Vacuoles and vesicles in the rat junctional epithelium: a study with serial ultrathin sections

To investigate whether the electron-lucent structures resembling vesicles and vacuoles in the rat molar junctional epithelium (JE) are in fact intracellular or extracellular, a study using serial ultrathin sections was carried out. In one series of experiments, the animals were not treated before the tissues were conventionally fixed; in another, anesthetized animals were administered horseradish peroxidase 20 min before the tissues were fixed. A large number of electron-lucent structures resembling vesicles and vacuoles were detected in both the peripheral and central cytoplasm of the JE localized at enamel and connective tissue sites. These were 70 to 800 nm in diameter and had a lucency similar to that of the extracellular space in untreated specimens fixed with conventional fixative. Serial ultrathin sectioning revealed that the electron-lucent structures gradually became part of the extracellular space in the following sections. There were also found in the middle portion of the cytoplasm in specimens pretreated with horseradish peroxidase. Numerous vacuole-like structures containing peroxidase-positive materials were found to be contiguous with the extracellular space. A small number of vesicles, also containing peroxidase-positive materials, did not appear in the previous or following sections. These results indicate that almost all electron-lucent structures resembling vesicles and vacuoles in the JE are located at the end of a long infolding, and are still in contact with the extracellular space.