Palatogenesis in hamster. II. Ultrastructural observations on the closure of palate

Formation of secondary palate in hamster was studied with electron microscopy. Prior to assuming horizontal position, the palatal shelves were covered by a two to three cell layer thick epithelium which was separated from the underlying mesenchyme by an intact basal lamina. Epithelial cells were attached to each other by desmosomes. Early hemidesmosomes could be identified as thickenings of the cytoplasmic membrane opposing the basal lamina. Epithelial cells, like other embryonic cells, contained only few organelles but were rich in polyribosomes. As the horizontal shelves approached each other towards the midline, lysosomes and tonofilaments appeared in the superficial and basal cells of the epithelia. Superficial cells showed degeneration and eventual lysis. Fusion of the opposing epithelia occurred between the deeper cells by means of newly formed desmosomes. The epithelial seam resulting from fusion of the epithelia was limited on each side by a continuous basal lamina. Its subsequent thining and eventual fragmentation resulted from the loss of cells by autophagy. There was no evidence of mesenchymal invasion of the epithelial seam. Mesenchymal macrophages appeared in the later stage of palatogenesis and were responsible for phagocytosis of cellular debris. Formation of the soft palate was basically similar to that of the secondary hard palate and occurred by fusion of the opposing shelves. Similarly, anterior closure of the palate occurred by fusion of the lower end of the nasal septum to the primary and secondary palates. Hyperplasia of the opposing epithelia, prior to their fusion, was often seen. It is suggested that formation of the palate occurs in predictable and coordinated fashion and that timely appearance of lysosomes causing lysis of intervening epithelia is of great significance in normal palatogenesis.     

Correlation between mandibular retrognathia and induction of cleft palate with 6-aminonicotinamide in the rat.

A single injection of the niacin antimetabolite 6-aminonicotinamide (6-AN) late in gestation produces cleft palate in the rat. In order to achieve an understanding of the mechanism of induction of cleft palate, craniofacial growth and palate development were studied in Sprague-Dawley rats after treatment with 6-AN on day 15 of gestation. The rats were maintained on a high niacin diet (95 ppm) and subjected to three different teratogenic levels of 6-AN. The first group was injected with 8 mg/kg, the second was fasted and injected with 8 mg/kg and the third was treated with 16 mg/kg. The lowest teratogenic dose, 8 mg/kg, produced mild mandibular retrognathia on day 16, delayed shelf elevation a few hours and resulted in small rostral and small caudal clefts of the secondary palate. The moderate dose, 8 mg/kg with fasting, produced more severe mandibular retrognathia, delayed shelf elevation about 24 hours and resulted in 37% full clefts and 63% partial clefts of the palate. The highest teratogenic dose, 16 mg/kg, produced severe mandibular retrognathia, delayed shelf elevation by more than 24 hours and resulted in 100% full clefts of the palate. In each 6-AN group, the most severe mandibular retrognathia was present between days 16 and 17, the critical time for palate closure in the rat. Treatment with 6-AN also produced abnormality of the epithelial cells of the palate, the toothbuds and the nasal septum. Molar and incisor toothbuds were small and malformed, and the epithelial surfaces of the palate and the soft tissue nasal septum did not fuse.     

Morphological observations in normal primary palate and cleft lip embryos in the Kyoto collection

Normal developmental events during human primary palate formation and alterations associated with cleft lip remain poorly defined. The purpose of this study was to analyze serially sectioned human embryos to identify morphological changes during normal palatal closure and alterations associated with failure of palatal formation. Normal and cleft embryos from the histological collection at the Congenital Anomaly Research Center at the University of Kyoto were studied and photographed for detailed evaluation. Seven serially sectioned cleft lip embryos of stages shortly after primary palate formation (Streeter-O'Rahilly stages 19, 20, and 22) with unilateral or bilateral clefts with varying degrees of clefting were studied. In the normal Kyoto embryos, initial nasal fin (epithelial seam) formation was observed between the medial nasal process and the lateral nasal and maxillary processes at stage 17. During stages 18 and 19, the nasal fin epithelium was replaced by an enlarging mesenchymal bridge, as the maxillary processes united with the medial nasal processes to form the primary palate. The most prominent features observed in the cleft embryos were a reduced thickness of mesenchymal bridging between the medial nasal and maxillary processes, with an excessive amount of epithelium at the junctions between these processes. With ingrowth of the maxillary processes, greater cell dispersion and apparent extracellular matrix accumulation were observed in the medial nasal region. During closure of the primary palate, terminal branches of the maxillary nerve crossed the mesenchymal bridge to the medial nasal region. The partial clefts had reduced maxillary ingrowth and smaller union areas with the medial nasal process. Detailed studies of experimental animal models are required to identify regional growth required for contact between the facial prominences, to clarify the mechanisms of mesenchymal ingrowth and epithelial displacement during palatal formation, and to identify local and/or general factors causing alterations that lead to primary palatal clefting.     

