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The ability of bacteria to penetrate acid-etched and unetched human dentin was examined in vitro using a split chamber. Results indicate that bacteria can grow through or be filtered by pressure through acid-etched dentin. Unetched dentin, while permitting fluid filtration, restricts bacterial penetration. Penetration of dentin tubules by bacteria reduces the rate of fluid filtration across dentin.

Two independent techniques (filtration and surface tension) were developed to measure the functional tubular radii of 11 dentin and 2 glass discs. Anatomic radii were determined using SEM for comparative purposes. The functional radii of the dentin discs ranged from 5 to 40% of the anatomic radii. This differences was due to the fact that SEM visualizes only the surface while the functional techniques measure the radii within dentin tubules. It is concluded that the functional methods are preferred whenever the dynamics of fluid flow through dentin are under consideration.

Epithelial outgrowths from hamster cheek pouch explants were cultured for varying periods of time up to 22 days. Growth of the epithelial sheets was monitored, employing colcemid for demonstrating mitotic activity and tritiated thymidine for DNA synthesis. Mitoses and thymidine uptake were observed among epithelial outgrowths at a considerable distance form the original explant. The epithelial nature of the growing cell sheets was confirmed, employing electron microscopic techniques. The cells exhibited the presence of tonofilaments, desmosomes, ribosomes, Golgi, mitochondria, and rough endoplasmic reticulum. The cultured explants were treated with cyclic nucleotides in order to investigate their modulatory effects on epithelial cell differentiation. Dibutyryl cAMP induced marked mitotic inhibition (46.3%) in our assay, which was increased to 57% with the addition of theophylline. Dibutyryl cGMP showed only a mild (5%) stimulatory effect on mitotic activity. Dibutyryl cAMP enhanced keratinization in the epithelial cell outgrowths with the biogenesis of keratohyalin granules, whereas dibutyryl cGMP did not produce any observable alterations.

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