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.     

Strain differences between C57BL/6 and SWV mice in time of palate closure and induction of palatal slit and cleft palate

Palatal slit, which occurs spontaneously in C57BL/6 (C57BL) mice, is increased in frequency among C57BL fetuses from dams treated with triamcinolone acetonide, but is not induced in SWV fetuses. On the other hand, C57BL is more resistant than SWV to cleft palate induction by triamcinolone. Using these C57BL and SWV mice, the relation of palate stage and chronological age was examined from 1 P.M. on day 14 to 9 A.M. on day 16 in untreated embryos, and the condition of the palate after triamcinolone treatment on day 12 was examined at 9 A.M. on day 16. In untreated embryos, horizontalization and fusion of the palatal shelves occurred earlier in C57BL than in SWV embryos, but fusion of the primary palate with the secondary palate occurred later. After triamcinolone treatment, the development of the palate was delayed in both C57BL and SWV embryos. These results suggest that the times of normal palate closure are related to the differences between C57BL and SWV mice in their susceptibilities to palatal slit and cleft palate induction and that triamcinolone produces palatal slit and cleft palate by delaying palate closure.     

Palate development after fetal tongue removal in cortisone-treated mice

Morphological studies of cortisone-induced cleft palate have shown retardation in the rotation of palatine shelves from a sagittal to a transverse plane. Cortisone also reduces fetal muscular movements, which may explain why displacement of the tongue from between the palatine shelves is delayed. Previous work with extrauterine development of control fetuses demonstrated that fetal membranes and tongue were major obstacles to shelf rotation. Thus, removal of these obstacles might permit rotation and fusion of palatine shelves in cortisone-treated fetuses. In the present experiment, fetuses from cortisone-treated strain CD-1 mice were released from uterus and membranes and allowed to develop for eight hours in a fluid medium with the umbilical cord left intact. Compared to 4% fusion in utero, there was palatal fusion in 20% of fetuses released from membranes. When the fetal tongue was removed during extrauterine development, the frequency of fusions increased to 61%. Fusion appeared normal by the criteria applicable through light microscopy. Thus, cortisone induces cleft palate primarily through interference with shelf rotation. The palatine shelves of treated fetuses retain their ability to fuse when they can come in contact during the normal time for palate closure.     

Tissue phosphatase changes following triamcinolone associated with cleft palate in rats.

One theory of the development of cleft palate in rats involves the action of lysosomal enzymes secreted by epithelial cells at the time of fusion of the palatal shelves. To test this theory we studied the biochemistry of the palates of fetal rats daily between days 14 and 19 (from 3 days before to 3 days after palate closure). Triamcinolone was administered once im on gestation day 14 to Wistar rats; 0.5 mg/kg body weight produced approximately 50% cleft palates. Pooled control palatal tissue was compared with pooled experimental tissue; that from fetuses with clefts being pooled separately from those not affected. Acid phosphatase and beta-glucuronidase were assayed. Concentration vs. time curves for both enzymes were very similar. Prior to the time of palate closure both enzymes were present in low concentration. Between days 16 and 17, the normal time of closure, there was an abrupt increased in enzyme concentration, with experimental tissue showing a significant elevation over control tissue on days 17 and 18. Alkaline phosphatase was also present in small amounts before closure and significantly higher in control tissue on day 17. Protein was depressed in palates having clefts on day 17; thus the ratio of enzyme activities to protein synthesis was significantly elevated at a critical time. Unaffected experimental palates had a normal ratio. These results suggest imbalanced acid phosphatase, beta-glucuronidase, and alkaline phosphatase activity compared with protein synthesis at the time of palate closure following triamcinolone in rats.     

Unilateral clefts: a new defect of the rat secondary palate

As shown in a previous study [Schüpbach et al, 1984], different types of total and partial clefts of the secondary palate can be produced through amniocentesis performed in Sprague-Dawley rats at day 16.2 of gestation. Among these were a small number of unilateral clefts that were examined in the scanning electron microscope and in Epon sections. The 410 treated amnions yielded a total of 395 viable fetuses. Total clefts occurred in 48.9% of viable fetuses examined at day 17.8 and in 21.8% of those examined at days 19.3. Partial clefts were observed in 14.1% and 18.5% of viable fetuses examined at days 17.8 and 19.3, respectively. Unilateral clefts were observed in 3.8-10.2% of the partial clefts and in 0.5-1.8% of all viable fetuses. The eight animals with unilateral clefting included fetuses with a total unilateral cleft in the anterior hard palate. Morphological observations suggested that under conditions of delayed palatal closure total unilateral clefts may be the result of initial elevation of one, and delayed elevation of the other, shelf and partial unilateral clefts probably represent the result of an incomplete retrograde closure.     

