Alterations in the epithelial surface of A-Jax mouse palatal shelves prior to and during palatal fusion: a scanning electron microscopic study

Mouse models in palate development and orofacial cleft research: Understanding the crucial role and regulation of epithelial integrity in facial and palate morphogenesis

Cleft lip and cleft palate are common birth defects resulting from genetic and/or environmental perturbations of facial development in utero. Facial morphogenesis commences during early embryogenesis, with cranial neural crest cells interacting with the surface ectoderm to form initially partly separate facial primordia consisting of the medial and lateral nasal prominences, and paired maxillary and mandibular processes. As these facial primordia grow around the primitive oral cavity and merge toward the ventral midline, the surface ectoderm undergoes a critical differentiation step to form an outer layer of flattened and tightly connected periderm cells with a non-stick apical surface that prevents epithelial adhesion. Formation of the upper lip and palate requires spatiotemporally regulated inter-epithelial adhesions and subsequent dissolution of the intervening epithelial seam between the maxillary and medial/lateral nasal processes and between the palatal shelves. Proper regulation of epithelial integrity plays a paramount role during human facial development, as mutations in genes encoding epithelial adhesion molecules and their regulators have been associated with syndromic and non-syndromic orofacial clefts. In this chapter, we summarize mouse genetic studies that have been instrumental in unraveling the mechanisms regulating epithelial integrity and periderm differentiation during facial and palate development. Since proper epithelial integrity also plays crucial roles in wound healing and cancer, understanding the mechanisms regulating epithelial integrity during facial development have direct implications for improvement in clinical care of craniofacial patients.

Cellular and developmental basis of orofacial clefts

During craniofacial development, defective growth and fusion of the upper lip and/or palate can cause orofacial clefts (OFCs), which are among the most common structural birth defects in humans. The developmental basis of OFCs includes morphogenesis of the upper lip, primary palate, secondary palate, and other orofacial structures, each consisting of diverse cell types originating from all three germ layers: the ectoderm, mesoderm, and endoderm. Cranial neural crest cells and orofacial epithelial cells are two major cell types that interact with various cell lineages and play key roles in orofacial development. The cellular basis of OFCs involves defective execution in any one or several of the following processes: neural crest induction, epithelial-mesenchymal transition, migration, proliferation, differentiation, apoptosis, primary cilia formation and its signaling transduction, epithelial seam formation and disappearance, periderm formation and peeling, convergence and extrusion of palatal epithelial seam cells, cell adhesion, cytoskeleton dynamics, and extracellular matrix function. The latest cellular and developmental findings may provide a basis for better understanding of the underlying genetic, epigenetic, environmental, and molecular mechanisms of OFCs.

TGFβ3 regulates periderm removal through ΔNp63 in the developing palate

The periderm is a flat layer of epithelium created during embryonic development. During palatogenesis, the periderm forms a protective layer against premature adhesion of the oral epithelia, including the palate. However, the periderm must be removed in order for the medial edge epithelia (MEE) to properly adhere and form a palatal seam. Improper periderm removal results in a cleft palate. Although the timing of transforming growth factor β3 (TGFβ3) expression in the MEE coincides with periderm degeneration, its role in periderm desquamation is not known. Interestingly, murine models of knockout (-/-) TGFβ3, interferon regulatory factor 6 (IRF6) (-/-), and truncated p63 (ΔNp63) (-/-) are born with palatal clefts because of failure of the palatal shelves to adhere, suggesting that these genes regulate palatal epithelial differentiation. However, despite having similar phenotypes in null mouse models, no studies have analyzed the possible association between the TGFβ3 signaling cascade and the IRF6/ΔNp63 genes during palate development. Recent studies indicate that regulation of ΔNp63, which depends on IRF6, facilitates epithelial differentiation. We performed biochemical analysis, gene activity and protein expression assays with palatal sections of TGFβ3 (-/-), ΔNp63 (-/-), and wild-type (WT) embryos, and primary MEE cells from WT palates to analyze the association between TGFβ3 and IRF6/ΔNp63. Our results suggest that periderm degeneration depends on functional TGFβ3 signaling to repress ΔNp63, thereby coordinating periderm desquamation. Cleft palate occurs in TGFβ3 (-/-) because of inadequate periderm removal that impedes palatal seam formation, while cleft palate occurs in ΔNp63 (-/-) palates because of premature fusion.

Convergence and extrusion are required for normal fusion of the mammalian secondary palate

The fusion of two distinct prominences into one continuous structure is common during development and typically requires integration of two epithelia and subsequent removal of that intervening epithelium. Using confocal live imaging, we directly observed the cellular processes underlying tissue fusion, using the secondary palatal shelves as a model. We find that convergence of a multi-layered epithelium into a single-layer epithelium is an essential early step, driven by cell intercalation, and is concurrent to orthogonal cell displacement and epithelial cell extrusion. Functional studies in mice indicate that this process requires an actomyosin contractility pathway involving Rho kinase (ROCK) and myosin light chain kinase (MLCK), culminating in the activation of non-muscle myosin IIA (NMIIA). Together, these data indicate that actomyosin contractility drives cell intercalation and cell extrusion during palate fusion and suggest a general mechanism for tissue fusion in development.

A study of the mouse palate shows that the fusion of tissues during development involves convergence and displacement of epithelial cells, coupled with cell extrusion driven by the contractile activity of actomyosin.

