Rapid disappearance of the medial epithelial seam during palatal fusion occurs by multifocal breakdown that is preceded by expression of a smooth muscle actin in the epithelium

Breakdown of the medial epithelial seam (MES) is essential to allow bridgin g of the mesenchyme during palatal fusion. Evidence exists for three mechan isms for this breakdown that are incompatible at the level of individual ce lls in the seam. To determine if breakdown of the seam was regionally restr icted, 3-dimensional reconstructions were generated using volume rendering software from 1 mu serial sections in the sagittal plane of rat palates fix ed during the process of fusion. The earliest break detected in electron mi crographs was cell separation and in reconstructions was a discrete defect, with a rounded outline, nearer to the nasal than to the oral margin of the seam. Further breakdown produced a pattern of rounded defects along the na sal margin of the seam resulting in interconnected columns of cells prefere ntially attached to the oral epithelium. Computer generated slicing of reco nstructed seams showed that groups of cells evident in cross-sections as is lands at this stage of breakdown of the MES could be artifacts. Unequivocal islands of epithelial cells formed later in fusion had a rounded outline, an incomplete basal lamina and a halo of cells containing phagocytosed apop totic debris. The pattern of breakdown indicated that the MES breaks down u nder tension. Laser confocal microscopy of sections and whole-mounts of pal ates demonstrated alpha-smooth muscle actin preferentially localized in the epithelial cells of the palatal shelves immediately before and during form ation of the seam. Expression in epithelial cells of the isoform of actin n ormally restricted to smooth muscle cells engaged in tonic contraction supp orted an interpretation that the epithelial cells of the seam may be capabl e of generating tension during the palatal fusion event.