Vascular injection of the macromolecular tracer, horseradish peroxidase (HRP), was used to study the permeability of the odontoblast cell layer in developing and mature rat molar teeth, and to investigate the effect of cavity preparations on the permeability of this epithelioid cell layer in adult animals. HRP injected into the vascular system of normal animals 28 days of age and older was localized histochemically (from 5 to 90 min after injection) throughout the extracellular spaces of the maxillary dental pulps; however, the tracer did not penetrate beyond the tight junctions at the apical region of the odontoblast cell layer, and was absent from the predentin and dentin. In contrast, HRP injected into very young neonatal animals (e.g., day 3) resulted in free passage of HRP between odontoblasts and into the overlying predentin and dentin. When Class V cavities had been prepared in adult maxillary molars after HRP was injected into the blood stream, HRP reaction product penetrated the predentin and dentin immediately beneath the cavity preparation; however, adjacent, un-traumatized areas of predentin and dentin in the operated teeth were devoid of reaction product. These results provide evidence that: (1) a physiological barrier develops between the distal segments of odontoblast cell bodies in normal rat molar teeth between days 15 and 28 of postnatal life, and this barrier prevents the passage of macromolecules from the pulp into the predentin and dentin; and (2) this barrier is perturbed following routine restorative procedures in adult animals.
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