The effect of hypervitaminosis A on rat palatal development.

Retinoic acid or retinyl acetate was administered to pregnant rats in doses sufficient to induce a 90% incidence of cleft palate. In another study, a delay in the reorientation of the palatal shelves was observed to be longer with the more potent teratogen, retinoic acid. On day 16 of gestation, 24 hours after final dosage with vitamin A, the synthesis of DNA and protein was studied in fetal carcass, mandible, and palate, and that of sulfated mucopolysaccharides (S-MPS) and glycoproteins (GP) in fetal head, mandible, and palate. Increases in DNA synthesis in fetal palate and in GP synthesis in fetal palate were found; thus, the mechanism of action of vitamin A in inducing cleft palates in rats may be caused by interference with the normal biochemical synthetic pattern of the palatal shelves.     

Development of the fetal mouse palate in suspension organ culture

Explanted palates of day 12 and day 13 mouse fetuses were cultured in a chemically defined serumless medium for 48-72 h by a suspension culture technique. The palate of day 12 fetuses closed successfully within 72 h and that of day 13 fetuses within 48 h. Both macroscopically and histologically, the in vitro fusion of palatal shelves simulated the palatogenetic process in vivo. This novel technique for culturing the fetal mouse palate may be of potential use for the study of palatogenesis and in developmental toxicology.     

Palatal slit: a new spontaneous defect of the palate of C57BL/6 mice

A hitherto undescribed palatal defect, here named "palatal slit," was observed during a teratological study of C57BL/6 fetuses. The defect, involving a failure of fusion of the premaxilla and palatal shelves, corresponds to stage 7 in normal palate closure. Adult C57BL/6 mice have been observed with the defect, so it does not represent a developmental delay that is repaired postnatally. Genetic factors of an unknown nature seem to be involved in the occurrence of palatal slit, which does not appear to be related developmentally to cleft palate. Some preliminary information on the defect is reported here.     

Primary veloplasty or primary palatoplasty: some preliminary findings

Staged palatal closure was carried out in 30 children. The soft palate was closed at 9 months and the hard palate at 5 years. These patients were followed up for 7 years, and it was found that although the incidence of lateral crossbite was reduced in both unilateral and bilateral cases, the speech results were less satisfactory than those obtained with total palatal closure. In this series, there were two fistulae at the junction of the hard and soft palate. This was related to difficulty in closing this area in some patients at the time of the second operation. As a result, the procedure is not advised. An alternative palatal closure technique is described. This technique consists of nasal layer closure, careful dissection and reconstruction of the levator musculature, transverse division of the nasal layer, insertion of a buccal flap for lengthening, and closure of the oral layer with Veau flaps without dissection behind the tuberosities and with almost total closure of the lateral donor sites on the palatal shelves. In this way there is minimal scarring, particularly in the retrotuberosity area. This is felt to be important since it would seem from studies of facial growth that this is a much more significant area than the palatal shelves.     

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.     

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

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

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.     

Experimental study on protection of vitamin B6 on TCDD-induced palatal cleft formation in the mice

BACKGROUND: 2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin (TCDD) can cause a high percentage of cleft palate in fetuses when administered during organogenesis in certain strains of mice including the C57BL/6J. In this study, vitamin B₆ (B₆) was tested for antiteratogenic effects on TCDD-induced cleft palate in fetal mice. METHODS: The pregnant C57BL/6J mice were dosed with 24 µg TCDD/kg and/or 5, 10, 20, and 40 mg B₆/kg body weight on gestation day (GD) 10. The control group mice were dosed with 50 ml sesame oil/kg body weight on GD10. The mice were sacrificed on GD12.5, GD13.5, GD14.5, GD15.5, and GD17.5, respectively. The harvested embryos were examined to detect the incidence of cleft palate and the developing palatal shelves in a different phase were investigated morphologically and histologically among different groups. RESULTS: Total frequency of clefts is 55.56% in the TCDD group and 31.81% (5 mg), 44.44% (10 mg), 40.90% (20 mg), and 32.00% (40 mg) in the TCDD+ B₆ groups. There were no statistically significant differences among the TCDD and TCDD+ B₆ groups (p=0.743>0.05). CONCLUSIONS: It was demonstrated in this study that B6 could not antagonize 2, 3, 7, 8-TCDD-indued cleft palate. Birth Defects Res (Part B) 86:357–361, 2009. © 2009 Wiley-Liss, Inc.     

