Expression of 72-kDa gelatinase (matrix metalloproteinase-2) in the developing mouse craniofacial complex

Tissue remodelling is an important feature during embryogenesis. Although the matrix metalloproteinases are believed to participate in these processes, the relation between matrix metalloproteinases and tissue remodelling during craniofacial morphogenesis remains unclear. The purpose of the study was to look for the presence of enzymes involved in extracellular matrix degradation during craniofacial morphogenesis. Protein expression of the matrix metalloproteinase, 72-kDa gelatinase (matrix metalloproteinase-2, gelatinase A, 72-kDa type IV collagenase) was studied by gelatine zymography and by indirect immunofluorescence with conventional and confocal microscopy. In the anterior region of the developing mouse face, 72-kDa gelatinase was labelled mainly in the tips and peripheral regions of the nasal and facial prominences. Upon contact and fusion of the prominences, the staining was intensely localized to the zone of the fusion and the tips and peripheral regions of the nasal prominences and the maxilla. The labelling of 72-kDa gelatinase was also present in the peripheral regions of the mandible, second branchial arch, and the face around the developing eye. However, during lens vesicle formation, the staining of 72-kDa gelatinase was absent in the invaginated lens ectoderm. After the lens had completely detached from the surface ectoderm, the staining was resumed in the corneal epithelium and mesenchyme. Gelatine zymography was used to confirm the presence of active and latent 72-kDa gelatinase in the developing mouse craniofacial complex. Collectively, these data indicate that 72-kDa gelatinase may play a significant part in localized tissue remodelling during craniofacial morphogenesis and the aberrant expression or function of the enzyme could be involved in causing facial abnormalities.

Cell proliferation and expression of EGF, TGF-alpha, and EGF receptor in the developing primary palate

Growth factors such as epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) play an important role in cell proliferation during embryogenesis. The purposes of the study were to characterize the expression patterns of EGF and TGF-alpha and their receptor, EGF receptor (EGF-R), and to analyze regional patterns of cell proliferation during primary palate morphogenesis when facial primordia outgrow and fuse to form the premaxillary and upper lip regions. The expression of all molecules was studied with indirect immunohistochemistry with conventional and/or confocal microscopes in normal days 10 and 11 CD1 mice. 5-Bromodeoxyuridine (BrdU) and proliferating cell nuclear antigen (PCNA) were used as markers of cell proliferation. EGF, TGF-alpha, and EGF-R were found to have similar distribution patterns at all stages examined. In the anterior region of the face, the molecules were intensely localized at the tips and peripheral regions of the medial and lateral nasal prominences. Upon fusion of the facial prominences, all three molecules were present mainly at the fusion area and the tips and peripheral areas of the maxillary and nasal prominences. BrdU and PCNA were found to have distribution patterns similar to those of EGF, TGF-alpha, and EGF-R, with intense staining at the tips and peripheral regions of the facial prominences. These results show that EGF, TGF-alpha, and their receptor were expressed more intensely in regions of the developing primary palate where cell proliferation was most pronounced, and suggest that EGF, TGF-alpha, and EGF-R may play a role in cell proliferation during morphogenesis of the primary palate.

Distribution of basement membrane components in the mouse primary palate

Laminin, type IV collagen, and fibronectin are major components of the basement membrane of epithelial tissues. The purpose of this study was to characterize their distributions as the epithelial seam of the primary palate is disrupted and the mesenchymal bridge forms and enlarges. The distributions of these molecules were studied with indirect immunofluorescence in day 11 (9-20 tail somites), noncleft CD1 mice. During epithelial seam formation at 9-11 tail somites (TS), all three molecules were intensely localized in the basement membrane of the seam and adjacent stomodeal and nasal epithelia. During epithelial seam replacement by mesenchymal tissues at 12-18 TS, basement membrane components were first fragmented along the seam, then absent at the site of the mesenchymal bridge. At 19-20 TS, basement membrane components were intact around the margins of the primary palate. These results show that disruption of basement membrane components occurs rapidly and simultaneously with mesenchymal bridge formation in the primary palate.

