Secondary Palate Formation Research Articles

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.

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.

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.

Hypervitaminosis A and matrix alterations in maxillary explants from 16-day rat embryos.

Hypervitaminosis A in treated pregnant rats has been shown to interfere with normal palatal closure and fusion, as demonstrated by the presence of cleft palates in offspring. The observation that palatal shelves of excess vitamin A exposed fetuses are stunted and delayed in rotation suggests that vitamin A may inhibit a biochemical event crucial to the successful contact of the palatal shelves. Maxillary explants from 16 day Wistar rat embryos cultured in the presence or absence of 30 IU/ml retinyl palmitate were analyzed for DNA, glycosaminoglycan, and collagen synthesis. Maxillary explants cultured in vitamin A-containing medium showed an inhibition in DNA, GAG, and collagen synthesis in comparison to control explants. Excess vitamin A in the culture medium of maxillary explants also resulted in a reduction of intermolecular cross-links in collagen. The possible significance of the results in terms of cleft palate and normal secondary palate formation is discussed.

Autoradiographic study of macromolecular synthesis in the fusion epithelium of the developing rat primary palatein vitro

The facial processes involved in primary palate formation undergo epithelial fusion in a manner morphologically analogous to that observed during secondary palate formation. We have used whole embryo culture to analyze the synthesis of macromolecules (DNA, protein glycoprotein) in the primary palate, based on the incorporation of various labeled precursors. The results of this study demonstrate that changes in the synthesis of macromolecules occur during the fusion of the facial processes, which resemble those previously reported to occur during secondary palate development. These changes include cessation of DNA synthesis in cells in a restricted zone of the epithelium, concomitant with maintenance of glycoprotein synthesis. These findings indicate that the molecular events underlying the development of the primary and secondary palate may be similar.

Developmental aspects of secondary palate formation

Research on development of the secondary palate has, in the past, dealt primarily with morphological aspects of shelf elevation and fusion. The many factors thought to be involved in palatal elevation, such as fetal neuromuscular activity and growth of the cranial base and mandible, as well as production of extracellular matrix and contractile elements in the palate, are mostly based on gross, light microscopic, morphometric or histochemical observations. Recently, more biochemical procedures have been utilized to described palatal shelf elevation. Although these studies strongly suggest that palatal extracellular matrix plays a major role in shelf movement, interpretation of these data remains difficult owing to the complexity of tissue interactions involved in craniofacial development. Shelf elevation does not appear to involve a single motive factor, but rather a coordinated interaction of all of the abovementioned developmental events. Further analysis of mechanisms of shelf elevation requires development of new, and refinement of existing, in vitro procedures. A system that enables one to examine shelf elevation in vitro would allow more meaningful analysis of the relative importance of the various components in shelf movement. Much more is known about fusion of the palatal shelves, owing in large part to in vitro studies. Fusion of the apposing shelves, both in vivo and in vitro, is dependent upon adhesion and cell dealth of the midline epithelial cells. Adhesion betweeen apposing epithelial surfaces appears to involve epithelial cell surface macromolecules. Further analysis of palatal epithelial adhesion should be directed towards characterization of those cell surface components responsible for this adhesive interaction. Midline epithelial cells cease DNA synthesis 24-36 h before shelf elevation and contact, become active in the synthesis of cell surface glycoproteins, and subsequently manifest morphological signs of necrosis. Death of the midline epithelial cells is thought to involve a programmed, lysosomal-mediated autolysis...

Catecholamine‐sensitive adenylate cyclase in the developing golden hamster palate

The present study was initiated to determine whether specific hormones would influence adenylate cyclase activity within the maxillary-palatal complex during formation of the hamster secondary palate. Stages from initial appearance of the palatal processes to shortly after birth were studied. Highest basal adenylate cyclase activities occurred during the earliest periods of palate development. This basal enzyme activity began to diminish as palatal fusion occurred and remained lowered until birth. Activation of adenylate cyclase by fluoride was maximal at concentrations of 5-10 mM, and was observed throughout the span of palatal development. Fluoride activation of adenylate cyclase was greatest prior to fusion of the palatal processes, then decreased until birth when a slightly increased enzymatic stimulation was seen. Norepinephrine and epinphrine were the catecholamines most capable of inducing increased activation of adenylate cyclase at most periods of palatal growth. Increased enzyme activity in the presence of norepinephrine was more susceptible to antagonism by the beta adrenergic agent, propranolol, than to the alpha adrenergic agent, phentolamine. The remaining catecholamines, namely isoproterenol and dopamine, displayed a lesser ability to activate the enzyme, and adenylate cyclase was not equally responsive to these catecholamines at identical developmental stages. Other hormones, i.e. histamine, serotonin, thyrotropin, growth hormone, thyroxine and glucagon were generally ineffective in activating the enzyme. Phosphodiesterase activity was not detected until shortly before birth.