D-penicillamine-induced cleft palate in mice

The teratogenic potential of the lathyrogen, D-penicillamine (DP), was assessed in pregnant mice, especially with respect to its ability to produce cleft palate. The dosage and the duration of treatment as they relate to the induction of cleft palate were also studied. Two different doses of DP were administered orally for either 5 or 4 consecutive days during the critical period of palatal closure. D-penicillamine (DP) at a dose level which does not have any apparent maternal toxic effects produced cleft palate in the offspring, and this teratogenic effect depended more upon the duration of treatment than the dosage administered. Inhibitory effects on the formation of bone matrix were observed at the base of the palatal shelf. It is suggested that DP is potentially an osteolathyrogenic agent. The mechanism of induction of cleft palate in DP-treated mice was explored by histological studies using light microscopy. Delayed elevation of the palatal shelves was observed and is considered to be the cause of the induction of cleft palate. No other external malformations could be detected in DP-treated fetuses.     

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

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

Rescue of an in vitro palate nonfusion model using interposed embryonic mesenchyme

The authors previously established an in vitro palate nonfusion model on the basis of a spatial separation between prefusion embryonic day 13.5 mouse palates (term gestation, 19.5 days). They found that an interpalatal separation distance of 0.48 mm or greater would consistently result in nonfusion after 4 days in organ culture. In the present study, they interposed embryonic palatal mesenchymal tissue between embryonic day 13.5 mouse palatal shelves with interpalatal separation distances greater than 0.48 mm in an attempt to "rescue" this in vitro palate nonfusion phenotype. Because no medial epithelial bilayer (i.e., medial epithelial seam) could potentially form, palatal fusion in vitro was defined as intershelf mesenchymal continuity with resolution of the medial edge epithelia bilaterally. Forty-two (n = 42) palatal shelf pairs from embryonic day 13.5 CD-1 mouse embryos were isolated and placed on cell culture inserts at precisely graded distances (0, 0.67, and 0.95 mm). Positive controls consisted of shelves placed in contact (n = 6). Negative controls consisted of shelves placed at interpalatal separation distances of 0.67 mm (n = 6) and 0.95 mm (n = 7) with no interposed mesenchyme. Experimental groups consisted of embryonic day 13.5 palatal shelves separated by 0.67 mm (n = 11) and 0.95 mm (n = 12) with interposed lateral palatal mesenchyme isolated at the time of palatal shelf harvest. Specimens were cultured for 4 days (n = 19) or 10 days (n = 23), harvested, and evaluated histologically. All positive controls at 4 and 10 days in culture showed complete histologic palatal fusion. All negative controls at 4 days and 10 days in culture remained unfused. Five of six palatal shelves separated at 0.67 mm interpalatal separation distance with interposed mesenchyme were fused at 4 days, and all five were fused at 10 days. At an interpalatal separation distance of 0.95 mm with interposed mesenchyme (n = 12), no palates (zero of four) were fused at 4 days, but seven of eight were fused at 10 days. These data suggest that nonfused palatal shelves can be "rescued" with an interposed graft of endogenous embryonic mesenchyme to induce fusion in vitro.     

Pathogenesis of bromodeoxyuridine-induced cleft palate in hamster

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

Prevention by tiopronin (2-mercaptopropionyl glycine) of methylmercuric chloride-induced teratogenic and fetotoxic effects in mice

Previous investigations (Fuyuta et al., '76, '79) have shown that a single oral administration of 25 mg/kg methylmercuric chloride (MMC) to pregnant ICR mice on day 10 of pregnancy induced cleft palate in a remarkably high incidence in fetuses. Based on these findings, the present study dealing with the prevention of cleft palate by Tiopronin, (2-mercaptopropionyl glycine, Tp), was initiated. Twenty females in the positive control group were given 25 mg/kg MMC orally on day 10 of pregnancy and then given physiological saline intraperitoneally. Twenty females in the negative control group were given distilled water orally and then given saline intraperitoneally. Cleft palate was found in 98.1% of fetuses in the positive control group and none of them in the negative control group. Twenty females were pretreated with a single oral dose of 25 mg/kg MMC on day 10 of pregnancy and were posttreated with Tp intraperitoneally, immediately and at every 24, 48 and 72 hours after the MMC treatment. The doses of Tp were 320, 160 and 80 mg/kg/day. The incidences of cleft palate in fetuses were reduced to 1.49, 31.3 and 47.8% in the Tp-treated groups with the doses of 320, 160 and 80 mg/kg/day, respectively. Tiopronin could effectively prevent the expected incidence of cleft palate. Other types of abnormalities as well as fetotoxicity represented by reduced fetal body weight were also effectively prevented with the Tp-treatment.     

