdU)-induced cleft palate in hamster fetuses were analyzed. Morphological observations indicated that BrdU interferes with the growth of the vertical shelves and thus induces cleft palate. At an ultrastructural level, BrdU-induced changes were first seen in the mesenchymal cells. Eighteen hours after drug administration, the initial alterations were characterized by swelling of the nuclear membrane and the appearance of lysosomes in the mesenchymal cells of the roof of the oronasal cavity. During the next 6 hr, as the palatal primordia developed, lysosomes were also seen in the overlying epithelial cells. The appearance of lysosomal activity, which was verified by acid phosphatase histochemistry, was temporally abnormal and was interpreted as a sublethal response to BrdU treatment. Later the cellular alterations subsided; 48 hr after BrdU treatment, they were absent in both the epithelial and mesenchymal cells of the vertically developing palatal shelves. Subsequently, unlike controls (in which the palatal shelves undergo reorientation and fusion), the BrdU-treated shelves remained vertical until term. Biochemical determination of DNA synthesis indicated that although there was an inhibition of DNA synthesis at the time of appearance of palatal primordia, a catch-up growth during the ensuing 12 hr may have restored the number of cells available for the formation of a vertical palatal shelf. It was suggested that BrdU affected cytodifferentiation in the palatal tissues during the critical phase of early vertical development to induce a cleft palate.     

Growth of the secondary palate in the hamster following hydrocortisone treatment: shelf area, cell number, and DNA synthesis

The contribution made by mesenchymal cells during the later stages of palatal development was examined in control and hydrocortisone-treated hamster embryos. Cross-sectional area of the palatal shelf was measured, and the numbers of both epithelial and mesenchymal cells were counted. DNA synthesis was measured by 3H-thymidine incorporation and was used as an index of growth by cell proliferation. The observations in controls indicated that, unlike development during the initial 24 hr, the later period of vertical palate development, followed by reorientation of shelves and their closure, was characterized by a steady level of mesenchymal cell number and palatal shelf area. An absence of corresponding growth in the epithelial cell number suggests that the cells may accommodate the growth either by increasing their size and/or by stretching along the basal lamina. Hydrocortisone treatment did not alter the growth pattern of cell numbers or shelf area. However, it prevented the fusion between the opposing shelves, perhaps by affecting the cytodifferentiation of the palatal tissues. Although a continuous increase in the number of mesenchymal cells during the latter half of vertical shelf development, i.e., between days 11:00 and 12:00 of gestation, is not required for reorientation and fusion of the shelves, it is not clear from the data from the present study whether a critical number of cells and/or cell density is essential for reorientation and fusion of the palate. It was suggested that, for normal palatal development, information on cell cycle and positioning of mesenchymal cells within the shelf during the vertical development may be crucial for further understanding of subsequent events of palatogenesis.     

Gross and cellular analysis of 6-mercaptopurine-induced cleft palate in hamster

The present study analyzes the morphological, histochemical, and ultrastructural aspects of the pathogenesis of 6-mercaptopurine (6MP)-induced cleft palate in hamster fetuses. Gross and light microscopic observations indicated that 6MP stunts the growth of vertical palatal shelves and thus induces cleft palate. Ultrastructural analysis showed that, in contrast to controls, 6MP-induced alterations were first seen in the mesenchymal cells 24 hr after drug administration. The initial alterations were characterized by swelling of the nuclear membrane. During the next 12 hr, lysosomes were seen first in the mesenchymal cells and then in the cells of the medial edge epithelium (MEE) of the developing palatal primordia. The appearance of lysosomes was temporally abnormal and was interpreted as a sublethal response to 6MP treatment. Subsequently, the nuclear alterations and the lysosomes diminished; and 48 hr after 6MP administration, they were absent from the palatal tissues. Ninety hours after 6MP administration, unlike the controls (in which the palatal shelves were already fused), changes were seen at the epithelial-mesenchymal interface in the developing cleft palatal shelves. These changes were characterized by breakdown of the basal lamina and epithelial-mesenchymal contacts. Eventually, at term, the MEE of the vertical shelf stratified. It was suggested that 6MP affected cytodifferentiation in the palatal tissues during the critical phase of early vertical shelf development and thereby induced cleft palate.     

Synthesis and distribution of collagen in the rat palate during shelf elevation.

Previous studies have shown that drugs which block the crosslinking of collagen prevent formation of the secondary palate by inhibiting shelf elevation. In this paper, the authors establish that collagen synthesis increases significantly just prior to shelf elevation and that type I collagen is synthesized throughout this developmental period (gestational Days 14–16 in the rat embryo) by isolated shelves in vitro. The largest accumulation of collagen fibers, predominantly oriented in a rostral-caudal plane, was observed adjacent to the basement membrane of the oral palatal epithelium. The unique location and orientation of these collagen fibers suggest that they play a structural role in the elevation of the palate.     