Epithelial bridging of the primary palate: II. In vitro model mimics in vivo behavior

Previously, Forbes et al. [J Craniofac Genet Dev. Biol, 9:271-284, 1989] and Millicovsky et al. [Am J Anat 164:29-44, 1982], demonstrated that some of the epithelial cells of the primary palate formed extensive projections, bridging the medial and lateral nasal prominences. These connections are thought to aide in the fusion process by facilitating union of the prominences, a process known as secondary fusion [Millicovsky et al., 1982]. In order to study the epithelial cell and its behavior more closely an in vitro model was established [Gibson et al.: J Craniofac Genet Dev Biol, 1989], where epithelial cells in culture were shown to produce many of the morphologic characteristics observed in vivo. In the present study, an in vitro model is discussed which reproduces the epithelial projections observed in vivo. Epithelial cells, previously characterized, were obtained from the primary palate of 13-day-old rat embryos and sub-cultured as explants. Comparisons were made with the epithelial bridging observed in vivo of two species of animals. The results indicated sub-cultured epithelium as isolated cells, at either low or high density, rarely formed bridges. Primary cultures of epithelial explants also infrequently formed projections. However, sub-cultures of epithelial explants, plated as small clusters of cells with intervening spaces between cell groups, demonstrated extensive epithelial bridging. Epithelial projections did not form from cells that were directly attached to the plastic culture dish; only superficial, elevated cells formed projections. Significantly, the connections that occurred between explants did not attach to the plastic substratum. Instead, they appeared as line connections suspended by the medium. With time, the number of projections increased and epithelial cells could be seen along the projections forming an epithelial bridge. This study established a model of epithelial bridging in vitro for analysis of a process which has been shown to be an integral part of primary palate fusion.     

Localisation of acidic and basic fibroblast growth factors during mouse palate development and their effects on mouse palate mesenchyme cells in vitro

Summary The distribution of acidic and basic fibroblast growth factors (aFGF, bFGF) was mapped during mouse embryonic palate development. Generally, they localised most intensely in the basement membrane and epithelia rather than the mesenchyme. Localisation was predominantly restricted to the palatal nasal, and medial edge epithelia. Staining was particularly intense in the medial edge epithelia at the time of mid-line epithelial seam formation. Intense staining persisted in the epithelia of the degenerating seam and later in the oral and nasal epithelial triangles. Mouse embryonic palate mesenchyme (MEPM) cells cultured in vitro on a variety of substrata (on plastic, on the surface of a collagen gel and within a collagen gel) responded to treatment with aFGF or bFGF. These responses were modulated by the culture substratum. The FGFs stimulated MEPM cell proliferation on plastic and on collagen, but inhibited cell growth in collagen. The FGFs had little effect on protein production when cells were cultured on plastic, but caused a large reduction in on-collagen and incollagen cultures. This reduction was greater in collagenous than non-collagenous proteins. Generally, treatment with FGFs stimulated the production of glycosaminoglycans (GAGs), particularly hyaluronan (HA) and dermatan sulphate (DS). In addition, the size class of HA was shifted to a higher molecular weight form. These data indicate that aFGF and bFGF may play a role in modulating mesenchymal cell matrix biosynthesis, so facilitating palatal epithelial seam degeneration. Correspondence to: M.W.J. Ferguson     

Quantification and localization of ornithine decarboxylase in the embryonic palate.