The fetal cleft palate: II. Scarless healing after in utero repair of a congenital model.

The role of fetal surgery in the treatment of non-life-threatening congenital anomalies remains a source of much debate. Before such undertakings can be justified, models must be established that closely resemble the respective human anomalies, and the feasibility and safety of these in utero procedures must be demonstrated. The authors recently described and characterized a congenital model of cleft palate in the goat. The present work demonstrates the methodology they developed to successfully repair these congenital cleft palates in utero, and it shows palatal healing and development after repair. A surgically created cleft model was developed for comparative purposes. Palatal shelf closure normally occurs at approximately day 38 of gestation in the caprine species. Six pregnant goats were gavaged twice daily during gestational days 32 to 41 (term, 145 days) with a plant slurry of Nicotiana glauca containing the piperidine alkaloid anabasine; the 12 fetuses had complete congenital clefts of the secondary palate. Repair of the congenital clefts was performed at 85 days of gestation using a modified von Langenbeck technique employing lateral relaxing incisions with elevation and midline approximation of full-thickness, bilateral, mucoperiosteal palatal flaps followed by single-layer closure. Six congenitally clefted fetuses underwent in utero repair, six remained as unrepaired controls. Twelve normal fetuses underwent surgical cleft creation by excision of a 20 x 3 mm full-thickness midline section of the secondary palate extending from the alveolus to the uvula, at 85 days of gestation. Six surgically clefted fetuses underwent concurrent repair of the cleft at that time; six clefted fetuses remained as unrepaired controls. At 2 weeks of age, no congenitally or surgically created clefts repaired in utero demonstrated gross or histologic evidence of scar formation. A slight indentation at the site of repair was the only remaining evidence of a cleft. At 6 months of age, normal palatal architecture, including that of mucosal, muscular, and glandular elements, was seen grossly and histologically. Cross-section through the mid-portion of the repaired congenitally clefted palates demonstrated reconstitution of a bilaminar palate, with distinct oral and nasal mucosal layers, after single-layer repair. In utero cleft palate repair is technically feasible and results in scarless healing of the mucoperiosteum and velum. The present work represents the first in utero repair of a congenital cleft palate model in any species. The use of a congenital cleft palate model that can be consistently reproduced with high predictability and little variation represents the ideal experimental situation. It provides an opportunity to manipulate specific variables, assess the influence of each change on the outcome and, subsequently, extrapolate such findings to the clinical arena with a greater degree of relevance.     

Spontaneous complete clefting of the palates in a mouse fetus: a study by scanning electron microscopy and serial section reconstruction

A 15 day mouse fetus having spontaneous complete clefting of the primary and secondary palates was studied in comparison with its normal litter mates and with normal 14 day fetuses. Specimens were studied by scanning electron microscopy at various stages of microdissection, by light microscopy of thin serial sections and by serial section reconstruction of the anterior chondrocranium of the clefted specimen and one of its normal litter mates. Differentiation of tooth and bone tissue was slightly retarded in the clefted fetus but paranasal and oral landmarks, though distorted, were present. The clefted fetus had a smaller angle between cranial base and nasal capsule and a marked discontinuity between the primary and secondary palates. Cell surfaces on the medial edge of the secondary palate in the clefted fetus resembled cell surfaces of oral areas that do not normally fuse, i.e. they are polygonial, flat and bear few surface projections in contrast to the normal 14 day condition where these cells are spindle shaped, convex and have many microvilli. The observations support the concepts that clefting of the secondary palate is consequential to clefting of the primary palate, that maldevelopment of neural crest mesenchyme is not necessarily a contributing factor, that clefting of the primary and secondary palates is associated with a shorter anterior-posterior dimension of the head and that when fusion of palatal shelves fails to occur the cells of the medial edges modulate in the direction of a generalized type of surface epithelium.