Tissue fusion, the process by which two independent prominences become united to form one continuous structure, is common during development, and its failure leads to multiple structural birth defects. In this study, we directly examine the cellular and molecular mechanisms by which tissue fusion occurs using the mouse secondary palate as a model. Using live imaging, we find that fusion of the secondary palatal shelves proceeds by a progression of previously undescribed cell behaviors. Cellular protrusions and establishment of contacts between palatal shelves leads to the formation of a transient multicellular epithelial structure that then converges toward the midline, driven by cell intercalation. This convergence occurs together with displacement of the epithelium and epithelial cell extrusions that squeeze epithelial cells out from between the palatal shelves and mediate continuity of the structure. We show that in mice this morphogenesis requires an actomyosin contractility pathway culminating in non-muscle myosin IIA activation. Altogether, these data support a new model for tissue fusion during mouse embryogenesis in which convergence, displacement, and cell extrusion drive the union of independent structures.

Intra-amniotic transient transduction of the periderm with a viral vector encoding TGFβ3 prevents cleft palate in Tgfβ3(-/-) mouse embryos

Cleft palate is a developmental defect resulting from the failure of embryonic palatal shelves to fuse with each other at a critical time. Immediately before and during palatal fusion (E13-E15 in mice), transforming growth factor β3 (TGFβ3) is expressed in the palatal shelf medial edge epithelium (MEE) and plays a pivotal role in palatal fusion. Using Tgfβ3(-/-) mice, which display complete penetrance of the cleft palate phenotype, we tested the hypothesis that intra-amniotic gene transfer could be used to prevent cleft palate formation by restoring palatal midline epithelial function. An adenoviral vector encoding Tgfβ3 was microinjected into the amniotic sacs of mouse embryos at successive developmental stages. Transduced Tgfβ3(-/-) fetuses showed efficient recovery of palatal fusion with mesenchymal confluence following injection at E12.5 (100%), E13.5 (100%), E14.5 (82%), and E15.5 (75%). Viral vectors injected into the amniotic sac transduced the most superficial and transient peridermal cell layer but not underlying basal epithelial cells. TGFβ3 transduction of the peridermdal cell layer was sufficient to induce adhesion, fusion, and disappearance of the palatal shelf MEE in a cell nonautonomous manner. We propose that intra-amniotic gene transfer approaches have therapeutic potential to prevent cleft palate in utero, especially those resulting from palatal midline epithelial dysfunction.

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.

Palatal seam disintegration: to die or not to die? that is no longer the question

Formation of the medial epithelial seam (MES) by palatal shelf fusion is a crucial step of palate development. Complete disintegration of the MES is the final essential phase of palatal confluency with surrounding mesenchymal cells. In general, the mechanisms of palatal seam disintegration are not overwhelmingly complex, but given the large number of interacting constituents; their complicated circuitry involving feedforward, feedback, and crosstalk; and the fact that the kinetics of interaction matter, this otherwise simple mechanism can be quite difficult to interpret. As a result of this complexity, apparently simple but highly important questions remain unanswered. One such question pertains to the fate of the palatal seam. Such questions may be answered by detailed and extensive quantitative experimentation of basic biological studies (cellular, structural) and the newest molecular biological determinants (genetic/dye cell lineage, gene activity, kinase/enzyme activity), as well as animal model (knockouts, transgenic) approaches. System biology and cellular kinetics play a crucial role in cellular MES function; omissions of such critical contributors may lead to inaccurate understanding of the fate of MES. Excellent progress has been made relevant to elucidation of the mechanism(s) of palatal seam disintegration. Current understanding of palatal seam disintegration suggests epithelial-mesenchymal transition and/or programmed cell death as two most common mechanisms of MES disintegration. In this review, I discuss those two mechanisms and the differences between them.

Palatal fusion - where do the midline cells go? A review on cleft palate, a major human birth defect

Formation of the palate, the organ that separates the oral cavity from the nasal cavity, is a developmental process characteristic to embryos of higher vertebrates. Failure in this process results in palatal cleft. During the final steps of palatogenesis, two palatal shelves outgrowing from the sides of the embryonic oronasal cavity elevate above the tongue, meet in the midline, and rapidly fuse together. Over the decades, multiple mechanisms have been proposed to explain how the superficial mucous membranes disappear from the contact line, thus allowing for normal midline mesenchymal confluence. A substantial body of experimental evidence exists for cell death, cell migration, epithelial-to-mesenchymal transdifferentiation (EMT), replacement through new tissue intercalation, and other mechanisms. However, the most recent use of gene recombination techniques in cell fate tracking disfavors the EMT concept, and suggests that apoptosis is the major fate of the midline cells during physiological palatal fusion. This article summarizes the benefits and drawbacks of histochemical and molecular tools used to determine the fates of cells within the palatal midline. Mechanisms of normal disintegration of the midline epithelial seam are reviewed together with pathologic processes that prevent this disintegration, thus causing cleft palate.

Tails of the unexpected: palatal medial edge epithelium is no more specialized than other embryonic epithelium

OBJECTIVE

To determine whether palatal medial edge epithelium (MEE) is specialized in its ability to disappear compared with other embryonic, non-palatal, epithelium.

SUBJECTS

Embryonic tissues harvested from CD1 mice.

