Direct evidence of epithelial-mesenchymal interdependence in situ.

In the basal odontogenic zone of the mandibular incisor of the rat the cytological evidence of the toxicity of 40 mg cyclophosphamide per kg, 24 hours following a single intraperitoneal injection, was confined to the proliferating mesenchyme. Localised loss of precursors of odontoblasts cessation of basal odontogenesis and acellularity of the related part of the pulp ensured. The cells of the internal enamel epithelium bordering this region did not differentiate into ameloblasts and appeared atrophic. Upon recovery of the proliferating mesenchymal cell population odontogenesis was resumed basal to the zone of the cytotoxic injury. Pulpal connective tissue generating irregular dentine advanced from the reconstituted basal zone into the area of pulpal acellularity. This was followed by the differentiation of the peripheral stunted cells of the internal enamel epithelium into ameloblasts and locally delayed resumption of amelogenesis. These observations appear to offer direct evidence of epithelial-mesenchymal interdependence in situ.