Deficient and delayed primary palatal fusion and mesenchymal bridge formation in cleft lip-liable strains of mice

During mammalian primary palate formation, the facial prominences enlarge around the nasal pit, fuse and then merge to give rise to the tissue of the upper lip and premaxillary region. The mechanisms involved in successful primary palate formation and how they are affected in the cleft lip genotype remain poorly understood. The purpose of this study was to compare morphometrically internal development and growth of the primary palate in five different strains of mice. Two of the strains, BALB/cByJ, and C57BL/6J, have normal primary palate development, and three of the strains, A/J, A/WySn, and CL/Fr, have stable frequencies of cleft lip associated with genotype. In the present study, frequencies of 4, 23, and 24%, respectively, were observed on day 13. For palatal growth analysis, embryos were collected on days 10 and 11, staged by number of tail somites (TS), and the heads were photographed and serially sectioned for measurement of primary palate components. The heights of the epithelial seam and the mesenchyme bridge between the facial prominences were measured on serial sections and areas of contact were calculated. The position or depth of the maxillary prominence was determined from the number of frontal sections from its tip to the rostral end of the nasal fin. Analysis of measurements showed that in cleft lip strains enlargement of the epithelial seam and replacement of epithelia by a mesenchymal bridge were both delayed relative to somite stages. Measurements from day 11 embryos with complete failure of contact were excluded from the growth analyses. The mesenchymal bridge formed at 12--13 TS in noncleft strains, 14 TS in the A/J strains with higher cleft lip frequency, and 15--17 TS in A/WySn and CL/Fr strains with higher cleft lip frequency. Forward growth of the maxillary prominence was highly correlated with the primary palate measurements and mesenchymal bridge formation in all strains. In both cleft and noncleft strains, the primitive choanae open at 18--20 TS and the medial nasal region narrows with advancing embryonic development. As a result, cleft lip-liable strains have a narrower window in development in which a robust mesenchymal bridge must form, thus increasing the liability to cleft lip.

Targeted disruption of the Huntington's disease gene results in embryonic lethality and behavioral and morphological changes in heterozygotes

Huntington's disease (HD) is an incurable neuropsychiatric disease associated with CAG repeat expansion within a widely expressed gene that causes selective neuronal death. To understand its normal function, we have created a targeted disruption in exon 5 of Hdh (Hdhex5), the murine homolog of the HD gene. Homozygotes die before embryonic day 8.5, initiate gastrulation, but do not proceed to the formation of somites or to organogenesis. Mice heterozygous for the Hdhex5 mutation display increased motor activity and cognitive deficits. Neuropathological assessment of two heterozygous mice shows significant neuronal loss in the subthalamic nucleus. These studies show that the HD gene is essential for postimplantation development and that it may play an important role in normal functioning of the basal ganglia.

A morphometric analysis of cell densities in facial prominences of the rhesus monkey embryo during primary palate formation

The mechanisms by which the craniofacial complex undergoes morphogenetic change during primary palate formation remain unknown. Although changes in cell dispersion and extracellular matrix content are known to be involved in growth of embryonic primordia, little information about these parameters is available for the mammalian face. The purpose of this study was to analyze cell densities in different facial regions of rhesus embryos during primary palate formation. Eight serially-sectioned embryos of stages 13 to 18 in the collection at the California Primary Research Center, most labeled with 3H-thymidine, were analyzed by making regional cell counts at x400 with an ocular micrometer. Three embryos with autoradiographic label preserved in every section were used to analyze patterns of labeling through the depth of the entire face. The results showed that different regions of an embryonic face have significantly different cell densities. At the early stages, the cell densities were high in the maxillary prominences, and the lateral and medial nasal prominences; whereas cell densities were low in midline tissues. At later stages, cell densities were lower in the lateral portion of the maxillary prominences than in the nasal prominences. Serial counts through the face showed regional variation in individual prominences, but central high-density cores did not appear to be present. Labeling with 3H-thymidine was present in all facial primordia. Labeling indices were consistently higher in facial prominences than in midline tissues ventral to the brain. The results suggest that regional changes in cell densities could be used to reflect net changes in cell dispersion associated with altered balances between cell proliferation and extracellular matrix content in embryonic facial primordia.

A morphometric analysis of human embryonic craniofacial growth in the median plane during primary palate formation

As the human primary palate develops between embryonic stages 15 and 18, the facial prominences are part of a rapidly growing craniofacial complex that undergoes extensive morphogenetic change. The purpose of this study was to analyze growth in the medial plane in order to identify regional changes that occur during changes in craniofacial morphology. Photographs of midsagittal sections of 35 human embryos of stages 15 to 19 from the Carnegie Embryology Collection were enlarged, and landmarks were digitized for angular and linear measurements and for finite element modeling (FEM) analysis. The results showed magnitudes and directions of growth required to change average stage 15 morphology to later stages. As the facial and cranial components increased in size, shape change was most pronounced in the posterior cranial and orofacial regions. Increases in cranial linear dimensions were significantly larger than those in the cranial base regions. Between stages 15 and 18, the posterior cranial angle decreased, the forebrain and midbrain rotated superiorly toward the hindbrain, the orofacial angle increased, and the face grew above the thorax. The results suggest that morphogenetic growth changes in the cranial regions are closely associated with facial regions during primary palate formation.