METHODS

Organs were cultured in 2 ml of DMEM/F12 supplemented with 300 microg/ml L-glutamine and 1% penicillin/streptomycin. Organs were cultured under various conditions including opposing other organs and opposing an inert material for a period of 6 days. Tissues were then processed for histological examination.

RESULTS

MEE of shelves opposing nothing persisted, whereas MEE of shelves contacting another shelf disappeared. When a tail was placed against a palatal shelf the MEE disappeared, as did the epithelium from the tail, resulting in fusion between the shelf and tail. Furthermore, when palatal shelves were placed against an inert material the MEE disappeared, suggesting pressure alone is a sufficient stimulus to initiate disappearance of the MEE, and that the interaction between the two palatal shelves is not a prerequisite for the disappearance of MEE. Moreover, when two embryonic tails were cultured in close apposition they fused, as did paired limbs. Non-palatal epithelia also disappeared after contact with inert materials. Epithelial disappearance began within 24 h of contact, but there was an age limit.

CONCLUSION

These findings suggest that embryonic epithelium from non-specific sites around the body has the ability to disappear with mechanical contact resulting in fusion of tissues. MEE may not be as specialized as once thought.

Jag2-Notch1 signaling regulates oral epithelial differentiation and palate development

During mammalian palatogenesis, palatal shelves initially grow vertically from the medial sides of the paired maxillary processes flanking the developing tongue and subsequently elevate and fuse with each other above the tongue to form the intact secondary palate. Pathological palate-mandible or palate-tongue fusions have been reported in humans and other mammals, but the molecular and cellular mechanisms that prevent such aberrant adhesions during normal palate development are unknown. We previously reported that mice deficient in Jag2, which encodes a cell surface ligand for the Notch family receptors, have cleft palate associated with palate-tongue fusions. In this report, we show that Jag2 is expressed throughout the oral epithelium and is required for Notch1 activation during oral epithelial differentiation. We show that Notch1 is normally highly activated in the differentiating oral periderm cells covering the developing tongue and the lateral oral surfaces of the mandibular and maxillary processes during palate development. Oral periderm activation of Notch1 is significantly attenuated during palate development in the Jag2 mutants. Further molecular and ultrastructural analyses indicate that oral epithelial organization and periderm differentiation are disrupted in the Jag2 mutants. Moreover, we show that the Jag2 mutant tongue fused to wild-type palatal shelves in recombinant explant cultures. These data indicate that Jag2-Notch1 signaling is spatiotemporally regulated in the oral epithelia during palate development to prevent premature palatal shelf adhesion to other oral tissues and to facilitate normal adhesion between the elevated palatal shelves.

Epithelial cell proliferation and apoptosis in the developing murine palatal rugae

Epithelial cell proliferation and apoptosis during morphogenesis of the murine palatal rugae (PR) were examined histochemically by using anti-bromodeoxyuridine (BrdU) and the terminal deoxynucleotidyl transferase-mediated UTP nick-end-labelling (TUNEL) technique. Formation of the PR rudiment was observed as an epithelial placode in fetuses at 12.5 days post-coitus (dpc). During the PR formation, BrdU-positive cells were detected mainly in the epithelium of the interplacode and interprotruding areas in fetuses administered BrdU maternally at 2 h before killing. TUNEL-positive cells were detected only at the epithelial placode area in 12.5-14.5 dpc. At 16.5-18.5 dpc, the BrdU-positive cells were decreased in number in the epithelial cells at the interprotruding area of the PR. Only a few TUNEL-positive cells were observed in the protruding area of the PR at 16.5 dpc. These results suggest that cell proliferation and apoptosis in the palatal epithelium are involved spatiotemporally in the murine PR morphogenesis.

Medial edge epithelial cell fate during palatal fusion

To explain the disappearance of medial edge epithelial (MEE) cells during palatal fusion, programmed cell death, epithelial-mesenchymal transformation, and migration of these cells to the oral and nasal epithelia have been proposed. However, MEE cell death has not always been accepted as a mechanism involved in midline epithelial seam disappearance. Similarly, labeling of MEE cells with vital lipophilic markers has not led to a clear conclusion as to whether MEE cells migrate, transform into mesenchyme, or both. To clarify these controversies, we first utilized TUNEL techniques to detect apoptosis in mouse palates at the fusion stage and concomitantly analyzed the presence of macrophages by immunochemistry and confocal microscopy. Second, we in vitro infected the MEE with the replication-defective helper-free retroviral vector CXL, which carries the Escherichia coli lacZ gene, and analyzed beta-galactosidase activity in cells after fusion to follow their fate. Our results demonstrate that MEE cells die and transform into mesenchyme during palatal fusion and that dead cells are phagocytosed by macrophages. In addition, we have investigated the effects of the absence of transforming growth factor beta(3) (TGF-beta(3)) during palatal fusion. Using environmental scanning electron microscopy and TUNEL labeling we compared the MEE of the clefted TGF-beta(3) null and wild-type mice. We show that MEE cell death in TGF-beta(3) null palates is greatly reduced at the time of fusion, revealing that TGF-beta(3) has an important role as an inducer of apoptosis during palatal fusion. Likewise, the bulging cells observed on the MEE surface of wild-type mice prior to palatal shelf contact are very rare in the TGF-beta(3) null mutants. We hypothesize that these protruding cells are critical for palatal adhesion, being morphological evidence of increased cell motility/migration.