Alterations in the epithelial surface of human palatal shelves prior to and during fusion: a scanning electron microscopic study.

AbstractThe surface topography of palatal shelves was examined by scanning electron microscopy prior to and during secondary palate formation in 72 therapeutically aborted human embryos and fetuses ranging in age from 37 days to 12 weeks (post‐fertilization). Alterations in epithelial surface appearance were observed in the prospective region of fusion prior to contact between the shelves. First seen as an elliptical area along the anterior portion of the presumptive medial edge, a zone of alteration spread progressively along the anterior four‐fifths of the medial edge prior to shelf contact, and continued along the posterior one‐fifth of the shelf as fusion proceeded. The altered zone eventually extended onto the oral surface in the middle of the shelf but was restricted to a narrow zone along the medial edge posteriorly. Initially characterized by elongation and “intertwining” of superficial cells, alterations later included cell death, desquamation and possible active migration of surface cells. The shelf epithelium subjacent to the zone of alteration exhibited increased irregularity in thickness and numerous “invaginations” into the underlying mesenchyme. These alterations are thought to reflect intrinsic changes within the shelf and may play a role in the fusion process. Evidence for surface remodeling following fusion was also observed.

Epithelial changes in the presumptive regions of fusion during secondary palate formation.

Specific epithelial changes before fusion of the palatal processes and nasal septum of human fetuses were studied at the fine structural level and were compared with the changes in rodents and chick cervical sinus formation.

The role of the cartilaginous nasal septum in midfacial growth

1. 1. The prenatal development of the cranial base and cartilaginous nasal septum in the midsagittal plane was investigated in normal A/Jax mice and A/Jax mice with spontaneous bilateral clefts of the lip and palate. 2. 2. The emergence of the snout as a prominent rodent characteristic coincided with the extension of the angulated cranial base and was most rapid prior to and during closure of the palatal shelves. 3. 3. There was no significant difference in the amount of extension of the cranial base between the normal fetuses and those with bilateral clefts of the lip and palate. Therefore, the amount of extension of the cranial base was not identified as the important etiologic factor in the formation of a cleft secondary palate in A/Jax mice with bilateral cleft lip. 4. 4. No morphologic or chronologic differences in development of the cranial base were detected between the normal and cleft fetuses. 5. 5. The cartilaginous nasal septum was significantly shorter in the fetuses with bilateral clefts of the lip and palate. This shortness appeared to be the result of a localized anteroposterior deficiency in the anterior third of the septal cartilage. 6. 6. This finding in cleft A/Jax fetuses does not support the view that the nasal septum acts as a “growth center” which set the pace for the downward and forward thrust of the midfacial skeleton during early facial growth.

The distribution of glycogen in mouse and rat palatal processes during secondary palate formation: An ultrastructural study

The ultrastructure of palatal shelves in normal mouse embryos revealed the presence of glycogen reservoirs within the cytoplasm of mesenchymal cells during the shelf transposition stage of secondary palate formation. Shelves of cortisone-treated embryos, however, had considerably less mesenchymal cell glycogen through a comparable period of palatal process development. These findings suggest a relationship between glycogen and the internal shelf force responsible for shelf transposition as well as a possible local action of cortisone during cleft palate induction. In a separate investigation, ultrastructural observations of rat embryo palatal processes which normally fuse at 16 days to 16 1 2 days showed a remarkable decrease in epithelial cell glycogen between the 14 1 2 day embryo and the 15 1 2 day embryo as well as an increase of desmosomal junctions between cells in the older group. Since the shelves of the older group have a higher fusion potential in culture, these findings suggest a positive correlation between glycogen utilization and fusion potential as well as a similar correlation between the development of desmosomes and incidence of fusion.

Ultrastructure of mouse and rat palatal processes prior to and during secondary palate formation

Formation of the secondary palate by the union of two palatine processes or shelves involves a series of rapid changes in the epithelial and mesenchymal shelf components. These alterations are described at the ultrastructural level just prior to, during, and after complete union. The palatal shelves exhibit an irregular outer epithelial plasmalemma which becomes more regular just prior to contact. Desmosomes form at the contacting shelf surfaces and appear to bind the two processes together until mesenchymal union is ultimately made. The newly established epithelial wall gradually breaks down, probably due to an autolytic process. Lysosome-like structures are evident within the epithelial cell cytoplasm as fragmentation of the wall proceeds. The basal lamina separating the epithelium from the mesenchyme remains intact and the underlying mesenchymal cells are unchanged until final epithelial cell disintegration occurs. Mergence rather than fusion is evident in some areas along the epithelial wall, particularly in the soft palate region where, soon after epithelial wall lysis, the basal lamina repairs across the shelves leaving a depression into the young secondary palate which is gradually reduced. Concomitantly hemi-desmosomes appear along the newly formed basal lamina.