Ornithine decarboxylase (ODC; EC4.1.1.17), the key enzyme in polyamine biosynthesis, and intracellular polyamines increase rapidly and markedly in tissues and cells that are actively proliferating as well as differentiating and decrease as these processes cease. ODC activity has also been implicated as playing a role in the proliferation and differentiation of cells derived from the developing palate. Ornithine decarboxylase activity was thus quantified and ODC localized in the developing murine palate in vivo. Levels of ODC activity showed little variation during the ontogeny of the palate, averaging 126 pmol CO2/mg protein/hr. When difluoromethylornithine (DFMO), an irreversible inhibitor of ODC activity, was administered to pregnant mice throughout the period of palate development (days 11-14), palatal tissue ODC activity was reduced by 85%. No craniofacial malformations were observed, however. The lack of a teratogenic effect by DFMO treatment could be due to sufficient remaining ODC activity in craniofacial tissue and/or maintenance of intracellular polyamine levels by the activity of a polyamine transport system. The activity of this system was demonstrated by the ability of palatal tissue in vivo to take up radiolabeled putrescine. The presence of a polyamine transport system was previously suggested by the demonstration of such a system in palate mesenchymal cells in vitro. Dramatic temporal and spatial shifts in tissue patterns of immunolocalization for ODC in developing palatal tissue were also seen. Immunostaining for ODC was evenly distributed in oral, nasal, and medial edge palate epithelial cells on day 12 of gestation. The basal aspects of epithelial cells were, however, more intensely stained. Mesenchymal cells exhibited a peri-nuclear immunostaining pattern. On days 12 and 13 of gestation, the staining patterns for ODC in palate epithelial and mesenchymal cells were comparable. On day 14 of gestation, all regions of the palate epithelium, particularly the medial edge epithelia, were immunostained for ODC, whereas the intensity of staining in the mesenchymal cells was significantly reduced. This study represents essential initial observations toward understanding the role that ODC may play in normal craniofacial development.     

Analysis of cell migration, transdifferentiation and apoptosis during mouse secondary palate fusion

Malformations in secondary palate fusion will lead to cleft palate, a common human birth defect. Palate fusion involves the formation and subsequent degeneration of the medial edge epithelial seam. The cellular mechanisms underlying seam degeneration have been a major focus in the study of palatogenesis. Three mechanisms have been proposed for seam degeneration: lateral migration of medial edge epithelial cells; epithelial-mesenchymal trans-differentiation; and apoptosis of medial edge epithelial cells. However, there is still a great deal of controversy over these proposed mechanisms. In this study, we established a [Rosa26<-->C57BL/6] chimeric culture system, in which a Rosa26-originated ;blue' palatal shelf was paired with a C57BL/6-derived ;white' palatal shelf. Using this organ culture system, we observed the migration of medial edge epithelial cells to the nasal side, but not to the oral side. We also observed an anteroposterior migration of medial edge epithelial cells, which may play an important role in posterior palate fusion. To examine epithelial-mesenchymal transdifferentiation during palate fusion, we bred a cytokeratin 14-Cre transgenic line into the R26R background. In situ hybridization showed that the Cre transgene is expressed exclusively in the epithelium. However, beta-galactosidase staining gave extensive signals in the palatal mesenchymal region during and after palate fusion, demonstrating the occurrence of an epithelial-mesenchymal transdifferentiation mechanism during palate fusion. Finally, we showed that Apaf1 mutant mouse embryos are able to complete palate fusion without DNA fragmentation-mediated programmed cell death, indicating that this is not essential for palate fusion in vivo.     

Prenatal repair of experimentally induced clefts in the secondary palate of the rat

A refined technique of amniotic sac puncturing at day 16.2 (i.e., 16 + 2/10 days) of gestation was employed in order to produce a series of total clefts and rare forms of partial clefts in Sprague-Dawley rat fetuses. From a total of 410 fetuses of a precise, individually determined age, 95 upper jaws were examined in the scanning electron microscope and, in part, in serial Epon sections. All fetal heads were examined macroscopically. Total clefts were found in 48.9% of a total of 184 viable rat fetuses examined at day 17.8 of smear age and in 21.8% of a total of 211 fetuses examined at day 19.3. Partial clefts were observed in 14.1% and 18.5% of fetuses at days 17.8 and 19.3 of smear age, respectively. At day 19.3, 16.1% of the viable fetuses showed a very inconspicuous, small abnormality (with residual clefting and incomplete fusion with the nasal septum) in the region of the palatine foraminae. Morphological observations suggested that under conditions of detained palatal closure (1) fusion of the soft palatal shelves commences independently from and prior to fusion of the hard palate, (2) delayed palatal shelf fusion proceeding in the anterior direction may occur with or without remaining sickle-shaped clefts in the anterior hard palate, and (3) in fetuses with small sickle-shaped clefts, fusion of the palatal shelves with the nasal septum does not occur. The present data imply that an almost total prenatal repair and delayed closure of the secondary palate may occur in rats that, at day 16.2 of multiple analysis age, most certainly had a total palatal cleft resulting from tongue resistance.     