Pathogenesis of cleft palate in TGF-beta3 knockout mice

We previously reported that mutation of the transforming growth factor-beta3 (TGF-beta3) gene caused cleft palate in homozygous null (−/−) mice. TGF-beta3 is normally expressed in the medial edge epithelial (MEE) cells of the palatal shelf. In the present study, we investigated the mechanisms by which TGF-beta3 deletions caused cleft palate in 129 × CF-1 mice. For organ culture, palatal shelves were dissected from embryonic day 13.5 (E13.5) mouse embryos. Palatal shelves were placed singly or in pairs on Millipore filters and cultured in DMEM/F12 medium. Shelves were placed in homologous (+/+ vs +/+, −/− vs −/−, +/− vs +/−) or heterologous (+/+ vs −/−, +/− vs −/−, +/+ vs +/−) paired combinations and examined by macroscopy and histology. Pairs of −/− and −/− shelves failed to fuse over 72 hours of culture whereas pairs of +/+ (wild-type) and +/+ or +/− (heterozygote) and +/−, as well as +/+ and −/− shelves, fused within the first 48 hour period. Histological examination of the fused +/+ and +/+ shelves showed complete disappearance of the midline epithelial seam whereas −/− and +/+ shelves still had some seam remnants. In order to investigate the ability of TGF-beta family members to rescue the fusion between −/− and −/− palatal shelves in vitro, either recombinant human (rh) TGF-beta1, porcine (p) TGF-beta2, rh TGF-beta3, rh activin, or p inhibin was added to the medium in different concentrations at specific times and for various periods during the culture. In untreated organ culture −/− palate pairs completely failed to fuse, treatment with TGF-beta3 induced complete palatal fusion, TGF-beta1 or TGF-beta2 near normal fusion, but activin and inhibin had no effect. We investigated ultrastructural features of the surface of the MEE cells using SEM to compare TGF-beta3-null embryos (E 12. 5-E 16.5) with +/+ and +/− embryos in vivo and in vitro. Up to E13.5 and after E15.5, structures resembling short rods were observed in both +/+ and −/− embryos. Just before fusion, at E14.5, a lot of filopodia-like structures appeared on the surface of the MEE cells in +/+ embryos, however, none were observed in −/− embryos, either in vivo or in vitro. With TEM these filopodia are coated with material resembling proteoglycan. Interestingly, addition of TGF-beta3 to the culture medium which caused fusion between the −/− palatal shelves also induced the appearance of these filopodia on their MEE surfaces. TGF-beta1 and TGF-beta2 also induced filopodia on the −/− MEE but to a lesser extent than TGF-beta3 and additionally induced lamellipodia on their cell surfaces. These results suggest that TGF-beta3 may regulate palatal fusion by inducing filopodia on the outer cell membrane of the palatal medial edge epithelia prior to shelf contact. Exogenous recombinant TGF-beta3 can rescue fusion in −/− palatal shelves by inducing such filopodia, illustrating that the effects of TGF-beta3 are transduced by cell surface receptors which raises interesting potential therapeutic strategies to prevent and treat embryonic cleft palate.

Programmed cell death and cell transformation in craniofacial development

Fusion of branchial arch derivatives is an essential component in the development of craniofacial structures. Bilaterally symmetric branchial arch processes fuse in the midline to form the mandible, lips, and palate. The mechanism for fusion requires several different morphologic and molecular events prior to the completion of the mesenchymal continuity between opposing tissue processes. The ectodermal covering of the branchial arches is one of the cell types that has an important role during craniofacial development. The surface epithelia provide the initial adherence between the processes; however, this population of cells is ultimately absent from the fusion zone. The medial edge epithelium of the secondary palatal shelves is one example of such an epithelium that must disappear from the fusion zone of the secondary palate during development in order to complete palatal fusion. The mechanisms for removal of the epithelial cells from the fusion zone could include either programmed cell death, epithelial-mesenchymal transformation, or migration to adjacent epithelia. All three of these fates have been hypothesized as a mechanism for the removal of the palatal medial edge epithelia. The processes of programmed cell death, epithelial-mesenchymal transformation, and epithelial migration are reviewed with respect to both palatal fusion and results reported in other model systems.

Medial edge epithelium fate traced by cell lineage analysis during epithelial-mesenchymal transformation in vivo

Vital cell labeling techniques were used to trace the fate of the medial edge epithelial (MEE) cells during palatal fusion in vivo. Mouse palatal tissues were labeled in utero with DiI. The fetuses continued to develop in utero and tissues of the secondary palate were examined at several later stages of palatal ontogeny. The presence and distribution of DiI was correlated with the presence of cell phenotype-specific markers. During the initial stages of palatal fusion the DiI-labeled MEE were present in the midline position. These cells were attached to an intact laminin-containing basement membrane and contained keratin intermediate filaments. At later stages of palatogenesis the DiI-labeled MEE were not separated from the mesenchyme by an intact basement membrane and did not contain keratin. In late fetal development, DiI-labeled cells without an epithelial morphology were present in the mesenchyme. The transition of the DiI-labeled cells from an epithelial phenotype to a mesenchymal phenotype is consistent with a fate of epithelial-mesenchymal transformation rather than programmed cell death.

