ABSTRACT
The elevation and fusion of palatine are essential processes in the completion of the palatal development. It is believed that the mesenchyme plays a major role in the ascent of the palatal process, and that the palatal epithelium is involved in its fusion. The mechanism of fusion requires several different morphologic and molecular changes prior to the completion of the mesenchymal continuity between different palatine processes. The mechanism of removing the epithelial cells from the fusion zone could include either programmed cell death, epithelial-mesenchymal transformation or migration to adjacent epithelia. Retinoic acid has been known to induce cleft palate by disturbing mesenchymal growth and/or epithelial fusion. The effect of retinoic acid on the epithelium of the palatine process was studied in the fetus of the Sprague- Dawley rat with feeding 100 mg/kg of retinoic acid mixed in olive oil on the 10th day of fetal age and controlled with feeding pure olive oil. The epithelium of the palatine process was examined by PAS reaction and electron- microscopy on the 14th, 15th, 16th and 18th day of fetal age. The obtained results were as follows: 1. In the control group, glycogen was plentiful at the entire epithelium of the palatine process during the pre-fusion period(14th and 15th day), but it diminished in contact epithelium at the fusion stage(16th day). On the contrary, in the experimental group treated with retinoic acid, glycogen was plentiful and did not change from the 14th to 18th day of gestation. 2. In the control group at fusion stage(16th and 18th day), PAS-positive cells increased in the nasal and oral epithelium adjacent to the fusion site, and in the mesenchyme around the fusion site. 3. As a result of electronmicroscopic findings, the epithelium in the experimental group with retinoic acid seemed to be injured by retinoic acid; RER was composed of flattened cisternas and ribosomes were detached, mitochondrial crista and membrane were destructed and sacculated, and Golgi complex was extremely atrophied. According to the results, it seems that apoptosis as well as cell migration and transformation happen in the mechanism of cleaning the epithelium at the contact site, and that retinoic acid injures directly intracytoplasmic organelles and disturbs apoptosis, a sort of normal developmental process. More studies should be done to verify relations between apoptosis and large-sized glycogen granule in superficial epithelium.