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Changes in nasal morphology during primary palate formation in the C57BL/6J mouse embryo as revealed by transmission electron microscopy and histochemistry Nagy, Denis


Primary palate formation in the C57BU6J mouse embryo involves three facial prominences. These are the medial nasal prominence (MNP), lateral nasal prominence (LNP), and the maxillary prominence (MxP). The MNP and LNP come together and fuse forming a nasal fin which is a transient structure that persists for a time and then regresses. Mesenchymal cells invade the nasal fin and form a mesenchymal bridge. In this study, I look at the morphological changes in nasal fin epithelium and its basal lamina as the fin regresses. I also describe the spatial and temporal distribution of filamentous actin (F-actin) at the time of primary palate formation. Morphological changes in the developing nasal cavity are illustrated by performing 3-Dimensional reconstructions from serial sections. This information is incorporated into a model describing cellular events that take place during primary palate formation. C57BU6J female mice were time mated with detection of a vaginal plug being called Day 0 of pregnancy. Females were sacrificed at times ranging from Day 10 to 13, and embryos were removed from the uteri and placed in saline buffer. The embryos were staged developmentally according to tail somite (T.S.) number and then prepared for either fluorescence or transmission electron microscopy. A total of 25 embryos were collected ranging from 7 to 27 T.S. Specimens used in the F-actin study were fixed and then snap frozen in liquid nitrogen in preparation for cryostat sectioning. Sections were stained with a chemical probe for F-actin, NBD-phallacidin, which is a fluorescent phallotoxin that stains specifically only actin of the filamentous type. There appear to be different zones or domains of epithelium within the nasal cavity, nasal fin, and facial prominences as the primary palate is forming with respect to F-actin staining. This reflects different states of epithelial differentiation in these various regions. F-actin appears to be distributed uniformly all around the nasal cavity with higher concentrations in apical epithelium facing the lumen. Also at regions where the nasal cavity bends or evaginates, as detected by the 3- D reconstructions, F-actin content in epithelium at these locations increases dramatically. The facial prominence epithelium stains with reduced intensity especially in regions of presumptive fusion. Nasal fin epithelium of the fused LNP and MNP stains weakly with a disorganized appearance compared to epithelium in other areas. In the TEM study I found that prior to nasal fin formation the basal lamina of the MNP and LNP starts to break down before the prominences come into contact. Once the nasal fin forms, the zone of basal lamina destruction becomes larger. At the point of basal lamina destruction mesenchymal cell processes as well as epithelial cell processes touch and penetrate the lamina. Once the mesenchymal bridge has formed, new basal lamina beneath the base of the nasal cavity and oral cavity epithelia is formed. Comparisons were made of changes in the basal lamina during primary palate formation and in other developmental systems such as Mullerian duct regression, thyroid formation and secondary palate formation. Similarities and differences between these systems and the primary palate were observed. Also the actin distribution in the developing nasal cavity was compared to that seen in the salivary gland. The primary palate may be another example of a system where an epithelial component is transformed to a mesenchymal component with timing of basal lamina changes being important.

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