Studies of variant palatal rugae in normal and corticosteroid-treated mouse embryos

Fourteen- and 15-day mouse embryos treated with triamcinolone on day 11 of gestation were examined for the presence of variant rugae. Nontreated mouse embryos served as controls. Variant rugae found were classified into five types. All five types of variations (bifurcation, division, supernumerary, shortness and cross) were observed in triamcinolone-treated embryos, and shortness was most frequently seen. Supernumerary, bifurcation and division were ranked next, following by cross. Variant, rugae, except the cross, were also observed in non-treated embryos in low frequencies, but more than one-half of them were the bifurcation of the second ruga. Divided rugae ranked next, and supernumerary and shortness were found occasionally. Except for the bifurcated and supernumerary rugae, the greater part of the variant rugae were found in the fifth and fourth ruga in the triamcinolone-treated groups and in the fifth ruga in the nontreated groups. As the incidence of variant rugae in the triamcinolone-treated embryos was significantly higher than that in the nontreated, it was regarded as one of the changes induced by the corticoid. Based on the characteristic features of the rugal region, it is speculated that the formation of variant rugae is associated with the disturbance of normal epithelial-mesenchymal interaction which may be controlled by the nerve fibers appearing at the time of rugal formation. The relationship between the increased appearance of variant rugae and the failure of palatal shelf elevation was examined, but no direct evidence was obtained.

Morphogenesis of the secondary palate in mouse embryos with special reference to the development of rugae

Morphological studies of secondary palate formation, with special reference to the development of rugae, were carried out on Jcl:ICR mouse embryos. Three rugae were observed on the anterior part of the future oral surface of the vertically developing palatal shelves in 13-day embryos. Rugae increased in number as the development of the palatal shelves proceeded, and five to six prominent rugae were observed in 14-day embryos just prior to shelf elevation. The folding of these five to six rugae progressed in conjunction with the formation of a sharp, valley-like groove at the base of the anterior two-fifths of the vertical palatal shelves. As palatal shelves elevated, the groove disappeared gradually, and, accordingly, the folding of rugae loosened. In the groove region, the superficial epithelial cells were roundish, while the basal ones were elongated. Such characteristic features were no longer observed when the disappearance of the groove was completed. Eight rugae were observed on the future hard palate of 14-day embryos with already completed palatal fusion. An additional ruga was frequently found in 15-day embryos, and the pattern then was almost the same as that of an adult. Epithelial thickening and condensation at the rugae region, as well as mesenchymal condensation under the epithelium of the rugae, were confirmed in embryos both before and after elevation of the palatal shelves. There is a possibility that these structural characteristics observed in the epithelial and mesenchymal cells of the rugae and groove regions may be related to palatal shelf elevation.

Medial edge epithelium transforms to mesenchyme after embryonic palatal shelves fuse

The disappearance of palatal medial edge epithelium (MEE) after fusion of secondary palatal shelves is often cited as a classical example of embryonic remodeling by programmed cell death. We reinvestigated this phenomenon in 16-day rat embryos, using light and electron microscopy. We confirm reports that the periderm of the two-layered MEE begins to slough after shelves assume horizontal positions. In vitro, peridermal cells are not able to slough and are trapped during the adhesion process. In vivo, however, surface cells shed before the shelves in the anterior palate adhere, allowing junctions to form between opposing basal epithelial cells. Midline seams so formed consist of two layers of basal cells, all of which appear healthy. Even though its cells are dividing, growth of the seam fails to keep pace with palatal growth and it thins to one layer of cells, and then breaks up into small islands. The basal lamina disappears and elongating MEE cells extend filopodia into adjacent connective tissue. Electron micrographs reveal transitional steps in loss of epithelial characteristics and gain of fibroblast-like features by transforming MEE cells. One such feature, observed with the aid of immunofluorescence, is the turn of the mesenchymal cytoskeletal protein, vimentin. No cell death or macrophages are observed after adhesion and thinning over most of the palate. These data indicate that MEE is an ectoderm that retains the ability to transform into mesenchymal cells. Epithelial-mesenchymal transformation may be expressed in other embryonic remodelings (R.L. Trelstad, A. Hayashi, K. Hayashi, and P.K. Donahue, 1982, Dev. Biol. 92, 27), resulting in heretofore unsuspected conservation of embryonic cell populations.

Adhesion and fusion of the extraembryonic epiblast

A tissue culture model system has been devised to examine the attachment, expansion, and fusion of epithelial cell sheets. A normal embryonic epithelial tissue, the extraembryonic epiblast of the chick, is isolated mechanically and cultured on its natural substratum, the vitelline membrane. This persistently migratory tissue has distinct adhesive and non-adhesive regions. A serum-free chemically-defined culture medium has been formulated that permits determination of the effects of individual growth and trophic factors. Attachment of transferred epiblasts is dependent upon the presence of mineralocorticoids in the medium. This suggests that fluid transport is required for the cell sheet to make its initial attachment to the culture substratum. Expansion of the cell sheet following attachment, and the fusion of epiblasts advancing toward each other, does not require the presence of mineralocorticoid. No exogenous adhesive glycoproteins are required for attachment, expansion, or fusion. Antibody localization shows that endogenous laminin is present on the attachment surface of the specialized adhesive edge region of the extraembryonic epiblast. Following fusion of confronted epiblasts into one coherent cell sheet, the laminin disappears. Throughout these studies the adhesive and non-adhesive regions of the epiblast are identified by their characteristic distributions of actin microfilaments, localized using rhodamine-phalloidin staining.