Growth and morphogenesis of the human embryonic midface during primary palate formation analyzed in frontal sections

Although it is recognized that morphology of the craniofacial complex changes during primary palate formation, little information is available about the sites and amounts of growth in different regions. The purpose of this study was to analyze growth patterns of human embryonic heads in frontal sections and to identify regional growth associated with changes in craniofacial morphology and formation of the primary palate. Photographs of frontal sections of 31 human embryos of stages 16 to 19 from the Carnegie Embryology Collection were selected at seven different planes through each head, enlarged and traced, and landmarks were located for computerized morphometric measurements and for finite element modeling (FEM) analyses. Anatomical form change from the initial geometry of the average early stage 16 group to that of later stages was shown by numerical values and graphic displays of regional growth changes during midfacial morphogenesis. Between stages 16 and 19, extensive changes in midfacial morphology occurred as the frontonasal prominence elongated vertically (height increased by seven times) and narrowed to approximately half the width, with more pronounced changes in the anterior nasal region. The brain and the face became vertically separated, and the facial prominences became more frontally positioned relative to the brain. Changes in depth of the face were very limited during this period. The area of the maxillary region increased extensively, particularly in the anterior region, as the maxillary prominences grew forward, lateral to the nasal cavity, to contribute to the primary palate. The lateral nasal prominences increased in size with a predominantly horizontal growth pattern. These patterns of vertical growth of the midfacial tissues, narrowing of the nasal pits and frontonasal prominence, forward growth of the maxilla, and relative separation of the brain and face were identified as predominant features of embryonic craniofacial growth during primary palate development.

Computer reconstructions of human embryonic craniofacial morphology showing changes in relations between the face and brain during primary palate formation

During early development of the human facial region, the craniofacial complex undergoes extensive morphogenetic change. The purpose of this study was to generate computer reconstructions of serially-sectioned embryos in order to illustrate major changes in spatial relations that occur between cranial and facial tissues. Five human embryos of stages 16 to 18 from the Carnegie Embryology Collection, which were used in the frontal morphometric study, were selected to illustrate phases of morphogenesis. Serial photographs of frontal sections were enlarged, traced, and digitized for computer reconstruction. The craniofacial complex was divided into components that were similar to those analyzed in a morphometric study [Diewert VM, Lozanoff S: J Craniofac Genet Dev Biol 1993: 13:162-183] to facilitate visualization of changes in regional components. The major changes observed included changes in form of the brain and its ventral contour above the face; enlargement, elongation, and more ventral positioning of the facial prominences relative to the brain; and forward positioning of the eyes and facial tissues relative to cranial components. As the brain became separated from the face at advancing stages of development, the frontonasal prominences narrowed and became vertically elongated, the lateral nasal prominences enlarged and became medially positioned, and the maxillary prominences enlarged and extended anteriorly. Changes in cranial flexion and lifting up and back of the forebrain [Diewert VM, Lozanoff S: J Craniofac Genet Dev Biol 1993:13:184-192.] appear to contribute to withdrawal of the brain from between the facial prominences during early midfacial development before cartilaginous tissues of the chondrocranium develop.

Recent advances in primary palate and midface morphogenesis research

During the sixth week of human development, the primary palate develops as facial prominences enlarge around the nasal pits to form the premaxillary region. Growth of craniofacial components changes facial morphology and affects the extent of contact between the facial prominences. Our recent studies have focused on developing methods to analyze growth of the primary palate and the craniofacial complex to define morphological phases of normal development and to determine alterations leading to cleft lip malformation. Analysis of human embryos in the Carnegie Embryology Collection and mouse embryos of cleft lip and noncleft strains showed that human and mouse embryos have similar phases of primary palate development: first, an epithelial seam, the nasal fin, forms; then a mesenchymal bridge develops through the nasal fin and enlarges rapidly. A robust mesenchymal bridge must form between the facial prominences before advancing midfacial growth patterns tend to separate the facial components as the medial nasal region narrows and elongates, the nasal pits narrow, and the primary choanae (posterior nares) open posterior to the primary palate. In mouse strains with cleft lip gene, maxillary growth, nasal fin formation, and mesenchymal replacement of the nasal fin were all delayed compared with noncleft strains of mice. Successful primary palate formation involves a sequence of local cellular events that are closely timed with spatial changes associated with craniofacial growth that must occur within a critical developmental period.

