Fetal development of human oral epithelial pearls with special reference to their stage-dependent changes in distribution

Objective: To access detailed distribution and age-dependent changes of oral epithelial pearls. Design: Investigation and analysis with human fetal serial sections. Setting: Institute of Embryology. Methods: This study examined serial frontal sections of the upper and lower jaws of 19 human fetuses at 12 to 18 weeks and of the lower jaws of four late-stage fetuses. Results: The upper jaw contained more than 20 midline and more than 60 lateral pearls greater than 20 μm in diameter, whereas the lower jaw contained fewer than 30 pearls of the same size. Midline pearls in the upper jaw were often cylindrical or rugby-ball shaped, whereas all pearls in the lower jaw were small and spherical. Epithelial pearls in the upper jaw started developing along the upper midline until 12 weeks; lateral pearls and additional midline pearls (or strictly, paramedian pearls) developed until 15 weeks. In the lower jaw, however, pearl development started at 18 weeks and was almost always from the dental lamina. Some of the fetuses assessed had an open nasopalatine canal without a duct, but there was no fibrous connection between this canal and pearls. Similarly, the lip frenulum or incisive suture was not connected with these pearls. Conclusion: The timing and sequence of development suggest that postfusion rupture of the palate by midline pearls was unlikely.