Elevated glucocorticoid receptor levels in lymphocytes of children with the fetal hydantoin syndrome (FHS)

Our recent studies of the teratogenic mechanisms of phenytoin (DPH) and glucocorticoids in mice have indicated that DPH utilizes the anti-inflammatory pathway of glucocorticoids in producing congenital defects, such as cleft palate. This pathway is influenced by H-2 and H-3 histocompatibility-linked genes in the mouse, such that congenic strains have H-2 or H-3 alleles that confer susceptibility to DPH-induced congenital defects, and susceptible H-2 congenic strains have high glucocorticoid receptor levels. However, other H-2 or H-3 alleles confer resistance to these defects in their otherwise genetically identical congenic partner strains, and "resistant" H-2 alleles are associated with low levels of these receptors. To determine whether this animal work is applicable to the human, we have sought to investigate whether the level of glucocorticoid receptors in circulating lymphocytes of children with the fetal hydantoin syndrome (FHS) is as it is in the animals. We found that children with FHS had glucocorticoid receptor levels significantly elevated above those of unaffected children with similar DPH exposure in control families. The receptor level of affected children was also significantly elevated above that of fathers of children with the FHS and of fathers and mothers of control children. These findings are consistent with those documented in the animal models and suggest that an elevated level of glucocorticoid receptors in lymphocytes may be a marker for susceptibility to the FHS syndrome.

Arachidonic acid and male genital differentiation

Arachidonic acid (AA) prevents neural tube/defects, cleft palate, and micrognathia in the rat models of diabetic embryopathy and neural tube defects in the mouse embryo culture model. In this study, the involvement of AA in the male genital differentiation was described. These observations raise the complementary possibility that the AA-prostaglandin biochemical pathway may be a major mechanism mediating many embryonic events that involve cellular movement and fusion.

The arachidonic acid cascade is involved in the masculinizing action of testosterone on embryonic external genitalia in mice

We have evaluated whether the arachidonic acid cascade may be involved in the folding and fusion of the penis and scrotum in masculine differentiation, a possibility raised by recent observations of the involvement of the arachidonic acid cascade in the analogous embryonic processes of elevation and fusion of the palatal shelves and of folding and fusion of the neural tube. To test this hypothesis, during embryonic masculine differentiation in mice of the B10.A strain, we administered certain agents that produce blockade of masculinization. We report that arachidonic acid can reverse the inhibition of masculine development in male embryos produced by estradiol-17 beta or by cyproterone acetate, an androgen receptor-site blocker, and that such reversal can be prevented by an inhibitor of cyclooxygenase, such as indomethacin. We have also found that agents that block the arachidonic acid cascade at the level of phospholipase A2 (cortisone, phenytoin) or at the level of cyclooxygenase (indomethacin, aspirin) also block masculine differentiation and that such antimasculinization is reversed by arachidonic acid. The masculinization of male embryos is inhibited by indomethacin and aspirin, and the masculinization of female embryos produced by exogenous testosterone is prevented by indomethacin. These findings provide evidence that the mechanism by which testosterone organizes the genitalia involves a role of the arachidonic acid cascade leading to prostaglandins at a critical period of development and that interference with testosterone synthesis or action leads to a teratogenic deficiency of arachidonic acid during this time in the genital anlagen.

Hyperglycemia-induced teratogenesis is mediated by a functional deficiency of arachidonic acid

Congenital malformations now represent the largest single cause of mortality in the infant of the diabetic mother. The mechanism by which diabetes exerts its teratogenic effects is not known. This study evaluated whether arachidonic acid might be involved, a possibility raised by the role of arachidonic acid in palatal elevation and fusion, processes analogous to neural tube folding and fusion. This hypothesis was tested in two animal models of diabetic embryopathy, the in vivo pregnant diabetic rat and the in vitro hyperglycemic mouse embryo culture. The subcutaneous injection of arachidonic acid (200-400 mg/kg per day) into pregnant diabetic rats during the period of organ differentiation (days 6-12) did not alter the maternal glucose concentration, the maternal weight gain, or the weight of the embryos. However, the incidence of neural tube fusion defects was reduced from 11% to 3.8% (P less than 0.005), the frequency of cleft palate was reduced from 11% to 4% (P less than 0.005), and the incidence of micrognathia was reduced from 7% to 0.8% (P less than 0.001). The addition of arachidonic acid to B10.A mouse embryos in culture also resulted in a reversal of hyperglycemia-induced teratogenesis. The teratogenic effect of D-glucose (8 mg/ml) in the medium resulted in normal neural tube fusion in only 32% of the embryos (P less than 0.006 when compared to controls). Arachidonic acid supplementation (1 or 10 micrograms/ml) produced a rate of neural tube fusion (67%) that was not significantly different from that observed in controls. The evidence presented indicates that arachidonic acid supplementation exerts a significant protective effect against the teratogenic action of hyperglycemia in both in vivo (rat) and in vitro (mouse) animal models. These data therefore suggest that the mechanism mediating the teratogenic effect of an increased glucose concentration involves a functional deficiency of arachidonic acid at a critical stage of organogenesis.