A morphometric analysis of craniofacial growth in cleft lip and noncleft mice

Differences in face shape are considered a factor in cleft lip malformation. The purpose of this study was to analyze craniofacial growth in two strains: A/WySn with 28% cleft lip and C57BL/6J without cleft lip. Standardized photographs of 27 A/WySn and 25 C57BL/6J embryos with 34-46 somites (S) were taken in the superior, frontal, and lateral views. Landmarks were located and digitized for computerized analysis of growth change relative to somite number and at stages of face development before, during, and after primary palate closure. The results showed that both strains had similar overall growth patterns with increases in head width and face width, and decreases in nasal pit width. During early palatal closure in C57BL/6J mice, the nasal pit width was unchanged as brain width increased rapidly; and then later, the nasal pit width decreased as brain width increased slowly. However, during early closure in A/WySn mice, the nasal pit width decreased rapidly as brain width increased slowly; and then later, the nasal pit width was unchanged as brain width increased more rapidly. During early palatal closure, the narrower nasal pit width in A/WySn mice appeared to result from delayed growth of the supporting forebrain as the nasal pits become more medially positioned with normal face development. From the lateral view, the maxillary prominence depth was also smaller in the A/WySn strain during early palatal closure. This deficient forward growth of the maxillary prominences and the narrower positioning of the medial nasal prominences in A/WySn embryos appear to reduce the contact between the prominences and thus predispose this strain to cleft lip malformation.

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.

Developmental morphology of the solum nasi in the mouse lemur (Microcebus murinus)

The solum nasi of Microcebus murinus is characterized by the presence of a zona annularis, continuity between the anterior transverse lamina and the paraseptal cartilage, a continuous paraseptal cartilage, a palatine cartilage and a posterior transverse lamina. It lacks a fibula reuniens and possibly a cartilage of the nasopalatine duct as well as a palatine papillary cartilage. The morphology in M. murinus closely resembles that seen in Tupaia and Galago. This affinity results from the retention of primitive traits. However, Galago is reported to lack a zona annularis, thus displaying a specialization not shared with M. murinus. Therefore, the zona annularis provides a useful trait for distinguishing between the ontogenies of M. murinus and Galago.

A computer graphics program for measuring two- and three-dimensional form change in developing craniofacial cartilages using finite element methods

Allometric analysis of chondrocranial growth seeks to provide objective measures of morphogenetic form change during ontogeny of the primordial skull. Linear measures, typically employed to study differential growth, become problematic at the histological level since an external referencing system is impossible to achieve for microscopic anatomies in embryos. The purpose of this paper is to describe a computer graphics program which generates spatially invariant measures of two- and three-dimensional form change using finite element methods. Anatomical form change is viewed as a continuous deformation of an initial finite element representing an anatomical unit into a second configuration. The algorithm consists of isoparametric scaling of finite elements, strain matrix formulation, and size/shape variable derivation. The routine includes four segments serving to extract nodal data, generate the strain matrix relating the two morphologies as well as deriving corresponding size/shape variables, reference the major and minor axes of form change, and provide graphic display of the anatomical geometries. Applications are provided measuring two- and three-dimensional form change in the developing craniofacial cartilages of rats subjected to treatment with the known teratogen diazo-oxo-norleucine (DON). The finite element routine provides craniofacial form change variables which are expected in light of cellular alterations induced by DON administration. Finally, computational differences between this routine and similar approaches using finite element methods for analyzing biological form change are examined.