The fate of medial edge epithelial cells during palatal fusion in vitro: an analysis by DiI labelling and confocal microscopy.

Fusion of bilateral shelves, to form the definitive mammalian secondary palate, is critically dependent on removal of the medial edge cells that constitute the midline epithelial seam. Conflicting views suggest that programmed apoptotic death or epithelial-mesenchymal transformation of these cells is predominantly involved. Due in part to the potentially ambiguous interpretation of static images and the notable absence of fate mapping studies, the process by which this is achieved has, however, remained mechanistically equivocal. Using an in vitro mouse model, we have selectively labelled palatal epithelia with DiI and examined the fate of medial edge epithelial (MEE) cells during palatal fusion by localisation using a combination of conventional histology and confocal laser scanning microscopy (CLSM). In dynamic studies using CLSM, we have made repetitive observations of the same palatal cultures in time-course investigations. Our results concurred with the established morphological criteria of seam degeneration; however, they provided no evidence of MEE cell death or transformation. Instead we report that MEE cells migrate nasally and orally out of the seam and are recruited into, and constitute, epithelial triangles on both the oral and nasal aspects of the palate. Subsequently these cells become incorporated into the oral and nasal epithelia on the surface of the palate. We hypothesize an alternative method of seam degeneration in vivo which largely conserves the MEE population by recruiting it into the nasal and oral epithelia.     

Binderoid complete cleft lip/palate

A small subset of infants with complete cleft lip/palate look different because they have nasolabiomaxillary hypoplasia and orbital hypotelorism. The authors' purpose was to define the clinical and radiographic features of these patients and to comment on operative management, classification, and terminology. The authors reviewed 695 patients with all forms of incomplete and complete cleft lip/palate and identified 15 patients with nasolabiomaxillary hypoplasia and orbital hypotelorism. All 15 patients had complete labial clefting (5 percent of 320 patients with complete cleft lip/palate), equally divided between bilateral and unilateral forms. The female-to-male ratio was 2:1. Of the seven infants with unilateral complete cleft lip/palate, one had an intact secondary palate and all had a hypoplastic septum, small alar cartilages, narrow basilar columella, underdeveloped contralateral philtral ridge, ill-defined Cupid's bow, thin vermilion-mucosa on both sides of the cleft, and a diminutive premaxilla. Of the eight infants with bilateral complete cleft lip, one had an intact secondary palate. The features were the same as in patients with unilateral cleft, but with a more severely hypoplastic nasal tip, conical columella, tiny prolabium, underdeveloped lateral labial elements, and small/mobile premaxilla. Central midfacial hypoplasia and hypotelorism did not change during childhood and adolescence. Intermedial canthal measurements remained 1.5 SD below normal age-matched controls. Skeletal analysis (mean age, 10 years; range, 4 months to 19 years) documented maxillary retrusion (mean sagittal maxillomandibular discrepancy, 13.7 mm; range, 3 to 17 mm), absent anterior nasal spine, and a class III relationship. The mean sella nasion A point (S-N-A) angle of 74 degrees (range, 65 to 79 degrees) and sella nasion B point (S-N-B) angle of 81 degrees (range, 71 to 90 degrees) were significantly different from age-matched norms ( = 0.0007 and = 0.004, respectively). The ipsilateral central and lateral incisors were absent in all children with unilateral cleft, whereas a single-toothed premaxilla was typically found in the bilateral patients. Several modifications were necessary during primary nasolabial repair because of the diminutive bony and soft-tissue elements. All adolescent patients had Le Fort I maxillary advancement and construction of an adult nasal framework with costochondral or cranial graft. Other often-used procedures were bony augmentation of the anterior maxilla; cartilage grafts to the nasal tip and columella; and dermal grafting to the median tubercle, philtral ridge, and basal columella. Infants with complete unilateral or bilateral cleft lip/palate in association with nasolabiomaxillary hypoplasia and orbital hypotelorism do not belong on the holoprosencephalic spectrum because they have normal head circumference, stature, and intelligence, nor should they be referred to as having Binder anomaly. The authors propose the term cleft lip/palate for these children. Early recognition of this entity is important for counseling parents and because alterations in standard operative methods and orthodontic protocols are necessary.     