Light and electron microscopy study of fusion of facial prominences. A distinctive type of superficial cells at the contact sites

The contact site between the medial nasal prominence (MNP) and the lateral nasal prominence (LNP) during the period of primary palate formation in the mouse embryo was examined by light and electron microscopy. Throughout this period, a distinctive type of superficial cell was observed at the contact site. These superficial cells had a large nucleus and abundant cytoplasm as well as structural features characteristic of embryonic cells. At earlier stages, these cells were seen at the transitional region between the surface ectoderm and the epithelia of the nasal pit at the end of the isthmus, where initial contact of opposing MNP and LNP took place. At later stages, these superficial cells appeared to bridge the gap between MNP and LNP at the contact sites, which extended to the bottom of the valley formed by MNP and LNP. These cells were also observed on the surface near the contact sites, that is, the presumptive fusion area. These superficial cells displayed well-developed junctional complexes (intermediate and gap junctions, and desmosomes). Many filaments were observed subjacent to the plasma membranes of these superficial cells, some of which were associated with junctional complexes. These observations suggest that this kind of distinctive superficial cell may play critical roles in the contact of MNP and LNP throughout the fusion process.

Differentiation of cultured palatal shelves from alligator, chick, and mouse embryos

Palatal shelves from embryonic alligators, chicks, and mice were explanted at various stages of development and organ cultured in either chemically defined, serumless media or the same media supplemented with 10% fetal calf serum. Shelves from each vertebrate were either cultured singly or in contact, and heterologous combinations of palatal shelves from different animals were made: chick/mouse, chick/alligator, and mouse/alligator. Epithelial differentiation (particularly that of the medial shelf edge) was assayed by vital staining, histology, and scanning electron microscopy. In vitro medial edge epithelial differentiation, and consequently the mechanisms of palatal closure, were identical to those normally seen in vivo for each species, i.e., cobble-stoned migrating epithelia in the alligator, cell death and fusion in the mouse, and keratinisation and cleft palate in the chick. Differentiation was optimal in the chemically defined, serumless media and was independent of shelf contact in all three species. No heterologous combinations of palatal shelves closed with each other: Evidently, the modes of palatal closure in mice and alligators are sufficiently different to prevent them forming a chimeric palate, whilst neither is capable of inducing closure in a cocultured chick palatal shelf. These unified defined culture conditions make possible a large number of epithelial-mesenchymal recombination studies as well as specific inhibitor, teratogenic, and hormonal investigations.

Eyelid growth and fusion in fetal mice. A scanning electron microscope study

During the last phase of mammalian morphogenesis, between days 14 and 16 of gestation in the mouse, the fetal eyelids grow across the eye and become tightly fused with each other. This paper describes the surface pattern of fetal eyelids, revealed by the scanning electron microscope, during normal eyelid growth and fusion in the ICR/MI stock of mice. Fusion proceeds from both inner and outer canthi and progresses toward the middle of the gap. The first changes in cell shape and distribution occur at the inner canthus. On day 14, a large clump of rounded cells appears on the inner surface of the inner canthus. A day later, two clumps of rounded cells are positioned to either side of, i.e. above and below, the inner canthus. As fusion progresses, the diminishing gap fills with a profusion of rounded cells that are extruded, flattened, and sloughed off from the area of completed fusion. The profusion of rounded surface cells during eyelid growth and fusion appears to be a major characteristic in which the eyelid fusion process differs both from permanent fusions, such as the fusion of the neural tube, lip or palate, and from other temporary fusions, such as fusion of the digits to each other or of the pinnae to the scalp.

Primary palatal development in the chick

Seventy-nine chick embryos were examined by light microscopy and scanning electron microscopy to determine the mechanism of primary palatal development. Fusion between two discrete processes, the medial nasal and maxillary prominences, was found to be necessary for formation of a complete primary palate. This was one component of a three-stage process that included: (1) invagination of the nasal pit prior to the appearance of the facial prominences; (2) fusion between the medial nasal and maxillary processes caudal to the nasal groove; (3) rupture of the bucconasal membrane. The lateral nasal and maxillary prominences were found to be part of the same tissue mass. Mergence was proposed as a mechanism for the obliteration of the groove between these two localized prominences. These results were compared with those obtained by other authors for primary palate formation in rodents and man.

Secondary palatal development in the New Zealand white rabbit: a scanning electron microscopic study

Examination of surface topography in prefusion stages of secondary palatal development in rabbit embryos reveals a sequence of alterations in the surface cells of the epithelium along the medial margins of the palatal processes. A progressive increase in cellular protrusions resembling lamellipodia and filopodia, as well as cellular necrosis, is observed in those areas that undergo fusion. The changes precede fusion and are restricted to fusion sites. Prior to and at the time of epithelial contact between palatal processes, many long slender cellular protrusions are seen bridging the gap between the approximating tissues. The localization of the epithelial alterations and the appearance of similar cellular morphology in other embryonic epithelial fusion events strongly suggest: either an active role of the epithelial cells in the fusion of the secondary palate, or some common fundamental biochemical events that may facilitate or are responsible for the initial adhesion of such tissues.

