Fine structure of midline epithelium in the developing palate of the mouse

The midline seam of epithelium in the developing secondary palate of C57BL mice was examined with the electron microscope. Fetal heads in which the palatal shelves had fused throughout their length were fixed in paraformaldehyde prior to microdissection of the midline palatal tissue which was post‐fixed in osmium tetroxide. The midline epithelial seam was two to four cells thick in the center, while at the nasal and oral ends of the seam the cells increased in number to form a triangular area. The epithelial cells most closely resembled those of the stratum germinativum of the oral mucosa, except for the absence of hemidesmosomes. The epithelial sheet was delineated on both sides by what appeared to be a typical basal lamina which was interrupted in some locations, with and without epithelial degeneration. Epithelial break‐down and fibroblastic migration across the midline was observed during a later stage in palatal development. A striking feature of the midline epithelium was the presence of different morphological types of relatively large electron dense granules. The role of the basal lamina, epithelial microvilli and electron dense granules is discussed.