Cleft lip, cleft palate, and velopharyngeal insufficiency

This article provides an introduction to the anatomical and clinical features of the primary deformities associated with unilateral cleft lip-cleft palate, bilateral cleft lip-cleft palate, and cleft palate. The diagnosis and management of secondary velopharyngeal insufficiency are discussed. The accompanying videos demonstrate the features of the cleft lip nasal deformities and reliable surgical techniques for unilateral cleft lip repair, bilateral cleft lip repair, and radical intravelar veloplasty.     

Cytokeratin, vimentin and E-cadherin immunodetection in the embryonic palate in two strains of mice with different susceptibility to glucocorticoid-induced clefting

An immunohistochemical study analyzing the pattern of distribution of some intermediate filament proteins, keratin and vimentin and, one adhesion molecule, cadherin in different stages of developing secondary palate in two strains of mice with different H-2 backgrounds was undertaken to investigate differences between a strain that is susceptible to glucocorticoid-induced cleft palate (A/Sn) and one that is resistant to glucocorticoid-induced cleft palate (C57/BL). The heads of embryos were processed by standard immunohistochemistry with antipancytokeratin (KAE1), antikeratins 18 (K18) and 19 (K19), antivimentin, and anti E-cadherin antibodies. Immunostaining with KAE1 antibody showed differences between the strains. The reaction was stronger in the medial edge epithelia of palatal processes in the A/Sn strain at all stages of palatogenesis. The C57/BL strain showed a weak immunostain to KAE1. Antivimentin antibody stained the mesenchymal cells of palatal processes and K18 and K19 showed no reaction in either strain of mice. Anti E-cadherin antibody was detected in the medial palatal epithelium of both strains of mice and in all stages of palate development. No differences were observed in E-cadherin and vimentin immunostain in palatal epithelium between the strains. The different expression of some cytokeratins in the embryonic palatal epithelium suggests that these intermediate filament proteins may be involved in different susceptibility to glucocorticoid-induced cleft palate in the mouse. The decreased immunoreaction of cytokeratins observed in the resistant strain would facilitate the disappearance of this molecule during the transformation from an epithelial to a mesenchymal phenotype that takes place during the development of the palate. These results may be related to the loss of cytokeratin expression observed during epithelial-mesenchymal transformation in the embryonic palate.     

One-stage closure of the entire primary palate

Timing of the closure of the anterior palate and alveolus is a subject of debate. Late repair of this defect is complicated by high fistula formation and subjects the patient to the problems of palate fistula for extended periods of time. We have utilized a single procedure performed when the child is 3 months of age that completely closes the anterior hard palate and alveolus along with the cleft lip. Our series consisted of 61 consecutive patients with unilateral clefts of the primary and secondary palate. Mucosal turnover flaps from the vomer along with lateral nasal mucosal flaps provide the nasal lining. A buccal sulcus flap with a Veau flap completes the oral repair. Ninety-five percent (58 of 61) of the patients had complete and stable closure of their anterior palate and alveolus after 1 year. The incidence of fistula formation in our series (3 of 61) is much lower than that reported with the utilization of other protocols. Excellent exposure of the anterior palate and alveolar defect during lip repair, early restoration of anatomic relationships, establishment of a good nostril floor and sill, and very low fistula formation are among the benefits of this procedure. The increase in operative time is considered minimal in light of aforementioned advantages.     

Palatogenesis: morphogenetic and molecular mechanisms of secondary palate development

Mammalian palatogenesis is a highly regulated morphogenetic process during which the embryonic primary and secondary palatal shelves develop as outgrowths from the medial nasal and maxillary prominences, respectively, remodel and fuse to form the intact roof of the oral cavity. The complexity of control of palatogenesis is reflected by the common occurrence of cleft palate in humans. Although the embryology of the palate has long been studied, the past decade has brought substantial new knowledge of the genetic control of secondary palate development. Here, we review major advances in the understanding of the morphogenetic and molecular mechanisms controlling palatal shelf growth, elevation, adhesion and fusion, and palatal bone formation.     

The distribution of syndecan during murine secondary palate morphogenesis.

The distribution of syndecan, an integral membrane proteoglycan, has been immunohistochemically mapped during the course of murine secondary palate morphogenesis, gestational days 12-15. Syndecan has been shown to mediate cell adhesion and shape change and to be involved in epithelial-mesenchymal interactions during the morphogenesis of several structures. Changes in epithelial cell architecture accompany and may serve to direct the reorientation of the murine secondary palatal shelves from a vertical position on either side of the tongue to a horizontal and adhering position above it. Using a monoclonal antibody made to the core protein of the ectodomain of syndecan, staining was observed to correlate with epithelial cell shape, packing and degree of differentiation. Staining of condensing mesenchyme was also observed. Syndecan may be involved in modulating epithelial cell shape, architecture and fates during both major phases of secondary palate morphogenesis: shelf reorientation and midline epithelial seam dissolution.     