Facial development in normal and mutant chick embryos. I. Scanning electron microscopy of primary palate formation

Early facial development in normal chick embryos was studied by scanning electron microscopy, and compared to the abnormal facial development of a mutant in which primary palate formation does not occur, thus resulting in bilateral cleft lip. In both normal and "cleft primary palate" mutant embryos, subsequent to the appearance of the nasal placodes, the surrounding tissues elevate to give rise to the presumptive facial primordia. As the facial primordia grow forward, they gradually assume the configuration of a square which is most pronounced at five days development. In normal embryos, the square configuration is then lost as the facial primordia become aligned in preparation for primary palate formation. The primary palate is formed at six days development by fusion of the "free-ended" medial nasal processes with the combined lateral nasal and maxillary processes across the nasal grooves. Just prior to fusion, long, slender filaments extend from the apposing surfaces of the facial primordia in the regions of prefusion contact. It is speculated that these "prefusion filaments" may function in alignment or adhesion of the facial primordia. In "cleft primary palate" embryos, facial morphogenesis appears to arrest at five days development, so that the square configuration persists. The medial nasal processes never contact the lateral nasal and maxillary processes, but instead remain separated from them by wide nasal grooves. Furthermore, facial primordia of mutant embryos do not exhibit the "prefusion filaments" characteristic of normal embryos.

Extracellular coat in developing human palatal processes: electron microscopy and ruthenium red binding

The prefusion epithelium of human palatal processes was examined for evidence of specialization which might facilitate epithelial adherence with the opposing palatal process. A surface coat stained with ruthenium red (RR) was found on all apical aspects of the palatal epithelium. In the prefusion regions, RR staining was also observed in the spaces between the superficial cells of the epithelium and in necrotic cells. Adjacent oral and nasal epithelium excluded the RR below the level of the apical junctional complex. In the absence of RR, a dense material was observed in the most superficial intercellular spaces of the prefusion region. Many superficial cells in the area were in various stages of necrosis. The combination of degenerating surface cells and an accumulation of a poly-anionic substance such as glycoprotein may facilitate epithelial adherence between opposing human palatal processes.

Scanning electron microscopy of cell surfaces following removal of extracellular material

The application of scanning electron microscopy to the study of cell surfaces is limited in intact tissues, because extracellular material may often obscure the details of nonluminal surfaces. To remove connective tissue elements we have treated human skin and both kidney, and an autonomic ganglion of the rat with hydrochloric acid and collagenase. Regional variations in the basal surface of the nephron are noted following removal of the basement membrane. The basilar interdigitations of the cells of the proximal tubule appeared as parallel ridges encircling the tubule. Ridges on the parietal epithelium of Bowman's capsule were randomly arranged and alternated with smooth surfaces. The dermal surface of the human epidermis has an alveolar or honeycomb appearance due to the elevation of the epidermal ridges and numerous pits for the dermal pegs. At higher magnifications the basal surface of cells of the stratum germinativum possessed numerous and irregular projections. Neurons with their processes are evident in the autonomic ganglion. The soma of the neurons are enclosed by flattened satellite cells. Irregular spaces between opposed satellite cells are interpreted as regions for the passage of processes related to the ganglion cells. Nodes of Ranvier were clearly seen along nerve fibers. Some pitting of the nerve fibers was also noted. The HCl-collagenase method has the advantage of the removal of collagen and basement membrane while preserving the structural integrity of the cell surface.

Topographical changes along the neural fold associated with neurulation in the hamster and mouse

The topography of the ectoderm was examined by scanning electron microscopy during neurulation in hamster and mouse embryos. Stages from the appearance of the neural folds to closure of the posterior neuropore were studied. Progressive development of a zone of altered cellular morphology was observed along the crests of the neural folds. This zone evolved from an abrupt transition between surface and neural regions of the ectoderm to a narrow band of flattened cells which exhibited numerous membranous "ruffles" in the mouse, or blebs and presumably degenerating cells in the hamster, immediately prior to contact between the folds. These alterations were more prominent along the anterior than the posterior portions of the folds. Contact of the folds occurred first between the flattened cells with subsequent union of the surface cells. Stages of neural crest cell formation was observed subjacent to the zone of alterations in histological sections. It is suggested that the observed surface alterations may reflect changes in the membrane properties of the altered cells which are correlated with both neural crest formation and initial adhesion between the folds.

Scanning electron microscopy of epithelia prepared by blunt dissection

Simple dissection techniques of samples to be examined in the scanning electron microscope allow one to visualize easily the three-dimensional shape of epithelial cells in situ. Such preparations reveal a complex system of ridges and folds on the lateral surface of the cells whose intricacy can best be appreciated with SEM. In many epithelia there is a smooth apical band which corresponds to the region occupied by the junctional complex previously identified with conventional EM techniques. The secretion of chylomicra that result from a fatty meal can be observed. It is possible to study the distribution of concanavalin A binding sites on the lateral surfaces of the cells utilizing hemocyanin as a marker. In the case of the proximal tubule epithelium, the apical cell surface has many more binding sites than the lateral cell surface and there is a sharp demarcation at the level of the apical band. After blunt dissection the relationship of the basal surface of the cells with the basement lamina and the basement membrane can be appreciated as well. Possible physiological meaning of the morphological features observed is briefly discussed.

Surface coat of the palatal shelf epithelium during palatogenesis in mouse embryos

The palatal shelf epithelium of NMRI-mouse embryos was examined by electron microscopy during palatogenesis (day 14 + 2 hrs p.c. to day 14 + 10 hrs p.c.). A ruthenium red positive surface coat was observed on the epithelium of the vertically projected and horizontally rotated palatal processes. The surface coat on the tips of these processes was especially thick and was observed to be shed after fusion. The localization and the quantitative changes of the ruthenium red positive material during closure of the palatal shelves suggest that the surface coat may be involved in the initial process of palatal epithelial adhesion.