The fate of Meckel's cartilage chondrocytes in ocular culture

Modulation of the chondrocyte phenotype was observed in an organ culture system using Meckel's cartilage. First branchial arch cartilage was dissected from fetal rats of 16- and 17-day gestation. Perichondrium was mechanically removed, cartilage was split at the rostral process, and each half was grafted into the anterior chamber of an adult rat eye. The observed pattern of development in nonirradiated specimens was the following: hypertrophy of the rostral process and endochondral-type ossification, fibrous atrophy in the midsection, and mineralization of the malleus and incus. A change in matrix composition of the implanted cartilage was demonstrated with immunofluorescence staining for cartilage-specific proteoglycan (CSPG). After 15 days of culture, CSPG was found in the auricular process but not in the midsection or rostral process. In order to mark the implanted cells and follow their fate, cartilage was labeled in vitro with [3H]thymidine [3H]TdR). Immediately after labeling 20% of the chondrocytes contained [3H]TdR. After culturing for 5 days, 20% of the chondrocytes were still labeled and 10% of the osteogenic cells also contained radioactive label. The labeling index decreased in both cell types with increased duration of culture. Multinucleated clast-type cells did not contain label. Additional cartilages not labeled with [3H]TdR were exposed to between 20000 and 6000 rad of gamma irradiation before ocular implantation. Irradiated cartilage did not hypertrophy or form bone but a fibrous region developed in the midsection. Cells of the host animal were not induced to form bone around the irradiated cartilage. Our studies suggest that fully differentiated chondrocytes of Meckel's cartilage have the capacity to become osteocytes, osteoblasts, and fibroblasts.

An immunofluorescence study of chondrogenesis in murine mandibular ectomesenchyme

The temporal and spatial distribution of type I collagen, type II collagen, cartilage-specific proteoglycan (CSPG) and fibronectin in mouse mandible is described. CD-1 mouse embryos of 12-, 15-, and 18-day gestation were used, and matrix molecules were localized using indirect immunofluorescence. On day 12, accumulation of type II collagen, CSPG, and fibronectin within regions of condensed mesenchyme was noted. On day 15, intense staining for type II collagen and CSPG occurred. Fibronectin was less brilliant with its greatest concentration near the perichondrium. On day 18, the cartilage matrix was undergoing osseous replacement concurrent with loss of type II collagen and CSPG. Type I collagen was seen in the perichondrium, membranous bone and sub-basement membrane region in specimens of all ages. Synthesis and expression of extracellular matrix molecules reflect patterns of differentiation in mandibular mesenchyme.

Measuring histological form change with finite element methods: an application using diazo-oxo-norleucine (DON)-treated rats

Analyses of drug-induced anatomical malformations routinely rely on linear measurements as a data base. Morphometric approaches utilizing these measures become inappropriate at the histological level at which a constant external referencing system is impossible to achieve. The purpose of this study was to quantify anatomical form change in the craniofacial region of late embryonic rats induced by a known teratogen, diazo-oxo-norleucine (DON), independent of any global referencing system. A sample of 17 untreated specimens of 17-day gestation served as the control. A second group, equivalent in number and age, received 2.0 mg DON on day 15. Homologous landmarks were identified in each specimen and craniofacial regions were partitioned with respect to these bounding nodes into nasal, oral, and mandibular elements. Form change was viewed as the continuous deformation of a reference craniofacial region from a 15-day untreated specimen into each final 17-day geometry. An interactive graphics program generated spatially invariant measures of form change through finite element methods. A local coordinate system was established for each element. A point within each region of the 15-day reference specimen was selected and the spatial relationship between this point and bounding nodes was quantified through interpolation functions. Size and shape variables were derived from a Lagrangian strain tensor, and values were compared between groups. Results showed that all three craniofacial regions were smaller in size among DON-treated specimens, but only oral and mandibular region shapes were different from controls. The finite element approach was considered superior to other histological morphometric techniques since an entire geometry was described and a visual description of form change as well as spatially invariant measures of size and shape change were derived.

Development of human craniofacial morphology during the late embryonic and early fetal periods

After formation of the primary palate during the fifth and sixth weeks postconception (PC), human facial morphology develops rapidly and by 10 weeks PC the face has a typically human appearance. The objective of this study was to review major growth changes associated with development of face shape during this period. Morphometric evaluation of staged human embryos and fetuses in the Carnegie Embryological Collection showed that between 7 and 10 weeks PC when crown-rump (CR) length increased from 18 to 49 mm, facial structures grew predominantly in the sagittal plane, with a four-fold increase in length, a two-fold increase in height, but little change in width. These growth changes altered relations of oronasal structures and at 8 weeks PC the palatal shelves elevated. The sagittal position of the maxilla and the mandible to the anterior cranial base increased by 25 degrees and 30 degrees, respectively, and the mandible was prognathic during secondary palate closure in the first 2 weeks of fetal development. Both the mean cranial base angulation--which remained unchanged at 128 degrees--and the achieved maxillary position of 84 degrees were similar to the angulations present later, prenatally and postnatally. Therefore, human patterns of cranial base angulation and maxillary position appear to develop during the late embryonic period when the chondrocranium and Meckel's cartilage form the continuous craniofacial skeleton. The results suggest that rapid directional growth of the primary cartilages is important to development of normal human facial morphology and that interference with normal growth changes during this early critical period may produce irreversible effects on the face.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced cleft palate in the mouse: evidence for alterations in palatal shelf fusion