Experimental induction of palate shelf elevation in glutamate decarboxylase 67-deficient mice with cleft palate due to vertically oriented palatal shelf

BACKGROUND: Gamma-aminobutyric acid is an inhibitory neurotransmitter, synthesized by two isoforms of glutamate decarboxylase (GAD), GAD65 and -67. Unexpectedly, inactivation of GAD67 induces cleft palate in mice. Reduction of spontaneous tongue movement resulting from decreased motor nerve activity has been related to the development of cleft palate in GAD67(-/-) fetuses. In the present study, development of cleft palate was examined histologically and manipulated with culture of the maxilla and partial resection of fetal tongue. METHODS: GAD67(-/-) mice and their littermates were used. Histological examination and immunohistochemistry were performed conventionally. Organ culture of the maxilla was carried out as reported previously. Fetuses were maintained alive under anesthesia and tips of their tongues were resected. RESULTS: Elevation of palatal shelves, the second step of palate formation, was not observed in GAD67(-/-) mice. In wild-type mice, GAD67 and gamma-aminobutyric acid were not expressed in the palatal shelves, except in the medial edge epithelium. During 2 days of culture of maxillae dissected from E13.5-E14.0 GAD67(-/-) fetuses, elevation and fusion of the palatal shelves were induced. When E13.5-15.5 mutant fetuses underwent partial tongue resection, the palatal shelves became elevated within 30 min. CONCLUSIONS: These results suggest that the potential for palate formation is maintained in the palatal shelves of GAD67(-/-) fetuses, but it is obstructed by other, probably neural, factors, resulting in cleft palate.     

Effects of cyclophosphamide on the secondary palate development in golden Syrian hamster: teratology, morphology, and morphometry

Cyclophosphamide (CP), when injected in hamster mother between days 9 and 11 of pregnancy, was teratogenic in fetuses. On the basis of a morphological study it was deduced that CP delayed the reorientation of hamster palatal shelves by 16-20 h. In a subsequent experiment, in both control and CP-treated palatal shelves, the numbers of epithelial and mesenchymal cells were counted and cross-sectional area was measured. DNA synthesis, measured by 3H-thymidine incorporation, was used as an index of growth by cell proliferation. The results showed that during the vertical development of palatal shelves, the mesenchymal cells reached their peak number during the initial 24 hours, i.e., at the end of the second peak in DNA synthesis, and remained unchanged thereafter throughout reorientation. The shelf area also showed rapid increase during the initial 24 h followed by a spurt 2 h prior to reorientation. Cyclophosphamide prolonged the acquisition of these features by affecting the mesenchymal cells and consequently delayed the reorientation of the vertical shelves until such time that the number of healthy mesenchymal cells and shelf area were restored to the control values. The data lend further support to the hypothesis that the acquisition of a specific number of cells and shelf volume, during vertical palatal development, may be essential for palatal shelf reorientation.     

Human embryonic palatal epithelial differentiation is altered by retinoic acid and epidermal growth factor in organ culture

Reports of adverse human pregnancy outcomes including cleft palate have increased as the clinical use of isotretinoin (13-cis-retinoic acid) and other retinoic acid (RA) derivatives have increased, but the mechanisms by which their effects are exerted are not understood. Research in craniofacial development is generally performed in rodents, and mouse palatal shelves exposed in organ cultures to retinoids and epidermal growth factor (EGF) display altered medial epithelial cell morphology blocking normal union of apposing shelves. In the present study, precontacting human palatal shelves were maintained in organ culture for 2, 3, or 6 days and exposed to labeled thymidine (3H-TdR) during the last 16 hr. Retinoids and EGF were included in the media so that each shelf was exposed to one of the following: control, EGF, trans-RA at 10(-5)M, cis-RA at 10(-7) or 10(-9) M, or RA + EGF. After exposure of cultured human embryonic palatal shelves to 13-cis-RA and trans-RA with or without EGF, medial epithelial cells do not degenerate, cell surface morphology shifts toward a nasal type, glycogen deposits decrease, smooth endoplasmic reticulum (SER) increases, and basal lamina appear altered. In shelves exposed to EGF and trans-RA early in their development, DNA synthesis appears to terminate prematurely as compared to shelves cultured in control media, and this effect is accompanied by excessive mesenchymal extracellular space expansion. Exposure of shelves to EGF alone is sufficient to block degeneration and induce hyperplasia of the medial epithelial cells but does not induce other ultrastructural changes seen with both EGF and RA. The observed alterations in medial cell morphology could interfere with adhesion of the palatal shelves and may play a role in retinoid-induced cleft palate in the human embryo.     

In vitro development of the hamster and chick secondary palate

A series of experiments were undertaken to compare the in vitro behaviour of the medial edge epithelium (MEE) of hamster, in which palatal shelves normally fuse, and chick, in which they do not fuse. Homotypic pairs of hamster and chick embryo palatal processes, single palatal processes, and heterotypic palatal shelves of both animals were grown in vitro. The results indicated that contact between palatal shelves may not be crucial for MEE differentiation in mammals. The ability to acquire pre-fusion characteristics may be present in mammalian palatal tissue from their early development and may be expressed by cessation of DNA synthesis in the MEE, elevation of cAMP, and MEE cell death. Isolated chick palatal shelf cultured under identical conditions did not express these mammalian pre-fusion characteristics. When MEE of hamster and chick palatal shelves were placed in contact with one another, the intervening epithelia underwent cytolysis. This could be due to either the destruction of chick MEE by lysosomal enzymes liberated from adjacent degenerating hamster MEE cells, or by induction of cell death in chick MEE by hamster mesenchyme. Heterotypic palatal tissue combinations also suggest that release of lysosomal enzymes in the hamster MEE, which leads to its dissolution, may be the terminal event in epithelial differentiation prior to the establishment of mesenchymal continuity. It is suggested that an inverse relationship exists between DNA synthesis and cAMP levels during palatogenesis: when palate closes (as in mammals) the MEE is eliminated by increasing cAMP levels, whereas when palate remains open (as in birds) low level of cAMP preserve the integrity of MEE by supporting DNA synthesis.