Enhanced extrinsic innervation of nasal and oral chemosensory mucosae in keratin 14-NGF transgenic mice

The role of nerve growth factor (NGF) in neurotrophic support for the extrinsic innervation of the nasal and oral mucosae was investigated in keratin 14 (K14)-NGF transgenic mice in which NGF was over-expressed in K14-synthesizing cells. K14 immunoreactivity was localized in the epithelial basal cells of the whisker pad skin, the hard palate, the floor of the ventral meatus, and the anterior tongue that are stratified squamous epithelia, and also in basal cells of the vomeronasal, olfactory, and respiratory epithelia that are non-stratified epithelia. In transgenic mice, NGF expression was identified and confined primarily to the basal cells of stratified epithelia. The nasal mucosae including the vomeronasal, olfactory, and respiratory mucosae, and the glands associated with the vomeronasal organ received a greater innervation of protein gene product 9.5-immunoreactive extrinsic fibers in transgenic animals than nontransgenic controls. An increased density of calcitonin gene-related peptide-immunoreactive extrinsic fibers was observed in the nonsensory epithelia of the vomeronasal organ, the olfactory sensory and respiratory epithelia in transgenic animals. Our results indicated that the hyperinnervation of the nasal and oral mucosae by extrinsic neurons is due at least partially to target-derived NGF synthesis and release by K14-expressing basal cells.This work was supported by NIH grants NIDCD-00159 (T.V.G.), NIDCO-01715 (M.L.G.), and NINDS-31826 (K.M.A.).     

Mucociliary clearance in chronic sinusitis: related human nasal clearance and in vitro bullfrog palate clearance

Nasal mucociliary clearance was measured in both healthy subjects and patients with chronic sinusitis using saccharin granule technique. Nasal mucociliary transit time (ST) was significantly slower in the patients with chronic sinusitis compared with that in controls (p less than 0.005). Nasal mucus collected from each nasal cavity was used for in vitro bullfrog palate clearance studies and compared to the in vivo nasal ST. Mucociliary clearance rate (MTR) on frog palate was 12.5 +/- 2.5 mm/min in the mucus from control subjects, 6.1 +/- 1.5 mm/min in the mucus from the patients. The difference was statistically significant (p less than 0.005). The MTR on frog palate in the patients whose nasal ST was within normal range was significantly slower than that in controls (p less than 0.005), but not significantly different from that in the patients whose nasal ST was over the normal range. These results suggest that the nasal mucous properties which decreased the mucociliary clearance on frog palate did not contribute to the mucociliary clearance of the patients who had a normal one. No significant correlation existed between MTR on frog palate and nasal ST in both control and chronic sinusitis. In chronic sinusitis patients, decelerated nasal ST was recovered significantly by normal saline nebulization compared with the value before the nebulization (p less than 0.01). None of the significant change of ST was observed in control before and after the nebulization.     

Distinct functions for Bmp signaling in lip and palate fusion in mice

Previous work suggested that cleft lip with or without cleft palate (CL/P) is genetically distinct from isolated cleft secondary palate (CP). Mutations in the Bmp target gene Msx1 in families with both forms of orofacial clefting has implicated Bmp signaling in both pathways. To dissect the function of Bmp signaling in orofacial clefting, we conditionally inactivated the type 1 Bmp receptor Bmpr1a in the facial primordia, using the Nestin cre transgenic line. Nestin cre; Bmpr1a mutants had completely penetrant, bilateral CL/P with arrested tooth formation. The cleft secondary palate of Nestin cre; Bmpr1a mutant embryos was associated with diminished cell proliferation in maxillary process mesenchyme and defective anterior posterior patterning. By contrast, we observed elevated apoptosis in the fusing region of the Nestin cre; Bmpr1a mutant medial nasal process. Moreover, conditional inactivation of the Bmp4 gene using the Nestin cre transgenic line resulted in isolated cleft lip. Our data uncover a Bmp4-Bmpr1a genetic pathway that functions in lip fusion, and reveal that Bmp signaling has distinct roles in lip and palate fusion.