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes a high percentage of cleft palate in fetuses when administered during organogenesis in certain strains of mice including the C57BL/6J, but not in certain other strains (AKR/J). The purpose of the present study was to examine various biochemical and morphological aspects of TCDD-induced changes in the developing palatal shelves. Our results indicate that when TCDD (100 micrograms/kg) was given on individual days between days 8 and 10 of gestation, a high percentage of cleft palate was observed. Receptors specific for TCDD were detected in the C57BL/6J but not AKR/J palatal shelves. The amount of TCDD receptors is highest in the palatal shelves on day 13 as compared to other embryonic tissues including the liver. Examination of cryostat sections taken from embryos during the time of palatal elevation and fusion demonstrated that TCDD does not interfere with growth, elevation, or initial contact of the palatal shelves, but does interfere with firm adhesion and/or degeneration of the medial epithelial cells. Our results suggest that TCDD exerts a direct effect on the embryonic palatal shelves which results in formation of cleft palate.

Abnormal head posture associated with induction of cleft palate by methylmercury in C57BL/6J mice

Maternal treatment with methylmercury (MeHg) has been shown to induce a high frequency of cleft palate and produce growth retardation in rat and mouse fetuses, but the relation between these effects is unknown. The objective of this study was to determine if mandibular growth retardation was a factor that contributed to induction of cleft palate in C57BL/6J mice. Two doses of MeHg (10 mg/kg maternal body weight) were given subcutaneously on days 10 and 11 of gestation, and the fetuses were morphometrically studied on days 14, 15, and 18. Full clefts of the secondary palate were present in approximately half of the treated day 15 and 18 fetuses; therefore, the cleft palate (CP) and noncleft palate (NCP) groups were analyzed separately to facilitate identification of morphologic changes associated with the clefting. The results showed that, compared with controls, the day 14 MeHg-treated fetuses had significantly smaller placental weights, but only half of the fetuses had delayed palatal shelf elevation, reduced body weight, and delayed morphological development. However on day 15, the CP and the NCP groups had similar reductions in body weight and placental weight. A striking downward and forward positioning of the head was present in the MeHg-treated fetuses with the CP group more severely affected than the NCP group. Significant differences between the three groups (control, NCP, and CP) were present with mean head-to-body angles of 67 degrees, 60 degrees and 51 degrees, respectively. The absence of normal head lifting resulted in a relative mandibular retrognathia that when combined with a decrease in mandibular length produced alterations in spatial relations that were most severe in the CP fetuses. The results suggest that after exposure to MeHg, palatal closure is affected by altered tongue posture associated with the abnormal head positioning and shortening of the mandible that develop following placental and embryonic growth retardation.

A morphometric analysis of craniofacial growth and changes in spatial relations during secondary palatal development in human embryos and fetuses

Staged human embryos and fetuses in the Carnegie Embryological Collection were morphometrically analyzed to show craniofacial dimensions and changes in spatial relations, and to identify patterns that would reflect normal developmental events during palatal formation. Normal embryos aged 7-8 weeks postconception (Streeter-O'Rahilly stages 19-23) and fetuses aged 9-10 weeks postconception, in eight groups with mean crown-rump (CR) lengths of 18-49 mm, were studied with cephalometric methods developed for histologic sections. In the 4-week period studied, facial dimensions increased predominantly in the sagittal plane with extensive changes in length (depth) and height, but limited changes in width. Growth of the mandible was more rapid than the nasomaxillary complex, and the length of Meckel's cartilage exceeded the length of the oronasal cavity at the time of horizontal movement of the shelves during stage 23. Simultaneously with shelf elevation, the upper craniofacial complex lifted, and the tongue and Meckel's cartilage extended forward beneath the primary palate. Analysis of spatial relations in the oronasal cavity showed that the palatomaxillary processes became separated from the tongue--mandibular complex as the head extended, and the tongue became positioned forward with growth of Meckel's cartilage. As the head position extended by 35 degrees, the cranial base angulation was unchanged and the primary palate maintained a 90 degrees position to the posterior cranial base. However, the sagittal position of the maxilla relative to the anterior cranial base increased by 20 degrees between stages 19 and 23. In the late embryonic and early fetal periods, the mean cranial base angulation of approximately 128 degrees and the mean maxillary position angulation of approximately 84 degrees were similar to the angulations previously shown to be present later prenatally and post-natally. The results suggest that human patterns of cranial base angulation and maxillary position to the cranial base develop during the late embryonic period when the chondrocranium and Meckel's cartilage form the primary skeleton.

