Ultrastructural changes in rat buccal epithelium following elimination of masticatory movements.

We reported previously that rats kept for 5 weeks on a liquified diet by which masticatory movements were eliminated show widespread reductions of mitotic rates in oral epithelia, the rate in buccal epithelium being a third that in pair fed controls. This report deals with ultrastructural changes in the buccal epithelium of 7 rats subjected to the same experiment. In 43% of control basal cells and 24% of 1st row spinous cells, chromatin showed the full dispersal seen in buccal keratinocytes between the second and fifth spinous row. In all experimental animals, the proportion of these cells was elevated, averaging an 80% increase in the basal layer and more than a 100% increase in the first spinous row. The size of the nucleus was unchanged; the size of the nucleolus was 33% smaller in the basal layer and 30% in the 1st spinous layer. The number of lysosomes and keratohyaline granules per cell and the proportion of cells having these organelles were reduced in both layers, in proportion to the reduced nucleolar size. We conclude (1) that the reduced mitotic rate, entailing lengthened mean cell cycle time and lengthened mean stay in the progenitor compartment, allowed higher proportions of cells in this compartment to reach the chromatin dispersal characteristic of cells in the maturation compartment, and (2) that compared to controls formation of the nucleolus lagged behind the dispersal of chromatin. We suggest that the cells reaching this degree of dispersal might be the post-cycling cells.