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

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

Correlation between alterations in Meckel's cartilage and induction of cleft palate with beta-aminoproprionitrile in the rat

The lathyrogen beta-aminoproprionitrile (BAPN) induces cleft palate in rats when administered at a critical time in secondary palate formation. BAPN is known to inhibit the crosslinking of newly synthesized collagen, but its primary site of action in producing cleft palate is unknown. In this study time-mated Sprague-Dawley rats were given a single oral dose of 600 mg/kg BAPN at five known gestational ages in the 48 hours before palatal shelf elevation, and the fetuses were studied on days 16, 17 and 18. Evaluation of craniofacial relations and palate development in BAPN-treated heads revealed that delayed palatal shelf elevation and resulting cleft palate were related to retrognathia of the mandible. However, shortening of the mandible was due primarily to vertical and lateral bending of Meckel's cartilage. High and retruded tongue positions that were present with the deformities in Meckel's cartilage interfered with palatal shelf movement to the horizontal plane. The group treated with BAPN at 15 days 7 hours, approximately 24 hours before normal palatal shelf elevation, had the most severe defects in Meckel's cartilage, the longest delay in palatal shelf elevation and the highest incidence of cleft palate. Inhibition of crosslinking of collagen in Meckel's cartilage appeared to weaken the cartilage during the critical period in facial development when extention of the tongue and mandible beneath the primary palate is required to facilitate palatal shelf elevation.

Cortisone-induced cleft palate in the brachymorphic mouse

Previous studies have shown that the autosomal recessive gene brachymorphic (bm/bm), which is maintained on a C57BL/6J (C57) background, reduces limb growth and sulfation of cartilage proteoglycans. Hydrocortisone administered on gestational days 11-14 resulted in 20% CP in the C57 mouse, but 95% CP in the bm/bm mouse. The bm/bm mouse had a median effective dose for CP of 45 mg/kg, compared to 325 mg/kg for C57 and 40 mg/kg for A/J. Morphometric analysis indicated that the time of palatal elevation was delayed in the bm/bm relative to the C57 mouse both with and without hydrocortisone treatment. The amount of cytoplasmic glucocorticoid receptor protein present in the bm/bm palate on day 14 was the same as the amount found in the C57 palate, and was not elevated as it is in the A/J palate. The levels of cyclic AMP in the bm/bm palate on day 14 were 30-70% higher than that found in the C57 palate with or without hydrocortisone. These results suggest that both bm/bm and A/J exhibit a delay in palatal shelf rotation and elevated levels of cyclic AMP, which appear to be predisposing factors for cortisone-induced cleft palate. These strains differ in that elevated levels of steroid receptors are present in A/J palate, whereas lower levels are found in the C57 and bm/bm mice.

Differential changes in cartilage cell proliferation and cell density in the rat craniofacial complex during secondary palate development

During mammalian secondary palate formation sagittal growth of the lower face has been shown to be more rapid than that of the upper face, and the tongue and mandible extend beneath the primary palate. In order to identify factors contributing to this differential growth pattern, cellular and morphologic growth of the major cartilages of the upper and lower facial regions were studied in radioautographic sections labeled with tritiated thymidine. Evaluation of cell-density recordings, labeling indices, and structural dimensions revealed significant differences between Meckel's cartilage in the lower face, and the nasal cartilage and anterior cranial base cartilage in the upper face. After formation of the precartilaginous blastema, labeling indices were high in Meckel's cartilage (20-30%), but very low in the nasal cartilage and the anterior cranial base (0-2%). During secondary palate formation the volume of Meckel's cartilage increased more rapidly than the other cartilages and its growth was primarily in the sagittal direction. Between days 15 and 17, the increase in the length of Meckel's cartilage (165%) was approximately twice as great as the increase in the combined length of the nasal cartilage and the anterior cranial base (77%). During this period induction of cleft palate with some teratogens has been shown to severely retard growth of Meckel's cartilage and produce mandibular retrognathia that contributes to delayed elevation of the palatal shelves. Therefore, extensive cell proliferation in Meckel's cartilage, during a period of limited proliferation in other craniofacial cartilages, appears to contribute to its rapid growth and its differential sensitivity to growth inhibition.

Experimental induction of premature movement of rat palatal shelves in vivo

In order to enhance further knowledge of palatal shelf movement and the factors involved in palate closure, a method was developed for prematurely elevating palatal shelves in utero. Approximately 7 hours before expected shelf elevation, pregnant Sprague-Dawley rats were laparotomized and two medially directed squeezes were applied to the face of some of the embryos through the intact uterine wall. Palates from control (unsqueezed) and experimental animals were obtained immediately after the procedure and 2, 4, 7, 12 and 24 hours later. In the 0, 2 and 4 hours groups, 82% of experimental palates were elevated, whereas only 6% of control palates were elevated. At 0 hours only the hard palate in the experimental group had elevated, but at 2 and 4 hours almost half this group showed elevation of the soft palate as well, and, in addition, contact had been made between the elevated shelves.

Selective inhibition of mandibular growth and induction of cleft palate by diazo-oxo-norleucine (DON) in the rat

A high percentage of cleft palates can be induced in rat fetuses by a single injection of the glutamine analog diazo-oxo-norleucine (DON) on day 15 of gestation. The purpose of this study was to evaluate the effects of DON in vivo on craniofacial growth and spatial relations in order to identify factors that may contribute to the palatal defects. Sprague-Dawley rats in the experimental groups were given a single IP injection of 2.0 mg DON (6 mg/kg maternal body weight) on day 15 and were killed on day 16 or 17. Control fetuses were collected on days 15, 16 and 17. Fetal heads were fixed in Bouin's solution, embedded in Paraplast and serially-sectioned. Midsagittal and coronal sections were projected at 30 X and a series of linear and angular measurements were made. DON had limited effect on growth of the cranial base, nasomaxillary complex, and palatine processes, but dramatically reduced the length of Meckel's cartilage. Treatment with DON delayed shelf elevation approximately 24 hours, and tongue position remained high in the oronasal cavity. Growth retardation in Meckel's cartilage therefore may contribute to delayed shelf movement by retarding downward and forward positioning of the tongue-mandibular complex.

Palatal process movement in the rat as demonstrated in frozen sections

During mammalian secondary palate development, movement of the lateral palatine processes from the vertical plane to the horizontal plane involves a complex interaction of the palatine processes and the tongue within a dynamic growing oronasal cavity environment. This study of pre-fixation facial profile photographs and frozen sections was undertaken to evaluate external and internal changes in the oronasal complex during secondary palate elevation without the shrinkage known to be present with routinhistological preparation of embryonic tissues. Frozen sections of Sprague-Dawley rat embryos between 15 and 17 days of (conceptual) age were prepared by hexane quenching and cryostat cutting. The results showed that, during the stages of palate development prior to shelf elevation, the tongue and mandible became positioned beneath the primary palate, and the vertical dimension of the oronasal cavity increased by the lifting of the nasomaxillary complex. The tongue and mandible maintained contact with the primary palate, whereas a space developed above the tongue in the middle and posterior palate regions. As the vertical dimension increased the volume of the palatomaxillary processes increased rapidly, the tongue became squeezed, and the palatine processes bulged medially above the level of the tongue. After shelf elevation extensive contact between the palatine processes was present, and the tongue became flattened. The results of this study support the observations of Lazzaro (1940) that rapid increase in shelf volume owing to increased intercellular volume contributes to movement of the processes above the tongue. But, rapid increase in shelf volume occurred contemporaneously with the time when the tongue and mandible outgrew the oronasal cavity and became positioned beneath the primary palate. Therefore, it would appear that the simultaneous occurrence of a lower and more forward tongue position, and an increased palatomaxillary process volume without change in maxillary width, contributed to the medial movement of the processes above the tongue.

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.

Graphic reconstructions of craniofacial structures during secondary palate development in rats

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

The course of the palatine arteries during secondary palate development in the rat

The course of the palatine arteries was studied in 96 rat fetuses. The descending palatine arteries were a major blood supply to the incisive area of the primary palate. When shelves were elevated, the arteries became positioned more medially and changed from a semicircular to a V-shaped form in the pre-maxilla.